Automatic Overdrive Transmission Swap
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Automatic Overdrive Transmission Swap

This page is all about what I learned after I decided to swap the original TH400 transmission in my 1975 GMC Suburban for a TH700R4 transmission to gain the benefits of a modern overdrive transmission and help tame my "great for towing" 4.11 rear axle gears on the highway. As part of this project, I also had to build a custom Throttle Valve Cable Bracket so I could hookup the TV cable to the carburetor, and I had to learn about the Lockup TCC Wiring that you need to do.

Part of any swap to non-original parts involves researching what fits where, finding the right parts, and then getting it all to work together. You often have to modify existing parts of custom fabricate new parts to make it all work together like it should. This page is documenting what I have learned about this swap as I worked through the process. I hope you can learn something useful from this page, especially if you are contemplated this type of swap in a vehicle you own or work on. I have tried to explain my thought processes as I figured out what to use and why, along with some possible alternate results if the situation was a bit different. I do have other vehicles, and they could also benefit from a swap to an overdrive transmission, so I have tried hard to make the background research reasonably complete and to include any relevant details and tidbits I turned up along the way. The bolded lines in the charts below are the items that were relevant to my swap and are the most verified bits of data in here.


Transmission Features

The first consideration in any swapping project is "what is the most advantageous part to install". After all, you're going to do a lot of work here - both mental and physical - so you want to pick the parts that will give you the most benefits with an amount of effort you are comfortable with putting into your vehicle. The chart below compares what I feel are some of the more important features of the various various GM transmissions that are available: the gear rations, whether they can use a lockup torque converter, whether a separate transmission computer is required, and the available bellhousing mounting patterns. There is a separate section below for all of the transmission size data.

Transmission 1st 2nd 3rd 4th Lockup Converter Computer Required Bellhousing Pattern Stock Usage
Power Glide 1.76 1.0 - - no no Chevy V8
TH350 2.52 1.52 1.0 - no no Chevy or BOP V6/V8
TH400/3L80 2.48 1.48 1.0 - no no Chevy or BOP Big V8
TH700R4/4L60 3.06 1.63 1.0 .70 yes no Chevy V6/V8
TH200R4 2.74 1.57 1.0 .67 yes no Chevy and BOP V6
4L60E 3.06 1.63 1.0 .70 yes yes Chevy V6/V8
4L80E 2.48 1.48 1.0 .75 yes yes Chevy Big V8

The first consideration is gearing - primarily the fact that we want to use an transmission with an added 4th gear that is an overdrive gear. That overdrive gear will help keep the RPM down on the highway and is the primary advantage to switching to a later model transmission. All four overdrive transmissions fit this need, but with different gearing for the overdrive. The TH200R4 offers the most overdrive, followed by the TH700R4/4L60 and 4L60E, with the 4L80E coming in as the least advantageous overdrive here. (Please note that earlier versions of this page got the previous comment on "most advantageous overdrive" exactly backwards due to a brain-dead error on my part in editing this page. Kudos to a sharp-eyed reader named Ryan who spotted it and wrote in to let me know about it so I could fix it.)

Next is the gearing of the two lowest forward gears - third is always 1:1, so no differences there. The TH700R4/4L60, the 4L60E, and the TH200R4 all offer a lower first gear and second gear than my original TH400 with the TH700R4/4L60 and 4L60E offering the best choice with a 3.06 first gear. That's going to help get you underway easier, get you moving faster, and - in the case of my truck - haul heavier loads easier. In each of these cases, you also get a slightly deeper second gear to help keep the RPM drop between gears more manageable. In this case the 700R4/4L60 and 4L60E are the clear winners, though the TH200R4 is a close second. Some folks also say that the RPM drop between 1st and 2nd gear (which is directly related to how "far apart" the gear ratios are) on the TH700R4 is too much and that the closer gear ratios on the TH200R4 actually make for more useful power to the ground throughout 1st and 2nd gear.

Next is the usage of a lockup style torque converter. This allows to to couple the engine directly to the gears in the transmission during stable highway driving and thus eliminate any slippage in the torque converter. This eliminates heat in the transmission, allows for lower engine RPM's while cruising, and gives improvements in gas mileage. The converter is more expensive, but you'll save that back in gas mileage and reduced wear and tear on the engine and transmission. Some high performance transmission folks frown on using lockup type torque converters in high horsepower dragstrip applications, but they are very practical for street use, and many folks are quite successful in drag racing their later model automatic equipped GM vehicles. They also seem to withstand heavy duty truck use pretty well provided you have everything set up correctly. All four of the overdrive transmissions allow for the use of a lockup torque converter, so there is no advantage for any of the transmission designs here.

Next is the computer and wiring requirements. As the years progressed, even the transmissions in later model cars became computer controlled. For a low-buck retrofit application, being forced to use a computer for the transmission is undesirable complexity. The computer controlled transmissions do offer greater control over things, but like converting from a carb to fuel injection, you pay more for that control and you have to put up with the electronics to get there. With the computer controlled units, you can get a transmission that shifts smooth as silk at light throttle and will bark the tires like it has a full-on shift kit at full throttle! If the added wiring, cost, and complexity is a problem for you, then beware of the 4L60E and the 4L80E. The TH700R4/4L60 and the TH200R4 do not require a computer to control them. The wiring for a non-computer controlled transmission isn't terribly complicated, but does require some thought. In my initial swap, the TH700R4/4L60 and the TH200R4 were the winners because they don't need a computer. I am still toying with the idea of using a 4L80E behind the Buick 455 in my 1973 Electra at some point in the future, even if it is just daydreaming for now. In the end, there is no automatic advantage here either way - pick what you want and go with it.

