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Discussion Starter #1 (Edited)

Just thought I'd share my refreshing of my old TH400 transmission. I should preface this with saying it only has about 20,000 miles on it since it was last built by me 25 years ago. I was in college way back when I did this. However it's not just mileage that kills transmissions it's age. As this one developed a leak several months ago that intensified to about a gallon a month on the floor, plus when it was cold outside reverse wouldn't work till it warmed up some.

I just refreshened this transmission because I plan on retrofitting my Caprice Classic with a 4L80E. However I'm not ready for that at this point so I want to just spend the minimum on this TH400 to make it happy once more. There shouldn't be too much wrong, other than seals with a low mileage transmission.

Enough of the waffling, picture time.

Here's the tranny out of the car and the outside cleaned. This hasn't seen the light of day in 25 years.

This is the medium length version with a 9" tail shaft. There is a more common shorter one and much less common longer one.

First step is see if I can use my old transmission to engine stand adapter I made a while back for the Ford C6's. I contrived this ugly contraption together with a pile of old scrap steel I had leftover from several other projects.

The mount holes on the C6 tranny's are up high and I needed to make a little bracket adapter or two.

That's the side adapters...

And the front adapter.

And there we have it. Interestingly I made this for the C6 with a C.G. without the torque convertor. However the GM TH400 is balanced (C.G.) with the torque converter. So it's a little back heavy without the torque converter installed. But still more than doable rotating it by hand on the stand.

I've rebuilt transmissions on a bench before and it's a total pain in the... Well you know. No more of that needless struggling. This makes it infinitely easier to work on.

It's by no means elegant but it works.

Before I continue I will say this is not a step by step, I'm only highlighting certain parts. Since I don't plan on keeping the transmission I didn't get super involved in it. I did a complete step by step tear down, rollerizing and building up, and assembly of two C6's. I will also probably do the same for the 4L80E. Just not this one.

First thing to do is drain the thing. The transmission fluid probably looked good because it kept getting new fluid almost on a daily basis from hideous leaks.

However this transmission in this application takes a beating. This looks like a transmission with 120K miles on it, not 20K miles. The reason why it takes a beating, aside from fun pressing the go pedal is my Caprice Classic convertible has a curb weight of ~ 5000 lbs. It was also stuck with a 2.73 gear ratio all its life till recently. (see Ford 9" conversion in another thread)

As a result of the sheer weight plus a low axle ratio the propshaft loads the transmission something fierce. Add to that the engine that is no longer stock (couldn't originally spin its tyres on wet leaves) now develops around 600 ft/lbs of torque at the crank starting around 2500 RPM. That much torque and a hard to turn propshaft and the thing in the middle of that (TH-400) suffers.

All that gray sludge is, if you're new to transmissions, is the friction material on some of the parts wearing off. Much in the same way your brake pads wear. The difference is this friction material ends up in the ATF (darkening it) rather than being washed out of your wheels during a car wash.

This is why it's so important to change your transmission fluid about 30K miles. I know some newer cars say 100K or even never. But those cars are made disposable. Sorry newer car owners. Even our 1996 Impy I believe says 100K miles but if you wait that long chances are the transmission is done. Done as in stick a fork in it, it's done.

This poor Turbo 400 has only 20K miles on it and I should have changed the fluid and filter every 7K miles or so from the hard life it leads.

The fluid is still a bit brown, but I can imagine what it would be like if it hadn't leaked so much and the old fluid remained.

Showing pictures of nasty parts being pulled from the transmission isn't very interesting nor does it make for good pictures. Here's the TH-400 completely dismantled and cleaned. This is a tedious process and every part needs to be inspected for more problems and ultimately more parts I need to order. There were just a few. I'll cover the highlights in putting it back together.

I start with assembling the sub assemblies like this forward clutch. The forward clutch should have little to no wear because the only time it engages or disengages is when you select a drive range (D,2,1) and you shouldn't be neutral slamming the trans anyway. These frictions and steel clutch plates all look fine with no wear as measured with a caliper.

Here's the forward clutch assembled. I use Vaseline when assembling the piston seals into the pistons and bores and then I soak the frictions in a Ziplock bag with ATF. It's easy with less mess.

And this point I found the problems with the trans. Some seals were turning hard and shrinking whilst some others were turning very soft and spongy and expanding. Hence the no reverse when cold and the myriad of leaks it developed.

This is the direct clutch. This and the intermediate clutch takes a beating on shifting. The frictions were half used up but still serviceable so they went back in.

