Like I said I did not find a copy of the tech bulletin that Valeo supposedly put out in the 90's. There is a brief description of the concave theory I found on the web but it was too brief to understand.(I did find a post that read -- The Valeo bulletin states they did this to start the clutch engagement on the OD of the facings where you have the highest mean radius. Mean radius, think of putting the pull on a breaker bar as far out on the bar as possible. Same pull results in higher torque applied to the bolt. And they add that as the castings heat up they wind up flatter but not going convex. Convex is a slip producing condition).

I can only state that the original clutch assembly I removed had 33K miles on it. The presure plate, clutch disc and dual mass flywheel looked great. The disc was worn down slightly (qpprox .030) and both sides were parallel. The flywheel and pressure plate still have evidence of the original radial tooling groove marks. I measured the concave profile on a CMM and clearly both parts had .019 total deviation from a flat plane (.019 concave). I then removed fom the boxes my NOS dual mass flywheel and pressure plate. I measured them the exact same way and the result was .009 total deviation from a flat plane (.009 concave). the original parts were vintage 1990 and the new parts were superceded part numbers 1995 (Valeo upgraded the parts in the early 90's) . Again as I noted the new clutch behaves no different then the old. My only reason for disassembly was a rear main leak. While I was there I just replaced everything. I just finished this task this spring and only put several hundred stop and go miles on it.

 

.009 concave isn't much but I personally think it should be perfectly flat. I have three used dual mass flywheels and all of them have a severe amount of concaveness to them, WAY more than .009. When I did the clutch job on my '96 LT-4 I couldn't believe how badly the disc was worn near the outer edge, it was down to the rivets but the inner edge of the disc had hardly any wear at all. There was over 1/8 inch concaveness on the flywheel. And the pressure plate was almost as bad. The car has 106,000 miles on it. So I bought a used dual mass flywheel on eBay it was just as bad. So then I bought yet another used flywheel on eBay and it was just as bad too. At that point I said **** it all and replaced it with a solid flywheel and sprung disc and put the ZF doc shim kit in the trans and called it a day. I'm never ever going to **** with those shitty dual mass flywheels ever again. If the friction surface warps that bad over time when they have their full thickness (which isn't very thick to begin with) I can only imagine how fast and how bad it will warp again after an 1/8 inch or more is removed from it to make it flat again. I'll bet that the places that can resurface the dual mass flywheels don't machine them concave, I'll bet they machine them flat like a flywheel is supposed to be. There is no reason at all to have a concave surface on the flywheel, I don't know of any factory or racing flywheel that is supposed to have a concave surface. It just plain doesn't make any sense to have the outer edge of the clutch disc contact the flywheel and not have the inner edge of the clutch disc contact it until the outer edge of the clutch disc wears away enough to allow the inner edge of the disc to contact the flywheel because that's what will happen with a concave flywheel.

 

Sounds like you had a go at it. I only have history on the one I took out and the new one I had in a box. If it starts to slip in the future i know where to look.