Next is the transmission bellhousing bolt pattern. In my case I am bolting the transmission to a Chevy engine, so I could use any of the transmissions without any problems. If I was trying to swap an overdrive transmission behind a different engine - perhaps the Buick 455 in my 1973 Electra - then it would be a different story. I would either be restricted to using the TH200R4, or I would have to use an adaptor to be able to mount a Chevy transmission to a BOP engine bellhousing. The adaptors are an added expense, but beyond that, they do not really affect the choice of transmission for it's basic features, so I consider this one a draw overall with no advantage to any particular overdrive transmission.

Last is the stock usage - this is a rough way to compare the power handling levels of these transmission in stock form. If you are planning on rebuilding your transmission completely or buying it through a specialty rebuilder who can "beef up" the transmission, then this doesn't matter quite as much, though there still are limits. All of these transmission can be built to handle some serious abuse - more than most street engines can dish out, though abusing a transmission with big block torque is a tall order for transmissions that was used almost exclusively behind smaller V8 and V6 engines. On the flip side, if you are not planning on rebuilding or modifying your chosen transmission, then this matters a lot. If you stuff a transmission meant for a V6 behind a healthy V8, chances are pretty good that the transmission won't live very long or work as well as you'd hoped. This is exactly what I plan on doing for my swap - grabbing a transmission and bolting it in with little more than a fluid and filter change. In this case, the TH700R4/4L60, 4L60E, and 4L80E are the clear winners as you can find them in versions that originally came behind V8 engines in stock form and they are all capable of handling decent power levels for street usage. In fact, the internals of the TH700R4/4L60 and 4L60E are basically the same regardless of what engine they came behind, with only some small differences in the number of clutches and the specific shift tuning. There are special heavy-duty truck cases, but based on my research on the Internet, case breakage is apparently extremely rare in a TH700R4/4L60, so this is not a major concern for me. Also, there are some high-horsepower versions of the TH200R4 that came in applications such as the Grand National, but these are somewhat rare finds in the junkyard. Oft rumored, never seen outside of some small grainy photos in your favorite car magazine when they "wandered through their local junkyard" and saw one. If you're the person who always finds these sorts of gems for dirt cheap, feel free to write me and tell me how many you've found so I can be jealous. :-)

Looking over this list of features and comparisons and pondering them for my specific application, the computer controlled transmissions were pretty much discarded right away because of the added and unwanted complexity they bring to this particular swap. That leaves the TH700R4/4L60 and the TH200R4, with the edge given to the TH700R4/4L60 because of it's lower first gear and availability behind V8 engines in stock form. So the hunt began to find the perfect TH700R4/4L60 that would bolt into my vehicle with as few custom modifications as possible.


Size Matters

No, really - it does. Sure, you can beat metal out of the way and weld up new mount pieces, but it's all extra work compared to just bolting something in place. When performing any transmission swap, you need to consider the overall length of the transmission, the length of the main case, the length of the extension housing, the location of the mounting pad relative to the bellhousing face, and the mounting pattern of the transmission mount. These are all important in determining of the transmission you are looking at will in fact fit in your vehicle, and what modifications will be required to make it work. Note that while it's a pretty huge chart, the sizing information below is but a subset of the information available on the Internet, so don't despair if your transmission isn't listed here. Go search for the size information and you should be able to find it without too much trauma.

The graphic below shows which dimension is which. All dimensions are in inches.

  •  Dimension "A" is the overall transmission length and varies with the length of the extension housing used.
  •  Dimension "B" is the length of the main transmission case.
  •  Dimension "C" is the distance from the bellhousing to the center of the transmission mounting pad. Note that mounting pad is usually part of the extension housing, but the graphic shows it on the main case. This means that dimension "C" is usually longer than dimension "B"
  •  Dimension "D" is the distance between the center of the mounting bolts - this is the "width" of the mounting pad.
  •  Dimension "E" is the width of the transmission at the bellhousing and combined with dimension "B", helps you roughly compare the size of the various transmissions.

Transmission A B C D E Notes
Power Glide 25 23/64" 16 5/16" 20 9/16" 3 3/4" 19" Short shaft
Power Glide 27 9/16" 16 5/16" 20 9/16" 3 3/4" 19" Long shaft
TH350 27 11/16" 21 5/8" 20 3/8" 3 3/4" 19 1/8" 6" Extension housing
TH350 30 11/16" 21 5/8" 20 3/8" 3 3/4" 19 1/8" 9" Extension housing
TH350 33 27/32" 21 5/8" 20 3/8" 3 3/4" 19 1/8" 12" Extension housing
TH400/3L80 28 3/8" 24 5/16" 26 15/16" 4 1/4" 20" 4" ("C") Short extension housing
TH400/3L80 33 27/32" 24 5/16" 27 15/16" 4 1/4" 20" 9" ("D") Medium extension housing
TH400/3L80 37 7/8" 24 5/16" 27 21/32" 1 3/4" 20" 13" Long extension housing
TH700R4/4L60 30 3/4" 23 3/8" 22 1/2" 3 3/4" 20" All, except Corvette
TH700R4/4L60 29 7/8" 23 3/8" 22 1/2" 3 3/4" 20" Corvette, 1982-1992
TH700R4/4L60 29 1/2" 23 3/8" 22 1/2" 3 3/4" 20" With TH350 6" extension housing
TH700R4/4L60 30 1/2" 23 3/8" 28" 3 3/4" 20" With extension housing 24214689 and using mount on extension housing
TH200R4 27 11/16" n/a 27" 3 3/4" 19 1/8" Integral extension housing
4L60E 30 3/4" 23 3/8" 22 1/2" 3 3/4" 20" All 1993-1995, except Corvette
4L60E 29 7/8" 23 3/8" 22 1/2" 3 3/4" 20" Corvette, 1993-1995
4L60E 30 3/4" 21 3/4" 23 3/16" 3 3/4" 18 1/4" All 1996-later, except Corvette and LS1
4L60E 31 5/32" 21 3/4" 23 19/32" 3 3/4" 18 1/4" LS1, 1998-later
4L60E n/a n/a n/a n/a n/a Corvette transaxle, 1996-later
4L80E 32 11/16" 26" 30 3/8" 4 1/4" 20" 1991-later, Std 2WD
4L80E 31 11/16" 26" 30 3/8" 4 1/4" 20" 1991-later, HD 2WD
4L80E 33" 26" 30 3/8" 4 1/4" 20" 1991-later, HD Long
4L80E 29" 26" 30 3/8" n/a 20" 1991-later, 4WD

