Clutch slave failures...WTF?

[99Super]

Hey all!

Frank has been running great since built but I'm having a very frustrating issue.
Two weeks ago, I attended an AutoX, but at the end of my second run, my clutch slave
failed as I was returning to grid.

I assumed it was something I did when I built the car as I has shortened the nose of the slave a bit, to be just like the LPR slaves.
This allows one to get the slave in and out without fully compressing the clutch. But the failure was with the outer O-Rings, not the inner.
I replaced the slave with a new LUK unit and was a bit surprised that it also went in without compressing the clutch further.
Last weekend at the spring Maryhill Hillclimb, I was returning to grid after my first run and the slave failed again in the same manner.
End of my weekend

Is there something causing the slave to extend too far? A combination of a thinner flywheel (from surfacing) and a thicker friction disc (SPEC Stg 2+)?
The thicker disc appears to make the PP compress further, requiring more throw? The PP is a stock AP unit for a 9000T.

I'm thinking of spacing out the PP by adding a washer between the PP and flywheel.
Also thinking of adding a stop to the pedal to prevent overextension.

Has anyone else had this issue? I have a bag of rebuild kits, so I'll rebuild this slave, but I really need to figure out WTF is going on here.

37 years of 99s/900s and these are my first slave failures.

[squaab99t]

How are the Master or the flex line? Could be sending junk to the slave? I added a pedal stop when I ran my Quarter Master setup. It just did not need much travel to disengage.

[Jordan]

Are you sure you are overextending the slave and it isnt just an o ring failure?

[Sam]

Can it be you have the wrong part number? I can't remember if an NG 900 or 9000 slave cylinder would fit at all.

Also Luk quality has been sliding as of late. They were the provider for the clutches in the new jeep wrangler which basically use a modified tacoma aisin 6 speed. It took them 4 revisions to get it right with numerous reports of fluid fires and incidences of the clutch disc shooting through the case.

[Geoff]

Yeah, I've had that exact same issue on a 99. The slave popped apart when I stepped on the clutch, I installed a new aftermarket slave cylinder, it popped, I rebuilt it....
I had the same thing with the lightened/re-faced (thin) flywheel and being able to get the cylinder in without having to use the clutch tool.

I put some washers between the cylinder and the transmission case to try and get it closer to the pressure plate so it wouldn't have to extend so far. It was a total pain to get installed and then the cylinder popped again. I ran out of time and patience and haven't revisited it so let me know if you come up with a better solution.

[99Super]

Thanks for the ideas.
I don't think it's just an O ring failure. The first was an OE slave with OE rebuild kit and the failure was obviously an overextension. The second (1 week old LUK) failed the same way. In both cases there were chunks of cut/damaged O ring left on the piston.

I think that the SPEC disc is MUCH thicker than the stock unit and when the PP is bolted down, the slave has to extend much further by the end of the pedal stroke. The fingers are nearly flat before the slave extends. I think I need to add some shims to increase the stack height and reduce the necessary travel. I need to try to figure out Jordan's MCS clutch chart as I don't feel that I'm getting maximum clamping.
Does anyone have a new(ish) disc handy that can measure the thickness for me? All my spares are still in Boise...and I'm not.
Barring that, I'll be installing a travel stop to my pedal.