TH400 Rebuild or Leave as is?

I decided to make a separate thread for the Transmission to make it less confusing if more questions arise. Ok so I when I pulled my v12 from the junkyard car I took the TH400 with it, I want your opinion, what would you do, should I rebuild it or “risk” it and just install it as is? I know that the car has roughly 80k miles on it, the fluid is nice and red and it looks like the car was in the yard because it was a project someone gave up on, half the body had Bondo and primer on it and the engine bay was filled with mouse nests and droppings aka it sat for a long time. I know that these TH400’s have a reputation for being tough as nails and I figured that this transmission is underworked in these cars as it was originally intended for heavy-duty GM trucks up to 8000lbs so I am willing to bet that it is still good, I dont mind tearing it out once its in as I will be trying to tune the governor for better performance so it will likely come out atleast once anyways. Trying to save a bit of money/heartache as I am of the opinion that Automatic transmissions are “black magic fuckery” and am too scared to rebuild one. Or should I just pony up the cash and get a pro to look at it?

Rebuild it now.

The TH400s were very tough in terms of handling weight and torque but they are not universally known for long life.

Back in my days at GM dealerships it was very common for them to need overhaul at 80k-100k miles.

I’ve several cars with the TH400. A couple went wayyyy over 100k miles. One over 200k miles. A couple others, about 100k. The Th400 in my XJS needed o/h at 80k miles. The TH400 behind my present V12 needed o/h at about 100k…although exact miles is uncertain due to prior speedometer failures


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It was originally intended for GM’s full size/luxury cars…although it was indeed used in many truck applications.


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Ok, Thanks for the quick info! By chance would you know a good rebuild kit to use? and a manual? I have the Green shop manual for XJ6/XJ12 it tells you how to rebuild one but I am sure a second “opinion” would help. Are most rebuild kits universal to TH400’s or is the Jag one special?

I think any 400 kit will do…but there are varying quality and price levels

I have mine done by pros. Trans overhaul is something I never learned.

If you replace the torque converter (and you probably should) make sure to a Jaguar spec (or otherwise upgraded) unit. The Jaguar spec TCs have a higher stall speed than found in most applications (2100 rpm as I recall) and are heavier duty to withstand high RPM.


Ok will look into some pro shops, I live in southern California so good transmission shops and hot rod shops are a dime a dozen, what do you usually pay for a refresh?

I have looked around and I cant seem to find the answer, I plan on swapping this H.E. V12 and TH400 into a series 1 XJ6, I wanted to know if the TH400 will fit the transmission tunnel without modification? or will i need to make some space. Also does anyone know if the shifter will work correctly or do I need to get one out of a TH400 car? Thanks!

Don’t ask !

15 years ago it was about $1500 as I recall

A couple years ago about $2000

In both cases the cost includes R&R of the transmission.

I wanted to R&R the transmission myself and have a pro do a ‘bench overhaul’ but I couldn’t find even one repair shop willing to do so. Can’t say that I blame them, really.


Pretty sure you’ll need a shifter from a 400 car due to the spacing of the detents.

When I did my V12 conversion I summarily used the shifter from the V12 donor car. I didn’t bother with any experimenting so I can’t absolutely say that your existing shifter won’t work.


Hi William. I cannot help you with th400 details but I can share my experience with transmission rebuilding with you as I went through a transmission failure in a cts with a six speed automatic recently. First no local transmission shop will rebuild your transmission without the r&r being done in their shop. I found that most of the shops actually sent the transmissions to large scale rebuilders and then charged a markup plus the labor charges for r&r. When I contacted the large rebuilders they were happy to rebuild the transmission but no warranty unless installed by a recognized shop. At this point because the quotes were higher than the value of the older cts I decided to do it myself despite having no previous transmission experience. In reality the most difficult part of the job was the removal and the replacement of the transmission. (Modern car+very tightly packed in plus awd).
The failure in my car was an output shaft so the repair was only replacing the shaft plus seals but to reach the shaft it required completely dismantling the box. I subscribed to an online source for the shop manual. The other very important thing is youtube and google are your friends. I would expect you to find multiple videos of good old boys rebuilding 400’s if you search. There will likely be little tips that are extremely helpful. I just started at one end and carefully dismantled it piece by piece keeping as many components together as possible and arranging them in order on a clean work bench. Take pictures of everything as you go and make notes as there may be bearings,thrust washers etc that need to be oriented in the correct way. Once it is completely dismantled you reassemble it with the new wear parts from your rebuild kit. I have NEVER done any transmission work previously but I am mechanically inclined. I found it fiddly fitting the clutch packs etc but nothing was really difficult. I did have to purchase one or two special tools like monster snap ring pliers but nothing too expensive. If your mechanical skills are good you should have no problem rebuilding your yourself. It does require a large work space to set out parts in a logical order. Buy a good quality name brand rebuild kit. Good luck.

Thank you! I am fairly certain that I can rebuild it as I too am mechanically inclined, I have rebuilt engines, suspensions, differentials before but never an automatic transmission, as I stated in my first post it always seemed like “black magic fuckery” so I strayed away from it, but seeing how much money I can save doing it myself, there is no way I am taking it to a shop when I know I can do it myself. I priced it out and a “good” stock rebuild kit was about $150 from summit along with a few of the normal mods that are done to the TH400’s like the “Shift improver” valve body, adjustable vacuum modulator, and the governor tuning kit. it equals out to be about $320

Good deal! I expect you will be shocked at just how straight forward it is.

Yea I have been watching videos and it doesn’t seem too hard, probably a couple weekends worth of work for my first time.

Good video:

I found this type of tool very useful for handling the huge snap rings safely. Wear eye protection when removing them. They can do a lot of damage.

Thanks! I will look into that. From the videos I have watched the big snap rings cap be taken out with a screwdriver fairly safely, I have ordered some upgraded spiral snap rings as the factory ones on the TH400 are known to be a little on the weak side for higher rpm shifts.

You have to love old GM stuff. There are good quality aftermarket upgrades for just about everything. I had a 76 xj6 years ago. It is still the favourite driver of all the cars I have owned. If I were to come across an excellent condition xj12c I would be very tempted but realistically I expect I will run out of time before running out of projects. Good luck on yours.

Thanks Geoff, I’ll update this thread with some pics when I get started. I too want an XJ12c really badly but they are fairly rare and going up in value, so I will probably have to stick with a series 1 XJ6 v12 conversion. I recently got my Series 3 on the road, I saved it from the Junkyard back when they were allowing you to purchase whole cars. I got a rebuilt title so not perfect but I don’t expect to ever sell the car as It was the car that taught me a ton of patience and how to “properly” repair something as I am sure your series 2 or first classic Jag did with you. It took me about 4 years to get it where it is now and I love every minute of driving it! even when something breaks…every 5 seconds :laughing:

A TH400 is approximately the easiest automatic transmission there is to D-I-Y rebuild.

All the clutches in front, all the gears in back. No pistons in the case that need dedicated tools to remove.

I suggest buying the ATSG overhaul manual for the 400, plus a copy of Cliff Ruggle’s TH400 book.

There are some “special tools” that make the job go easier. A puller for the pump, a music-wire-with-handle to “shoehorn” the piston seals into place, and a press of some sort–arbor, hydraulic, even a screw-and-framework will do–to compress the springs on the piston retainer so you can remove the snap-rings.

If it were me, I’d also install a TransGo “Shift Kit”. Unlike the more-famous “performance” shift kits that produce harsh shifts, the TransGo shift kit is more of a durability-improver.

Thanks for the info I have the rebuild kit and the shift improver kit. along with the manuals.