I had already settled on a TH600R4/4L60 transmission based on the features, now I had to figure out which one was an acceptable donor for my uses - if any. My original TH400 transmission was a "short shaft" version measuring 28 3/8" in length with the transmission mounting pad located 26 15/16" from the bellhousing and the actual transmission case length being 24 5/16". The measurements I made of the TH400 transmission I removed from my truck were within 1/16" of the published specs. I am attributing that to my errors with the tape measure in my haste to get mine measured so I could get back into a warm house, and I am simply using the published specs.

First up, let's tackle overall length and case length. Overall length is important for the final driveshaft length and the case length is important for the fitment to the transmission tunnel. In my case, the overall length will be either 2 1/8" or 2 3/8" longer, depending on what extension housing I end up using. This will require shortening the driveshaft by a matching amount, so I know that no matter what I will be paying a visit to the local driveshaft shop, which I would be doing anyway - see the section on Inputs and Output for details on why. For the fitment to the transmission tunnel, the main case in on a TH700R4/4L60 is actually 15/16" shorter than the TH400 I am replacing and is the exact same width at the bellhousing. That means the transmission should fit nicely without any major clearance issues. This is all good news and there are no major problems to solve here. If I was opting to go with the 4L80E, the situation would be reversed - the main case on the 4L80E is 1 11/16" longer than the TH400, so there could easily be transmission tunnel clearance issues in your vehicle that require some creating hammering to make it all work. Depending on the specific original and donor transmissions, the full length transmission is anywhere from a tad shorter to tad longer, making that part more a less a wash and a change in driveshaft length all but certain.

That leaves the location of the transmission mount pad relative to the bellhousing. This distance is important because of the transmission crossmember that holds up the back of the transmission. Moving it to a new location on the frame could be difficult, and getting/making a custom fabricated piece to use the existing mounting holes the frame is either expensive, time consuming, or both. The typical TH700R4/4L60 places the transmission mounting pad a full 4 7/16" closer to the engine than the TH400 does. It's also  2 1/8" further rearward than the mount point for the TH350, so no factory transmission mount would work here. The mounting point for the typical TH700R4/4L60 is effectively right between the existing mounting points I could come up with using factory parts. That's bad. Another interesting detail that should be noted is that the original mounting point for the TH700R4/4L60 is not on the extension housing as it is on the TH400 transmissions I am used to dealing with. It is located on the main transmission case like on the TH350 - notice that for the TH700R4/4L60 dimension "C" (22 1/2") in the chart is 7/8" shorter than dimension "B" (23 3/8").

Here are some pictures illustrating this. The first two pictures are of a TH700R4/4L60, and you have to look close to tell, but if you stare at them for a while you will notice the distinct lack of a mounting pad on the extension housing in most cases. The next five pictures are of the used TH700R4 transmission I purchased to use in my swap with the special extension housing explained in the next section - they should give you some idea of the overall size and shape of the transmission. The final four pictures have excellent views of the mounting pad on the case as well as the mounting pad on the special extension housing that I will be using. In the first photo in that final group of four (taken from "underneath" the rear of the transmission looking up), the case mount has two bolts loosely threaded into it and the tailshaft mount has the remains of the original transmission mount still attached to it in. The last three photos show the rear section of the transmission after I had it mounted on an engine stand in the garage and turned over for a better view.

TH700R4Cutaway.jpg (45769 bytes) TH700R4PassengersSide.jpg (31328 bytes) TH700R4Pictures01.jpg (1449026 bytes) TH700R4Pictures02.jpg (1407006 bytes) TH700R4Pictures03.jpg (1372855 bytes) TH700R4Pictures04.jpg (1434503 bytes) TH700R4Pictures05.jpg (1536559 bytes) TH700R4Pictures06.jpg (1414903 bytes) TH700R4Pictures07.jpg (1637975 bytes) TH700R4Pictures08.jpg (1647857 bytes) TH700R4Pictures09.jpg (1490431 bytes)

One final note - if you decide to use an adaptor to mate your transmission to a different engine than it was intended to bolt to, make sure you find out all of the installed dimensions of the extra pieces required! This is because the adaptor has some thickness to it - typically about an inch - and that thickness will move the entire transmission rearward by that same amount. That moves the transmission mounting pad, decreases clearance to the transmission tunnel, and will affect the final length of your driveshaft - so double check things to make sure they still work as expected and to make sure everything fits and bolts together properly.



