Rebuilding your TH400 - fun and functional

Hi- Recently rebuilt the TH400 in a 92 XJS and a few thousand miles later there have been no faults. About to do another in the XJ12, so it’s time to share what I’ve learned.

This project is not cheap. My methods are not the cheapest either. I always buy the tools and rarely “make do”, but you could cut corners.

First you need “The Book” on rebuilding:

How to Rebuild & Modify GM Turbo 400 Transmissions (S-A Design Workbench Series)

It’s fully color, step by step.

You probably need a lift. You could do this on very tall ramps or jackstands using a very short transmission jack, but it’d be a huge pain in the ass.

You will need an engine support brace. Something like this:

You will need a transmission jack. I went with this one because it goes lower down than the cheaper kind:

I also ended up buying 3/8 impact sockets and extensions to remove all the transmission to engine bolts. It’s all pretty simple… hang the engine off the crossbar, remove the rear transmission mount, tilt the engine back and you can reach all the bolts. Do the bottom two bolts last so if won’t get hairy.

As for rebuilding, follow the book. However I would add…

Buy the bushing installer kit:

Also get a bushing puller kit:

I got fancy and also got a transmission case holder for the engine stand:

Finally, the book talks about “wave plates”. Jaguar TH400 didn’t use wave plates, and instead use dish plates. I found a broken one in my transmission and had to get a new one.


Hello John - great informational post - I have a question - I was of the understanding that the transmission had the internal parts of the GM TH400, and that the torque convertor was a six bolt mount, instead of the three bolt mount, when used for the jaguar, and the external housing was built to fit the V12, as compared to the V8 for the GM products - so what is throw me a bit, is that you had to by an internal part labeled as a Jaguar part (the dish plate) I am not a transmission specialist, but I will be having to rebuild the transmission for my 1986 XJS V12 Coupe so very pleased for all that you posted - Tex.

Hi- the case is the obvious difference but there are more that mostly don’t matter to us. There are actually lots of variations between TH400s across the spectrum, not just Jaguar. Different valve bodies, different valves, different torque converters, different grades of metal for otherwise identical metal parts! The TH400 in a Cadillac is much different than a GMC Diesel. They shift faster slower higher lower softer harder. But if you put back in the same parts you take out, you’re good.

Luckily they all use the same rebuild kit. The only part that was Jaguar specific, and typically replaced, was that dish plate. You could reuse yours but mine was in three pieces when it came out!

I left out the rebuild kit I used:

The kit does not include a case plug, and I broke mine so a new one was needed:

I put in a three bolt torque converter from B&M:

B&M 20404 Tork Master 2400 Torque…


Thank you for the additional information - always glad to get educated - Tex.

A word on torque converters…

If you plan on taking out V12 to the 6000-6500 RPM it is easily capable of make sure you get a torque converter built and balanced to survive at that speed.


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I didn’t mess with the governor so it’s definitely not going to 6000 :slight_smile: The torque converter I linked is really for stock only, it’s slightly higher stall.

And a couple of things I forgot now that I’m about to do it again….

The torque converter bolts were metric but the new torque converter used standard. I had to buy new bolts.

The torque converter bolts were ridiculously tight. They appeared to have red lock title on them. I was forced to remove the engine oil pan and baffle to get an impact wrench on those torque converter bolts.

A flywheel turner was handy:

Lisle 23800 Flywheel Turner

While the exhaust and driveshaft was off, I cleaned up under the car. There were big aluminum heat shields covered in oil from long ago repaired leaks. They were bathed in kerosene.

The engine oil pan, transmission, fly wheel, cover, and rear transmission mount all had some rust. I gave them to my local powdercoater. Blast and powder coat was $60. I don’t think anyone works cheaper so I use him all the time.

The second catalytic converter on one side was very much stuck to the downpipe, so I had to take the entire downpipe out and replace to two exhaust seals. If you take the upper catalytic converters/down pipes out, you’ll have to do the same thing.

On the XJ12 the exhaust is in poor condition so I just bought a new one for $1600 (barf). New downpipes new tips… anyone under my car will be impressed :slight_smile:

More tools (as I find cleaning garage…)

These are NOT affiliate links btw. Get yours where’ve you can find the best price.

Seal tool…. I’ve used the plastic kind before and prefer it I think, but this is what I used this time:

Streetside Tools SST-0015 -…

To align the pump housing you can get a big dryer vent clamp, or use the transmission case… but I was wasting money and used this:

Alignment Tool Accurate SST-1097…

Speaking of wasting money, this pops the pump out of the case… really not necessary but I was planning on doing 3 transmissions:

Streetside Tools SST-1055 - GM -…

I put a drain plug in the pan:

B&M 80250 Automatic Transmission…

To easily compress springs in drums, I got this…. It’s Chinesium and had to be tweaked to make work. It’s ok for $38

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