Inspection of a 904 torqueflite

This was some of the information I was looking at.
It says at the bottom of it you can just run a lower gear with no problems. ............................it’s an older post I found somewhere ........



I just went through 904 h*ll trying to match parts together. Let me share my story. . .

A long time ago, in a land far away. .. (ok it was only 6 years ago) I had a duster, that I wanted to put a 904 in. High 11 sec 340. nothing too special. From day one, the 904 gave me problems. The trans would flare the 2~3 shift, but only if I really wound it up (say over 6,000 rpm) To make a long story short, I replaced the following parts in no particular order. Valve body, pump/stator, high gear drum, input shaft/rear drum/ clutches/steels/bands/2nd gear servo/ int. band lever/ etc. . .basically every part in the trans. I finally gave up and put a 727 back in it. Well 6 years have passed, I have coleen steels, raybestos blue plate special race clutches, and an $800 turbo action converter, that I just can't give up on. So I decided to jump back in for my 416" EFI duster I'm doing right now.

At the time, I called every trans shop in the country. Nobody could figure out what it was. So this time, I really spent a lot of time trying to figure out what it was myself.

Well at the time when I put the trans together, in my infinate wisdom (I was only 20 by the way), I decided to put 5 rear (thin) clutches in my 4 clutch high gear drum. I'd seen this in pro trans' and other race type stuff, so I machined down my pressure plate, and made it all work. Now upon close inspection this time, I realized that the bottom clutch of the high gear clutch pack, was borderline to engagement on the rear clutch splines. What I figured out was happening, is that at low RPM, as the piston moved in the high gear clutch to clamp the clutchpack, even when the bottom clutch wasn't splined (indexed), it would simply rotate in to place and apply. But at high RPM the bottom clutch was spinning so fast, that it just kept skipping over the teeth on the rear clutch. Since it wouldn't spline itself, being the bottom clutch, it kept the entire clutch pack from clamping, thus causing the flare, until the point that the force of the piston caused the clutch to finally engage into high gear.

So because of all that, I have spent a lot of time looking and understanding the seemingly millions of 904 variations there are out there.

Here are a few things I've learned.


Any 4 clutch drum, will fit any 2 ring pump, as long as it is not a machined 5 clutch high gear drum pump, then there is potential for problem.

5 clutch drums will only fit machined
(clearanced) pumps.

If you plan on using a 5 clutch drum, you must have the correct rear clutch/input shaft to go with it. (this goes for 4 clutch stuff too, there are 3 clutch pack "colt" style stuff so pay attention!) If you don't use the matched rear drum/front drum the splines on the rear drum, will not completely engage the bottom clutch on the high gear drum.

Non lock up input shafts, CAN NOT simply be pressed into the mating rear clutch for a 5 clutch high gear drum. So if you're thinking of pressing out the input shaft of a non-lock up drum, to press into the lock up rear drum, you can't do it. The reason is, the snap ring location between the 2 are different. If you put the non-lock up input shaft, into the lock up style rear drum, it could move around .060. Which means the endplay could also move that much.)

From what I can tell there is only 1 input shaft/rear drum combo that will work to make a true non-lock up 5 clutch high gear drum combo work. It is a non-lock up 3 ring style, which uses a completely different pump all together.

There are several different high gear drums. Some even have different diameters which necessitates a different sun shell. There are 1 and 2 bushing types. There are also 2 different diameter stator supports. They only vary by about .050, so you have to really pay attention. The high gear clutch must match the pump/stator diameter.

Low gear sets can be switched out of late style 904's. but the thrust plate that go between the snap ring, and the sun shell must stay with sun gear. There are at least 2 distictly different thickness thrust plates in the sun shells.

You have to be very careful mis-matching parts together. The sealing ring heights on the different stack ups, can vary by as little as .100" from one style to the next. So what looks like it may work with no problem, if you have the end play set loose, it could cause some huge problems.


My advice to anyone that wants to run a 904 is to stick with an early style 4 clutch. The newer you get, the more variations and suttle changes there are. The low gear set can be simply swapped into your trans (78 and later 904/998/999trans have 2.74/1.58 gear sets) Just make sure you take the sun gear/thrust plates/spacer/planetary's and thrust washers as you will need all of those parts to do the swap. The 904 will hold a lot of power. Their light weight/small design vs. the 727 gives it a significant performance advantage on the right application. Just make sure you be careful when you start swapping parts because they aren't as forgiving as a 727 when it comes to part/year swaps.

Anyway. I hope all of my pain and time can be useful to someone out tHere