Solutions to Mounting Problems

In the chart above, I have included the measurements for the TH700R4/4L60 transmission with the "24214689" extension housing. This is the easiest solution I have found when you are replacing a TH400 with a TH700R4/4L60 because it adds a second mounting pad on the extension housing that you can use, and that new mounting pad is much close to the original TH400 mounting positions. Depending on the specific TH400 extension housing your vehicle came with originally, this extension housing places the trans mount anywhere from 1 1/16" (short extension housing), to 11/32" (long extension housing), to just 1/16" (medium extension housing) further rearward. I have the worst case scenario in my application, using the short extension housing, so have to deal with the mount being 1 1/16" further rearward that the original TH400 mount. Those of you with the medium extension housing only need to elongate the existing holes by 1/16" with the proper size drill bit, those of you with the long extension housing need to go a bit more at 11/32" - just a little bit more than 1/4".

I did some quick measurements on my transmission crossmember, and I will be able to drill completely new holes in my crossmember at the right location (using my new drill press, of course!) to make this work and the bolts should clear the rear edge - barely. The problem is that this will leave the mount very close to the rear of the crossmember, with some of the back edge of the mount hanging off the crossmember. Because of this I am also contemplating doing some basic cutting, welding, and extending to make the center 6" or so of my crossmember extend another 1" or so to the rear. A few simple cuts with the cutoff wheel combined with a few filler pieces of metal welded in place should do the trick.

My first clue leading me to this solution came from a Chevy Hi Performance article on doing overdrive transmission swaps - in their "required parts" chart they list an extension housing (GM part number 8673406) as a required part along with a TH400 style transmission crossmember. The text of the article also had more useful information. I could not find any references to that part number, so I was still a bit stumped on this one.

The TH700 is equipped with a removable tailshaft housing that has varied in length for different vehicles over different years. The tailshaft housing to look for if you’re replacing an original TH400 trans came stock on mid-’80s police cars (Caprice) and the early Impala SS. This tailshaft housing puts the mounting pad almost in line with the original TH400 pad and usually requires only slight elongation of the mounting holes to fit. If you’re replacing a TH350 trans, you can relocate the factory crossmember mounting holes rearward approximately 1½ inches or swap in an original TH400 crossmember.

However, this contradicted the mounting pad location data I had gotten from pretty much everywhere else on the Internet, This apparent conflict got me pondering the situation, and until I could get a real live TH700R4/4L60 transmission to look at, I had to go back to searching about the internet for answers. Those searches led me to some notes for GM part number 24214689 on a webpage devoted to GM installation instructions for their "over the counter" version of the 4L60. Here is the relevant snippet from that page explaining what this part is for.

Extension Housing
Extension Housing
     An extension housing (P/N 24214689) is available for the 4L60 transmission that moves the rear transmission mount to the same position as a TH400 (short-style) or 200R4 transmission. This extension housing (“tailshaft”) can eliminate transmission crossmember modifications when replacing a TH400 or 200R4 transmission with a 4L60 assembly.
     Note: When replacing a TH400 transmission, the driveshaft yoke must be changed to fit the 4L60 output shaft

I even found another page offering this part for sale and it listed dimensions as well as some other useful tidbits. Even better - I could now calculate the actual mounting pad location and overall length of the transmission based on the dimensions I had already found. The chart above reflects those calculations, but they are marked with a "?" because they are as of yet unverified.

# 49. 700R4 tail housing with transmission mount (7 3/8 inches long). This will move the mount to the rear 5 1/2 inches. This moves the rear transmission mount to the same position as a TH400 (short-style) or 200-4R transmission. This extension housing can eliminate transmission crossmember modifications when replacing a TH400 or 200-4R transmission with a 700R4 assembly.
These are almost impossible to find. I've seen these at the GM Dealer for $145.08, $191.25 elsewhere. When in stock $60.00

More hunting revealed that part number might have been used at least on 1982 to 1984 Caprices with the TH700R4 transmission, maybe even more models. I also did a part number search at for "24214689" and got a hit, and the description listed "8673406" as the new part number - the same part number from the original Chevy High Performance article. Jackpot! I think this is going to be the winning combination. If I can find a used trans with this on it, great. If not, I'll have to find what I can and then swap this on as needed. That way I can avoid doing any more modifications than I have to. I can also swap speedometer gears while I have the extension housing off so the speedometer is plausibly correct - the donor transmission will be geared for whatever car it came out of, and 4.11 rear ends aren't exactly common. I can use this handy page to calculate what I need.

The transmission mount itself should also be pretty easy - the TH700R4/4L60 uses the same mount as a TH350, and once you find the right TH700R4 to use, it should bolt up with only minimal elongation of the mounting holes for the transmission mount in the transmission mounting bracket. In addition, there are some very nice mounts available that can bolt up the TH350 or TH400 and to the single or two bolt crossmember mounts - check them out at Jegs.

Here are some comparison photos of my original TH400 transmission side-by-side with the TH700R4 transmission I purchased to swap into my vehicle. As you study the pictures, you should be able to see the various details noted in the text of this article and get an idea for how similarly sized these two transmission really are.

TH700R4ToTH400Comparison01.jpg (1501044 bytes) TH700R4ToTH400Comparison02.jpg (1500191 bytes) TH700R4ToTH400Comparison03.jpg (1499482 bytes) TH700R4ToTH400Comparison04.jpg (1479380 bytes)

Here's a close-up of the new transmission mount I'm using.