This little roller one way sprag is the key to making a TH400 live behind a built up big block. The early 70's to later TH-400's use a light duty version of this and it's rubbish. The light duty roller clutch/sprag I kept blowing on the TH-400 since my '73 Caprice Classic had the light duty version installed by then. I kept going through transmissions, even had them rebuilt by so called reputable shops and they would still blow. This transmission is actually out of a 1 ton box truck that lived a long time. I installed this in my Caprice and it blew apart on the first full throttle run.

I was 19 or so at the time and didn't know a darn thing about anything really and so I bought a couple of books on the TH-400 and learned about this and had to hunt down an early style (late 60's up to 70 I think) direct drum and install a high number count element one way sprag. So I did and never in 25 years had a problem with the transmission till now with its leaks and no reverse when cold.

Direct clutch with new seals and the old frictions and steels assembled.

The only bushing I replaced in the transmission was the torque converter bushing, just in case it was worn as the front seal was one of the leaks.

I don't trust ATP seals as I had problems with them in the past so I opt for a National Seal on the pump and tail shaft.

I removed all that hideous US Navy gray paint from the transmission. I don't know what I was thinking all those years ago.

This looks a bit nicer.

Clean inside and out. Ready for final assembly.

I'll stop here and pick this up in the next segment.


p.s. if there are any questions or comments please feel free to respond.

25 Posts
Discussion Starter #2
Hello again, picking up where I left off.

Some of the plastic parts were just roasted from heat and age. The downshift connector (downshift is electric and not cable on a TH400) was brittle and just fell apart. One of the extra parts I had to get.

I needed a new speedo drive and driven gear ratio to match the new rear axle gear ratio change. Plus the old speedo gears were brittle as well.

The only other hard parts that needed changing were the two large thrust washers in the planetary's. The TH400 had Torringtons in the other key locations, had GM rollerized these two as well the transmission would be considered rollerized and it would help reduce parasitic loss.

You can see the old washer in the middle has just about all the copper worn off and it was steel on steel. The reason why these wear is because the planetary's gears are cut on an angle to reduce gear whine. The problem is under load the angle cut gears either want to push away or suck together depending on power flow and the little washer in the middle get sandwiched, rather hard, between two rotating parts of different speeds or between the fixed case and a rotating part. The heavier the vehicle and/or the more torque the engine has the more the transmission suffers on parts like this.

I flipped the steel case saver washer/spacer around to a good side and used that with the new copper plated thrust washer.

This is where one of the large washers that wore down goes.

Then that whole assembly can go into the case.

This is the rear band and only comes on in reverse and LO (1) range. It should have no wear unless you're towing something heavy and using the transmission as a brake to slow you down constantly. Since I don't, this one doesn't have wear. If it's worn it could also be from slipping which usually means low pressure and you have to diagnose that further.

This was the other heavily worn thrust washer and its replacement. If I was keeping the transmission I would rollerize it like I did the C6's and just rid the transmission of these.

This washer is next down in the hole onto the planetary with its tabs facing down. I use Vaseline to hold things together.

Sun gear and shaft go in next.

Like so.

The forward planetary assembly goes in next.

Everything should mesh together nicely.

The centre support (right) goes in next along with the snap ring. The centre support also contains the piston that engages the intermediate clutch.

The snap ring has yet to be installed but if everything is set in place correctly the snap ring should fit right in.

There's a black 12 point bolt with a through hole all the way through it. This is tightened next and help holds the centre support. The hole through the bolt allows the pressurizing of the piston in the centre support. The holes to the left and right pass through the centre support unto the rings and pressurize the direct clutch when needed. The piston in the direct clutch has two areas. One for 3rd gear and then using both for reverse as reverse has a higher ratio gear than 3rd so you need more surface area to squeeze the clutches tighter to keep them from slipping.

This is the intermediate clutch (2nd gear) and probably takes the worst beating. Again about half the life left and still serviceable so back it went. There were a couple little hot spots, but nothing to get excited about.

The small band and forward clutch go in next. The small band only gets used when you select 2nd gear manually. This is 2nd gear with engine braking and should only be worn if you're using the engine to slow you down with a heavy load constantly.

Slowly getting there.

Had to the buy the entire thrust washer kit for the two I needed so I replaced all the rest even though they really didn't need it. What the heck. This is the forward clutch assembly and it goes in next.

You can use a punch if you're careful to rotate the forward clutch till all the teeth in the clutches mate.

