[DeTomaso] Clutches- long and short of it

[JDeRyke at aol.com]

MY standard explanation:
Before replacing any clutch slave, first check the disengagement your 
particular car gives. Do this by having someone push the clutch pedal clear to the 
floor (engine off & cold) while you reach in thru the bellhousing access hole 
with a flat feeler gauge. Ideally, with a stock system and no wear anywhere you 
will get 0.040" of clearance. Since a clutch disc has two friction faces, 
divide that figure by 2; this means that under ideal conditions (no heat, no wear 
in the linkage and the pedal completely flat on the floor) you have 0.020" of 
clearance on each side. This is not quite enough since clutch friction discs 
aren't that flat,   most 37-year-old Panteras do not have 0.040" clearance and 
no one shifts gears with the clutch pedal completely flat on the floor; we 
should but it doesn't happen.   

All this means that if there's not enough disengagement clearance, the clutch 
will drag against the flywheel or pressure plate. Drag means that the ZF 
synchros are being over-worked. ZF synchros are steel; they not only wear, they 
equally wear the integral gear-cone they touch when working. ZF gear-cones are 
not repairable- vendors have tried for years to re-do this part of otherwise 
good gears without success. A new gear pair from RBT Transmissions is around 
$2000 installed. Conclusion: shifting gears carefully with maximum disengagement 
pays dividends in longevity.

A typical long-throw slave cylinder will increase the average clutch 
disengagement distance to 0.040"-050" on a WORN system. This is still not really 
enough but helps alleviate clutch drag on the massively expensive trans synchros. 
The resulting 0.010-0.015"-longer clutch throw will not over-stress Centerforce 
diaphragm clutch fingers or a stock throwout bearing. The pedal will be 
slightly harder to push in.

I personally use a Tilton racing hydraulic throwout conversion that replaces 
the slave and its bracket, the bellhousing cross-shaft and its fork & 
bellcrank. With a stock clutch master cylinder, I get around 0.060" of disengagement. 
The system has been in our car for 15 years- I've tried it with both a 
Centerforce and a stock Long pressure plate with no problems. It's easier to push in 
than the stock system due to considerably reduced friction. Dennis sells a 
similar system, but hydraulic throwouts are not without problems, particularly 
during initial set-up. And if something goes wrong, the ZF must come out before 
you can trouble-shoot. So they're not for everyone. 
My 2¢- J DeRyke


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