TH700R4Mount01.jpg (1537802 bytes)

Here are the changes I had to do to my existing crossmember to make it work. The 1 1/16" distance I figured on having to move the mount was exactly right - too bad my initial drilling was 1 1/8" - just a tad too far. (It was user error on the drill press all the way on that one... Practice makes perfect, live and learn, etc, etc. :-) A quick elongation of the holes with a hand-held drill solved the problem and some final clean-up with a file made sure there were no sharp metal edges around the new holes to catch anything on - like my fingers! I also used a wire wheel in my hand-held drill to strip all the crud and what there was remaining of the original paint off the crossmember so I could see what I was doing for the test-fitting and drilling work. After I was sure it was right, I gave it another once over with the wire wheel to get all the last touches of crud off. Then I painted it a nice semi-flat black to prevent rust and make it look decent. I also tossed the top braces into my sandblast cabinet, cleaned them up, and painted them the same semi-flat black before I installed them. I think it looks rather nice - unfortunately, it makes the rest of the underbody really look crappy... :-)

TH700R4CrossmemberModifications01.jpg (1320778 bytes) TH700R4CrossmemberModifications02.jpg (1400142 bytes) TH700R4CrossmemberModifications03.jpg (1392379 bytes) TH700R4CrossmemberModifications04.jpg (1550058 bytes) TH700R4CrossmemberModifications05.jpg (1426028 bytes) TH700R4CrossmemberModifications06.jpg (1565432 bytes) TH700R4CrossmemberModifications07.jpg (1368344 bytes) TH700R4CrossmemberModifications08.jpg (1478783 bytes) TH700R4CrossmemberModifications09.jpg (1497622 bytes) TH700R4CrossmemberModifications10.jpg (1476666 bytes) TH700R4CrossmemberModifications11.jpg (1460406 bytes) TH700R4CrossmemberModifications12.jpg (1534093 bytes)


Inputs and Outputs

When sapping stuff around, you need to know what the inputs are and what the outputs are, not just where in space they are. Note that I am not getting into swapping torque converters for this discussion - you would need to use a torque converter than matches your chosen transmission - easy as that. For my swap from a TH400 to a TH700R4/4L60, that meant I had two important, but easy, things to take care of.

First, I needed to make sure I had a flexplate that would accept the smaller TH350-style torque converter. Most GM flexplates are "dual-pattern" so they can accept both styles, but some are not. If mine is not, I get to swap it out for a readily available dual-pattern unit. With the transmission already out, this is a trivial project - 6 bolts to remove to get it off, yank the flexplate, situate the new one, then tighten down the 6 bolts with a touch of Loctite for good measure.

Second, I would need to use a TH350-style output shaft on my driveshaft. That means I need to make a visit to the driveshaft shop and have them swap it - playing with driveshafts at home is virtually impossible, so contact a professional. Since the driveshaft will need to be shortened anyway, this is the perfect time to get this done. The extra cost is minimal, and it's not even an extra trip. I even have the proper transmission yoke - it came with my transmission - and if it's in good shape it can be transferred directly to the newly shortened driveshaft. Up here in the Seattle area, I use Drivelines NW for my work.

Speaking of driveshafts, virtually all of these swaps require changes to the driveshaft, some shorter, a few longer, and a lucky few require no changes at all. Once you get your transmission installed and settled down onto the mounts, you will need to work with your favorite local driveshaft shop to get the proper work done. They will want some details about your application and the exact measurements for your driveshaft. Depending on the shop they will ask you to take the measurements in specific ways to ensure the driveshaft fits and works correctly. Those measurements a key to getting a correct length driveshaft for your application! The distance between the rear of the transmission and the rear axle changes as the rear axle moves up and down, and at some point in the driveshaft there has to be a "slip yoke" that allows for this change in length. Usually the slip yoke goes directly into the back of the transmission, but other applications place it in the middle of the driveshaft in one form or another. On my application, there is a slip yoke installed into the back of the transmission connected by a single U-joint to a short piece of driveshaft. The rear of that short driveshaft is mounted in a support bearing with a second slip joint that is mounted on a second crossmember between the frame rails about 2' behind the transmission. Then there is a rear driveshaft section with U-joints at each and and a traditional slip joint at the front that connects to the short driveshaft piece. My understanding is that these multiple piece driveshafts are done to prevent flexing and odd harmonic problems with long driveshafts, and to allow for smoother operation at more extreme input and output angles. If you have ever looked under a large and long truck, you have probably seen one of these multiple-piece driveshafts with one or more support bearings near each "middle" U-joint. In any event, it is important that when the vehicle is at final ride height that the slip yoke be roughly centered in it's range of travel. If the slip yoke is too close to the transmission and does not have enough travel, you could bottom out the slip yoke at full suspension compression and break the transmission, bend the driveshaft, damage the rear axle, or all of the above. If the slip yoke is too far away from the transmission, it could fall out of the transmission under full suspension rebound (wheels at full droop), become loose and damage the output shaft of the transmission or the oil seal, or all of the above. In short - get it right! The typical way to do this is to make the measurements with the transmission mounted as desired and the rear suspension at normal ride height. I do this by putting the vehicle on jack stands with the rear set of jack stands under the rear axle so the rear suspension is compressed to normal ride height. Some shops will have to measure from "flange to flange" and subtract out half of the slip yoke's acceptable travel distance, other shops will have you install the slip yoke to the middle of it's travel and measure the length directly. Good shops can work with pretty much any measuring style you give them, provided you are clear about what you measured and accurate when you took your measurements. Whatever the measurement style you use, make 100% sure that your measurements are clearly understood by the shop you are working with.