To set the total end play you buy a selective thrust washer kit. This select washer thickness goes on the front pump. The two old ones on the left were worn a bit and the black ones come in the kit and are new. Now I end up using two of them because I am using an earlier style direct drum (for the higher capacity one way clutch) and the stack up dimensions are that much different between the years of TH400's.

The procedure is pick a close selective washer, use the new pump gasket, lightly torque the pump into place and for me I found inverting the transmission and letting all the weight of the internal parts push forward was much easier, then I zero the gauge.

Then I can stick a screwdriver in the front and lift up on the forward drum against the case and measure the end play.

The allowable end play GM gives is rather generous, however I don't like clunky transmissions so I try to get end play somewhere around 5 - 8 thou. In this case I had to lightly sand one washer to get me there. Yes I was lazy and didn't use the mill. Like I said if I was keeping the transmission I'd be doing a whole lot more to it, this will do as long as you sand evenly.

Once the end play is set the pump can get torqued down one last time with new sealing washers from the seal kit.

The rotating guts portion is complete.

Changing the seals on the accumulator and servo piston. The accumulator seal was so shrunken it wasn't even touching the bore inside the servo. Like I said, age kills a transmission just as much as mileage does.

Another accumulator/servo assembly to change seals on.

Gasket and spacer plate.

I didn't cover it but the whole valve body was taken apart, cleaned and each valve checked for smooth operation and not hanging up in the bore.

There are two different detent solenoids used (forced downshift). The open frame ones like this one do not need the metal gasket in the kit. Don't use it for this type of open frame solenoid.

I only use Wix filters as they are rated as one of the best.

Bottom end done.

The governor was cleaned and checked for smooth operation and installed.

The old modulator was checked with a vacuum gauge pump for leaks and it was fine so it was reused.

Assembled the extension housing and pan.


Here's a neat comparison to a C6 with a FE big block bolt pattern for one of our galaxies I did. This C6 was one of two that got the full treatment. All Red Eagle frictions, Kolene steels, I machined the drums for a higher friction steel count, wider Red Eagle band, rollerized the entire transmission (no more high friction flat thrust washers) and a custom made full size torque converter with a 2400 flash stall. All that means it behaves like a regular torque convertor would under normal driving conditions but step on the fun pedal and it will flash up higher putting the engine back in the power band sooner. Plus even the torque convertor is rollerized.

It's a nice setup, drive the car normally and it behaves like a stock Grand Marquis with smooth shifts, step on the throttle and the engine flares up with torque multiplication to help plant you in the seat. The 390 in the Ford I built is rated just north of 500 horse at 5600 RPM and completely mild mannered below that with a smooth idle. Fun car.

I will eventually do something similar but with a higher horsepower for my Caprice Classic.

And there it's nestled back in its home.

I've been driving the car as my daily driver once more for 3 weeks now, doesn't leak a drop of ATF, reverse works at any temperature and the shifts under normal driving are luxury car smooth with the seals on the accumulators changed. Floor it and it will still bark the wide 255 rear tyres on the 1-2 shift. For me this is exactly how I like my transmissions. Couldn't be any more happier. Well I could be if I had the 4L80E in it.

Speaking of which here it is.

These darn things are really hard to find in 2wd and not with a diesel bolt pattern. With the help of a good friend I got lucky and found this little gem for 380 bucks. It will be completely torn down and much will be done to it, including a wide ratio conversion similar to that of the 4L60 (700R4)/E transmission.

I did some preliminary homework and if you're looking for one you want a 1999 or newer. The majority of the fixes from the early years have been implemented in the 1999 models. In other words, most of the bugs are out of it.

This one came from a 1 ton with a gen VI 454 and it will bolt right up to my old Mark IV 454 in the Caprice Classic. The 1999 non diesel also sports dual pattern (extra bolt hole at top) for the newer LS engines as well.

I will say one thing, this is one heavy back braking transmission. It makes the TH400 and C6 feel very light indeed. That darn torque converter feels like GM filled it with lead. I did some online poking around the single clutch converters are 60 pounds and the triple heavy duty clutch converters are around 80 pounds. I think this is the heavy duty one as it weighs a spinal disc popping stupid amount.

Thanks for sticking around and reading all of this drivel. :wink:

Hopefully if anything it inspires someone to want to tackle their own projects, be it a transmission or anything really. I'm not a mechanic, just an electrical engineer by college and trade. If I can do it, you can do it. It just takes patience, common sense and dedication.

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