Th400 questions

I have 2 th400 torn apart for friends, each with different problems.

First trans is in a 71 Camaro, 6.0 LS with big turbo on E85. Car runs high 8s and is street driven regularly. Buddy said it was starting to kinda slide into 3RD gear. Trans has CK Performance forward transbrake valve body, billet fwd clutch hub, aluminum ATI direct drum (6 clutches) 4 intermediate cliches, etc. My question, the first 4 frictions were smoked all the way to the steel discs, but only on the side closest to the piston. What could cause that?!?!?!

My other 400 is out of a low 10 second Chevelle. This one has a homemade fwd manual valve body. My question, I always replace the Teflon sealing rings on the 2nd accumulator piston. The piston always fits well on assembly, but after some run time the pistons fit really loose. I seems to me the sealing can’t be all that good, I have noticed the 4l80E did away with the Teflon rings in favor of rubber o-rings. Is this something that can be improved? Or am I being to picky? Thanks


Well-Known Member
The direct clutch burning issue is most likely from the way the heat dissipates from the clutch to the steels, and into the drum. Typically they have a tendency to hot spot towards the middle of the clutch plates and nearest the top pressure plate and the hot spot diminishes as it gets closer to the apply piston. Not all apply pistons are created equal. Some of the steel and 4L80 aluminum pistons only have a small apply area and may cause local hot spots. You didn't mention your clutch or steel thickness. The minimum thickness I would use are .080 thk clutch disks and .077 thk steels. What are you using for the direct drum sealing rings ? Assuming you are using Peek or Teflon if its an aluminum drum. If they are solid white Teflon without the scarf cut than that may be an issue but there are different material solid Teflon sealing rings(speckled color) that work better at high temps than the solid white ones. Do you know if the valve body has fixed line pressure and know what its set too ?
As far as the Teflon sealing rings on the accumulator piston, those rings always seem to seal really well when oil gets behind them to force the seal out against the bore. They are sometimes difficult to air check because air doesn't have enough mass to push the seals out, even with some pressure behind it. So basically I wouldn't worry about it.
It’s a ATI aluminum drum with their piston. I set the clearance at about .070 (following the .010 per friction rule of thumb) orange Teflon sealing rings. The valve body is set up with fixed line pressure, according to Chris at CK it should produce about 200psi. I used 4 thick steels, I always keep them towards the middle of the clutch pack to absorb more heat.

As for the sealing rings, that makes sense. That’s kinda what I figured was happening. I noticed GM changed the pistons to use o-rings in the later models, kinda makes me wonder why unless they were trying to solve a issue. I guess if it ain’t broke I’m not going to fix it!