Transmission Converter Bolt Circle Output Shaft Splines Notes
Power Glide 10.75" 27  
TH350 10.75" 27  
TH400/3L80 11.5" 32  
TH700R4/4L60 10.75" 27  
TH200R4 10.75" 27  
4L60E 10.75" 27  
4L60E 11.00" 27 LS1, 1998-later
4L80E 11.5" 32  


Shifters and Linkage

There is the matter of the shifter on the column/console, and the indicator in the dash/console- GM overdrive transmissions added one more position to the shift pattern. The original shift pattern is P-R-N-D-2-1 and the overdrive transmission shift pattern is P-R-N-D-3-2-1. To fix this you may need to replace the shift lever detent plate with one from a later model or use an aftermarket or custom one that is set up for an overdrive transmission - it all depends on your original shifter. I would suggest that you try it out first to see how your original shifter works with your new overdrive transmission before you go nuts changing things. Sometimes you get lucky... If not, various sources offer these for the console shifters, but I have not yet found anything for the column shifters. If all else fails, you can take the junkyard route and use pieces from a later vehicle that originally came with an overdrive transmission - in my case that would be a 1982 or later Chevy pickup that came with a TH700R4 transmission.

The indicator label will also have to be changed to one appropriate for an overdrive transmission. You may be able to pirate the indicator label plate from a later model vehicle, if not, I'll need to make up a label of some kind to make this work - you definitely do not want your wife to get into the vehicle and not be able to find "Drive" because the indicator is bogus. :-)

The chart below is the best I can discern for shifter linkage location on each type of transmission based on info scattered on the internet and what transmissions are related to each other, and which transmission are known to be easily interchangeable. If you happen to have one of these and can confirm one of the lines with a "?", please let me know.

Transmission Shift Linkage Distance From Front of Transmission
Power Glide 8 5/8?"
TH350 8 5/8?"
TH400/3L80 9"
TH700R4/4L60 9 1/4"
TH200R4 8 5/8?"
4L60E 9 1/4"?"
4L80E 9?"

In my case, the shift linkage is a rather simple matter. The transmissions use the same style of shifter hookup on the transmission, the transmissions are roughly the same width, and the location of the shifter hookup relative to the front of the transmission is within 1/4". It should be a simple matter of bolting my original linkage to the new transmission, making sure it is adjusted properly, and lubricated everything for good measure.

The important thing I have been able to figure out is that there is about an inch difference between the shortest and the longest distances here, and that might just be too much for the stock style linkage to deal with. If your situation is more extreme enough, you may find that your linkage is binding and needs to be modified. This could be as easy and bending some stuff to clear, or it could mean drilling some new mounting holes in the frame for that part of the shift linkage. I found one web page that said you had to drill new mounting holes in the frame in swap that started with a Power Glide, so now you know to keep an eye on this.

This photo shows the shifter linkage inside the transmission pan with the linkage in the park position - note the extra distance to travel between Park and Reverse as compared to the other gears.

TH700R4InternalShiftLinkage01.jpg (1477534 bytes)

The shift pointer plate I found from a later model Chevy pickup worked fine, but as I originally hooked up my linkage, it won't go down into 1st gear on the shifter. I am not yet sure if the linkage is simply mis-adjusted, or if I have to change something inside the steering column. The linkage for the column is adjustable relative to the transmission, and I'm hoping I simply have it misadjusted.

TODO: Update this section once I have tried adjusting my shift linkage.


TV Cable

This is probably the hardest to understand and most critical part of a swap like this. I have some great details on my custom Throttle Valve Cable Bracket page about why this matters and how it all works together, so go read all about it there. The biggest thing to know is that the TV cable used on the overdrive transmissions is not a simple kick-down cable and it must be installed and adjusted properly before the vehicle is driven. Failure to do so can cause the transmission to completely burn out in as little as 100 miles - or less. Seriously. Pay attention, read up, and do this right.

This photo is the TV linkage inside the transmission pan - the small wire pointing "down" through the hole in the photo is the wire that sticks up through the pan rail area when the transmission is upright and installed in the car. This is where you clip the end of the TV cable onto. Attentive readers will also note the piece of lint in the picture that I had to clean off before I reassembled the transmission pan!

TH700R4InternalTVCable.jpg (1387717 bytes)

These pictures show the TV cable in relation to the rest of the transmission along with a decent exterior view of both sides of the transmission.

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Too Hot To Handle

Installing a transmission temperature gauge is just a smart idea to keep an eye on things to make sure there are no signs of impending doom. This is especially true after a swap where parts are being installed into situations that the factory was not really intending them to be installed and things can get weird in a hurry if you're not careful. Also, if you are slamming a junkyard transmission into a car with little of no internal work being done to it, this helps give you an idea of the internal condition of the transmission. A worn and slipping transmission will make a lot more heat than a tight new transmission will, and a gauge is the easiest and cheapest way to keep on this. Jeg's sells a nice transmission temperature gauge kit from B&M under part number 130-80212 that has the gauge, mounting enclosure, sender, wiring, and the stuff you'll need to hook everything up. VDO and others also offer transmission temperature gauges - check out the listing on

Speaking of heat, anyone who has even read up on automatic transmissions should know that heat is the #1 killer of automatic transmissions. TCI has a great chart showing transmission temperature vs. life expectancy that will really drive this point home. As noted above, a gauge will help you keep an eye on this, but how do you help keep the temperature down? Simple - add an auxiliary transmission cooler. Many places offer them, and the designs, styles, mounting, and plumbing options are immense. You can find complete kits for under $50 and installation is really easy, so there is just no excuse for not taking this simple step to protect your transmission.

If you drive in cold climates, you can even get a remote oil cooler thermostat to help the oil heat up faster. Perma-Cool and Derale seem to offer the same part, and Derale has a nice instruction sheet online showing hose routing and hookups. The only thing I do not like about these is the lack of a solid mounting bracket, though I could machine one myself, it seems kind of lame one is not offered and they recommend mounting it with tie-wraps.


