A413 Trans Rebuild part 4 – front clutch and drum assembly


Last Saturday evening’s portion of the project was to rebuild the front (direct) drum assembly. This turned out to be more of an adventure than I hoped, but I did manage to pull it off after some work.

Front clutch and drum assembly with callouts
The front drum assembly, sometimes called the front clutch or the direct drum, is assembled in the transmission directly behind the front pump, and is mated directly to the ring gear/front planet assembly. It consists of a cylindrical drum that is shaped a little like a bundt cake pan. The “outside” or drum of the cake pan is engaged by the forward clutch band; the cylinder inside the drum fits into the ring gear/front planet assembly. Inside this pan-shaped drum is a disc-shaped piston encircling the inner that in its retracted position lies on the bottom. The piston has a lip seal surrounding its outer circumference which seals it to the inside of the drum, and on the inner surface of the drum (around the “donut hole”) there is a corresponding lip seal. The piston is retained by a whopping big spring which itself is retained via a circular steel disc secured with a snap ring.

Front clutch assembly — friction discs and steels
Inside the space between the piston (at the bottom of the drum) and a “wave” snap ring at the top there is a series of “clutches” — alternating discs of friction material and steel. The friction material is cogged on the interior; the steel rings are cogged on the exterior.

Front clutch assembly in line with other driveline components
What happens is that the hydraulic logic of the transmission will decide to engage this front drum assembly. To do this, it routes transmission fluid under pressure into the chamber underneath the piston in the drum via slots cut in the inner cylinder. This forces the piston upward against the tension of the return spring, squeezing the friction discs against the steel discs, and ultimately against the pressure plate. When the clutches get squeezed together, it has the effect of locking the teeth on the inside of the friction discs to the cogs on the outside of the steel discs; locking the drum to the next item along the input shaft. When the hydraulic pressure is switched “off” via the valve body, the return spring — a large, steel coil colored red in the picture above — pushes the piston back down into its home position, thus allowing the friction discs to disengage from the steel discs, unlocking the drum.

Disassembly of this component takes place in two main stages: first (and easiest!) is removing the friction and steel discs from the drum. Second is releasing the return spring so that the piston can be removed to have its lip seals replaced.

Front clutch assembly — waved snap ring

Removing the waved snap ring — shown here in profile so you can see why it’s named that way — is a simple matter of prying radially inward with the blade of a screwdriver and then lifting it up. The “wavyness” takes up the space between the top of the pressure plate and the snap ring groove, allowing some clearance for the clutch discs to separate somewhat when they’re disengaged. It’s easy to work it out of the groove it locks into once you’ve got a start on it.

Front clutch assembly — pressure plate

The pressure plate, as you can see, looks almost exactly like the clutch “steels” — the only difference being that it is considerably thicker. Once you have removed it, you then remove alternating discs of friction material and steel, until the top of the piston is exposed. I used a magnet to pick them up a little more easily. When you’re done, they will look like the following picture if you lay them out in the order in which they were removed.

Front clutch assembly — friction discs and steels exploded a

Front clutch assembly — friction discs and steels exploded b

With that out of the way, it was time to turn my attention to getting the piston/spring assembly out. As you might guess from the picture, the spring is a whopper. It has a circular steel retainer, which is held in place by a snap ring around the inner cylinder inside the drum. At first, I thought I would just press down on the retainer with a pair of large C-clamps to gain the clearance needed to remove the snap ring, but I found that the clamps were too wide and would get in the way. I’m using 8″ C-clamps; my little 4″ jobbies weren’t strong enough to compress the spring (or large enough to clamp across the thickness of the bench top, either — I didn’t want to clamp directly to the bottom of the drum for fear of marring it.) If you have 6″ clamps, a thinner workbench, or both, you might be able to make a go of it with C-clamps alone. I on the other hand went rummaging through my scrap steel bin and came up with a small piece of angle iron to make a tool.

Now I have to say that I’m a beginning welder, and my fabrication here was a bit rushed, so the tool is crude….but effective!

Front clutch assembly — spring compressor front view
I think this was 1/2″ angle iron; I literally ran my grinder along the face shown here in the picture to weaken the metal along the length of the cut-out portion, cut the inside of the legs on either side with a hacksaw, and then nibbled the metal out of the cut-out with a pair of vice grips. The legs are spaced so that the cut-out clears the top of the drum’s inner cylinder, as well as the snap ring, and fit on either side of the spring retainer. You can get an approximate idea of how wide that is in this picture here:

Front clutch assembly — spring compressor front view with tape measure

Once I had the angle iron notched to clear the spring’s snap ring, I realized I would need to support it before I could put the necessary pressure on it to compress the spring. Unfortunately, the angle iron I had wasn’t wide enough to just weld a corresponding piece on the backside and have it clear the “back” of the snap ring, so this time I came up with a piece of heavy-gauge sheetmetal, and broke out my welder. I took some rough measurements to come up with the length of steel I would need, and then tacked on my support, and then bent it 90 degrees to come down as a third “leg” in a tripod (remember what I said about my welding skills…):

Front clutch assembly — spring compressor top view

Ugly, huh? I needed to adjust the feed and heat on the welder, and didn’t have a lot of opportunity to get it right.