Various and Possibly Interesting Details

This section is the grab bag of stuff that didn't really fit into other sections on this page. If it feels a bit random, that's because it is.

I talked a bit about transmission bellhousing bolt patterns, and there are some interesting details in here around the 3800 engine series and the TH700R4/4L60 transmissions. They actually came in one of two different Chevy bellhousing patterns - the traditional Chevy "90 degree" V6/V8 pattern, and a slightly different pattern for the Chevy "60 degree" V6 engines. The "60 degree" engines were lower powered, and some aspects of the internals transmissions that went behind them were not as beefy as their V8 cousins. For the bellhousing bolt patterns, there are some oddities in here due to how the various engines worked their way into different models, the most interesting being the 3800 V6 engines. These started life as the Buick V6 engines installed in various RWD vehicles in the 70's and 80's and they all used the BOP style bellhousing bolt pattern - I work on Buick's a lot and that's why I care about this engine and how it fits into the story. When GM modified the 3800 engines for FWD use in the mid 80's, they apparently changed the bellhousing design to something shared by other FWD engines - I'd guess something about mounting points and structural rigidity had something to do with the changes. Then, the 3800 engine managed to find it's way back into certain RWD configurations in the 90's - and instead of using the original BOP style bellhousing, GM decided to change it yet again to the "60 degree" V6 engine bellhousing bolt pattern used on other engines - presumably so they could share transmission cases with other RWD vehicles already in production. Some research when I did this said that the TH700R4 was available in a BOP bellhousing pattern, but based on further research, it appears that the RWD 3800 engine versions of the transmission were what folks thought had a BOP bellhousing pattern, but in fact it appears to be the "60 degree" V6 bellhousing pattern. Also, my understanding is that the 3800 engine is a "90 degree" V6 design, and if true, the fact that it got lumped into the "60 degree" engine family for the bellhousing bolt pattern is a bit odd. If you know differently, or simply have more details, I'd love to know about it so I can update this page.

Another side thought here is that the early overdrive transmissions had some teething problems and the factory did a bunch of work to improve them in the first few years. As an example, only post 1985 TH700R4 units (or those rebuilt to post-1985 specs) are considered acceptable in stock form - the 1982-1985 models had known reliability problems and each year saw various minor (and not so minor) changes in the internals of the transmission to make it better. Some folks say 1987 is the cutoff year, others go so far as 1988 or 1989. The basic rule is that the newer the transmission, the better the internals tend to be. All of the upgrades can be applied to the earlier transmissions if you go through and rebuild them, but if you plan on grabbing a transmission and slapping it into your car, this is something to ponder as you do your searching for a suitable donor transmission to start with. I'm sure a great many of those early year transmissions went for many miles without problems, but many did not. Fortunately, the early 1982-1987 donor vehicles are approaching 20 years on the road, and if they lasted this long, they are either going to last a while, or they have already been rebuilt and upgraded to the newer internal specifications. Based on this, and the fact that the "ideal" donor transmission to replace a TH400 was used in the 1982-1984 Caprice's, I opted to use a donor transmission out of a 1983 Caprice. I did buy it through a local wrecking yard that offers a 30-day warranty on all their transmissions, so there is a bit of extra piece of mind from that. It's just one more thing to keep in mind when contemplating this type of swap and when looking for a suitable donor transmission.

I added a drain plug while I had the pan off. I bought a magnetic 1/2" drain plug at the parts store and a matching 1/2"-20 nut to thread onto it. Then I marked the location I wanted to put it, drilled a 1/2" hole in the pan at that spot, and then welded the nut onto the inside of the pan. The weld just holds the nut in place and the gasket on the outside of the pan seals the entire thing up - so no worries about the weld not being oil-tight, it just has to be secure.

TH700R4DrainPlug01.jpg (1442821 bytes) TH700R4DrainPlug02.jpg (1489941 bytes) TH700R4DrainPlug03.jpg (1412064 bytes)

I built a simple lifting bar to help me get the transmission jacked up high enough to get it into my truck. The problem here was that I had to wheel the transmission under the car on the floor jack with it all the way down so it would clear the engine crossmember (by less than an inch!), but the jack didn't have enough lift to get the transmission up high enough to get it into the truck without putting some wood spacers between the jack and the transmission. This bar allowed me to do the job without having to bench press the transmission up into place. It's a simple piece of angle iron with two holes drilled in it that match the two lower from mounting holes on the transmission face and a pair of appropriately sized bolts and nuts is used to secure the bar to the transmission.

Here's how it works. You wheel the transmission under the vehicle on the jack normally and get it under the vehicle in the rough space where it needs to go. Then you turn it sideways and jack it up as far as it will go. Place a jack stand under the tail end and a small jack of your choice under the lifting bar at the front. Lower the main jack, insert enough wood spacers to get the total height you need, then lift the transmission off the lifting bar and jack stand. Remove the lifting bar, jack stand, and second jack from under the vehicle, turn the transmission back to it's proper orientation, then jack it up into place. It turns a two man transmission wrestling job into a single man piece of cake.

If you don't have room to turn the transmission sideways, just make the lifting bar longer and use supports at each side at the front and hold it up that way. Be creative as needed, just make sure that whatever you use has enough strength to support the weight of your transmission!

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Parts Needed

This is The Big Parts List. I have included all possible parts I could find, including some interesting (and possibly not so interesting) variations on the basic parts you will need. Some are redundant depending on how you choose to do the wiring and installation. Some will not be needed at all in any given situation. This chart is mainly for reference and for collecting all the possible part numbers in one place - it is not a shopping list of stuff to buy to complete the swap.

Part Part Number Source Notes
Donor TH700R4/4L60 transmission Varies Varies Used or new, as desired
Donor TH700R4/4L60 torque converter Varies Varies Used or new, as desired
Dipstick Assembly Varies Varies Used or new, as desired
Extension Housing 24214689 or 8673406 GM Only if trans is not so equipped already
TV cable 376800 TCI Only if old cable is not usable or missing
Dual pattern flexplate 399273 TCI Only needed if your flexplate isn't dual pattern
Transmission Crossmember varies varies Modify original or buy one custom-made
TH350-style slip yoke 14075214 GM Can get from driveshaft shop or it may come with your transmission
Lockup Wiring Kit 376600 TCI Can be wired without kit if needed and you have the proper parts and knowledge; additional parts may still be required for some situations
TCC solenoid (2-Wire) 8654126 GM Needed if your donor transmission has a 1-wire TCC solenoid and you want to use certain wiring options; your local transmission shop may be an alternate/cheaper source
TCC solenoid (1-Wire, Self-Grounding) 8654123 GM Needed if your donor transmission has a 2-wire TCC solenoid and you want to use certain wiring options; your local transmission shop may be an alternate/cheaper source
Pressure Switch (NO, Two Terminal) 8643710 GM Needed if your donor transmission does not have the correct "normally open/pressure closed" 4th gear pressure switch
Pressure Switch (NC, Two Terminal) 8642346 GM Different pressure switch variant that could be used in certain situations
Pressure Switch (NO, Self-Grounding) 8642473 or 8627332 GM Needed if you want to try and use the TCC Signal switch to control the TCC lockup indicator light
Pressure Switch (NC, Self-Grounding) 8642569 or 8634475 GM Different pressure switch variant that could be used in certain situations
Low Vacuum Switch (NO) 14014519 GM Needed to prevent TCC lockup under load; get as much as the harness as possible if buying a used switch
Low Vacuum Switch Wire Connector 12101914 GM Used to wire the low vacuum switch
Vacuum Delay Valve 14020691 GM Delays vacuum switch engagement; helps ensure TCC lockup only under stable engine operation
Adjustable Vacuum Switch (1-6 inches) 71V PATC Adjustable vacuum switch; has both NO and NC contacts
Adjustable Vacuum Switch (6-22 inches) 71V2 PATC Adjustable vacuum switch; has both NO and NC contacts; could be useful for a high vacuum over-ride switch for deceleration
Adjustable Vacuum Switch (Varies) DigiKey Search on "vacuum switch" to see what they have. These are switches made by Honeywell. You have to read the spec sheets on each one to find out what will work - the spec sheet from Honeywell on these switches if very enlightening and worth a read - the vacuum switches are at the bottom. They even have switches with MetriPak 280 style terminals on them. These are also much cheaper than the PATC switches - about half the price. It's also not clear if these are adjustable or not. DigiKey may not stock all variations that Honeywell makes.
Adjustable Vacuum Switch (Varies) DigiKey Search on "PSF109S" to see what they have. These are adjustable switches made by DesignFLEX. These are also much cheaper than the PATC switches - less than half the price. DigiKey may not stock all variations that DesignFLEX makes.
Cruise Control Style Brake Switch 25524845 GM Unlocks TCC when brakes are applied; also used as part of cruise control in some cases
Transmission Connector Plug (External) with wiring pigtail 12085533 GM Plugs into the transmission case connector from the outside and allows you to hook up your external wiring as desired
Transmission Connector Plug (External) without wiring pigtail 71XXX PATC Plugs into the transmission case connector from the outside and allows you to hook up your external wiring as desired
Transmission Connector Plug (Internal) ???????? GM Plugs into the transmission case connector from the outside and allows you to hook up your internal wiring as desired
Transmission Connector Plug Crimp-on Lug (Internal Connector) 12103347 GM These are the metal connectors that you crimp onto the wire ends and then push into the plastic connector body. If you can re-use your plastic connector body, you can use these. This one was labeled "M/P 280 Series 1.0-3.0 Gau" and appears to be for smaller wire. These are the ones I used.
Transmission Connector Plug Crimp-on Lug (Internal Connector) 12033821 GM These are the metal connectors that you crimp onto the wire ends and then push into the plastic connector body. If you can re-use your plastic connector body, you can use these. This one was labeled "M/P 280 Series 3.0-5.0 Gau" and appear to be for slightly bigger wire.
Transmission Case Connector 8634383 GM This is piece that clips into the wiring hole in the case, seals things up, and the internal and external connectors connect to it; you should not need to replace it except in rare situations; comes with a new o-ring
Wire, various colors, high temp varies varies I prefer to custom wiring everything from scratch when I do any wiring work so I have exact control over things and no slicing
TV Cable Bracket varies varies Buy or build as needed
TV Cable Geometry Corrector Kit 376715 TCI For Holley carbs
TV Cable Geometry Corrector Kit 376710 TCI For Edelbrock carbs
Speedometer Gears varies GM Only if needed to recalibrate speedometer
Shop Manual for Donor Transmission varies varies Optional, but highly recommended
Transmission Temperature Gauge varies varies Optional, but highly recommended
Auxiliary Transmission Cooler varies varies Optional, but highly recommended
External Thermostat for Auxiliary Cooler varies varies Optional, but recommended if you operate your vehicle in very cold climates

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

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Page last updated 01/02/2009 01:51:39 PM