Being the cautious sort, and not entirely trusting my weld, I tried to run a bead along the bottom as well. It turned out a little better…

Front clutch assembly — spring compressor underside

So the part that doesn’t come through clearly in the pictures is where I cut a couple of small notches on either side of the sheetmetal at the point where I wanted the bend to occur–this was just to weaken it slightly so that I could control the fold a little better. That actually worked out perfectly.

Next step was to set it up on the drum and see if it would work…

Front clutch assembly — compressing spring
Yep! Worked perfectly. Had to be careful with this though — that spring packs a whallop. You don’t want to have your face right over this thing should it let go.

Once I compressed the spring, I pried the snap ring loose, and slowly released the tension by loosening the vice. Here’s a shot of things after this was done.

Front clutch assembly — spring released

Notice how much higher that spring is when it’s fully uncompressed! You’ll see the snap ring dangling in the coils if you look closely, and the white circular shape at the bottom of the drum is the top of the piston. Just remove the spring, and the piston slips out with a slight twist.

Front clutch assembly — piston and lip seal top view

This is the top view of the piston; the lip seal is already removed and in this shot is dangling around my hand. The groove you see in the top of the piston is where the spring goes.

Front clutch assembly — piston and lip seal bottom view
This shot shows the underside of the piston — this is the side that is driven by the hydraulic force. The inner surface of the piston rides on the cylinder inside the drum, which has its own lip seal. This must be replaced carefully — they tend to get torn up when you slip them into position because of all the sharp edges.

Ken Mayer gave me an important tip on how to avoid tearing seals: take a 2-liter pop bottle, cut a strip of plastic out of the center section, and form an inverted funnel:

The FSM calls for numerous special tools tool for installing the O-rings and seals. I’ve used the cylindrical part of a bleach jug in the past, cut into a strip and rolled into a long cone or cylinder as required. This time I didn’t have another jug, so I scrounged through my recycling bin. I cut the top and bottom off a 2-liter soda bottle and slit the remaining cylinder longitudinally using a new single edge razor such that the cut is beveled. By rolling it into a long cone such that the sharp edge of the bevel won’t contact the O-ring, and greasing the O-ring, the O-ring slips onto the shaft and drops right into its groove without damage or distortion.

Here’s a shot of the “Mayer tool” in action — before I greased it, that is.

Front clutch assembly — Mayer tool

I used a little scotch tape on the inside of the funnel to help it hold its shape while I fussed with it. It worked great!

Once the seals were replaced, it was time to put things back together. I lubed up both of the seals as well as the bore inside the drum with some transmission assembly gel and then did exactly as the ATSG book said: put slight downward pressure on the piston while twisting it to push it into place in the bore.

I then recompressed the spring, reinstalled the snap ring, and then set about replacing the clutch friction and steel plates.

The friction plates, I am told, should be soaked in clean transmission fluid before assembly. You then install them in the order you see in the picture, above, ending with a friction plate on top right before the pressure plate. It was right about at this point I realized I had done something wrong: the pressure plate covered the groove for the waved snap ring! What the…

After some careful inspection, I realized that the piston wasn’t fully seated in the bore. Instead, I had installed it somewhat cockeyed, and it stayed that way even after I installed the spring! So I disassembled everything…again. And then I tried to remove the piston from the bore.

…and tried and tried and tried. No amount of twisting, pulling, prying, thumping the drum on the bench, etc., would free it. I took it inside and ran the drum under HOT water to see if I could get just enough expansion to free things up (I didn’t want to use a torch and cook the lip seals!) No luck.

Finally, realizing I had NOTHING to lose at this point, I took a hammer and a drift and smacked the top of the piston. Hard. Pop! It slipped into place.

I took it apart expecting a distorted piston, a ruined lip seal, a warped drum…something. But I guess I got lucky, because none of these things happened. I paid a little more attention when I reassembled it the second time, and used a little more force, and it went together correctly the next time.

Then, I reassembled the friction plates and steels and put in the snap ring. Next up, you’re supposed to use a feeler gauge under the “highest” portion of the waved snap ring to measure the clearance between it and the pressure plate. The range is supposed to be between .089 and .135″ or thereabouts. Mine was way, way over that. Now what???

Well, as it turns out, the steels for the rear clutch have the same OD, but aren’t as thick. Yep, I had mixed up the two different types of steels.

Finally, after I got that right, everything went back together as it should! Whew!


One Response to “A413 Trans Rebuild part 4 – front clutch and drum assembly”

  1. You actually make it seem so easy with your presentation but I find this matter to be really something which I think I would never
    understand. It seems too complicated and extremely broad for me.

    I’m looking forward for your next post, I’ll try to get the hang of it!

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