Diaphragm type clutch operation theory and adjustment

This applies only to all clutches with a diaphragm-type pressure plate. I’ve always had a good feel for adjusting clutches, but of various Datsuns of the same decade, some have a return spring on the release fork, and some don’t. It’s not like the spring is missing on those that don’t, they came like that. (This is all assuming the clutch is properly adjusted.)

If there IS a spring…

When your foot is OFF the clutch…the throwout bearing is not touching the diaphragm “fingers”, and the bearing is stationary while the pressure plate rotates, extending bearing life. When you push the pedal to shift, the stationary bearing hits the rotating fingers, causing wear, especially since the rotateable (is that a word?) portions of the bearing has significant mass and the pressure plate is rotating as fast as the engine…this wears away on the fingers.

If there IS NOT a spring…

When your foot is OFF the clutch…the throwout bearing is still touching the diaphragm “fingers”, although lightly, but the bearing will always spin with the pressure plate, causing wear to the bearing, but no wear on the fingers.

Which is the “better” arrangement, with or without a spring (the bearing always touching, or only during shifting) and why?

IMHO there both are “ok” with noone “better”. The releases without the fork pull back spring will allow the TB to be bumped back by the diaphragm fingers and the fore/aft clearance of the crankshaft so the fingers will touch the TB race intermittantly. If this wear were significant, you would probably see a circular groove in the face of the inner race of TB which I never have seen. Usually practice is to put a light coat of grease on the face of the TB as well as the fork grooves and the transmission input bearing housing where the TB rides. That should protect it against light pressure wear.

I think most of the wear will come as the diaphragm fingers slide radially on the TB inner race face as the pressure plate is retracted. That is where major force is being generated and happening on a regular basis. Even then I have not seen any annular wear on the face of the TB due to this. I have seen wear on the diaphragm fingers in an annular pattern but I have not analyzed closely whether this is due to intial contact or pressure plate retraction. I think I will dig up an old diaphragm pressure plate to see.

We’ll see who else pipes in on this.

Four years ago I replaced my old clutch after 40,000 miles because I had many autocrosses, and two major burnout sessions on it. There was a SLIGHT groove on the fingers, which is to be expected. Surprisingly the disk looked almost brand new, but since I had it all apart, I replaced everything required. I even found a crack on the pivot in the clutch fork after I cleaned it and then replaced that as well, I replaced the throughout bearing carrier because it had some fork-to-carrier contact wear. (Realize that the fork and carrier are from the 70’s decade) …and the work also gave me an excuse to get the flywheel lightened :slight_smile:

This new clutch has been perfect for about 14,000 miles when suddenly I’d had to stomp the pedal down as hard as I could to get it to BARELY disengage. Adjusting the clutch made it work better but feel “funny”, and I even replaced the master cylinder (hydraulic) since it’s easy to change and only costs $50 for the part, plus my time and $$$ for fluid. No the slave does not leak and there is no leak anywhere nor air in the line.

I finally pulled the tranny out, and saw the the pressure plate finger tips are SEVERELY worn. Again, the clutch was brand new (not reman’d), made by Sachs. The bearing spins freely, even if I try to push on it hard similar to the way the fingers would apply pressure to it. Extreme ring-like wear on the fingers is exactly how mine failed…no autocrosses, burnouts, etc. have been performed with my new clutch. My guess is they got so weak they finally decided to start elastically bending one day…the day I discovered something was wrong.

FYI…yes, my release fork does have a return spring which will pull the bearing away from the fingers when the clutch pedal is not depressed.

I think it’s a wash with one being no better or worse than the other.
With the former, the TO bearing rotates very smoothly anyway (or it should) and there is going to be very little slippage when the TO bearing engages the pressure plate fingers.
With the latter, there is no great pressure being exerted on the TO bearing balls or races and it’s doubtful that even without a spring the TO bearing would be rotating all of the time. Too much variation in clutch operating systems (cable, hydraulic, cabin clutch pedal springs, etc) to be sure one way or the other.

Either way, I don’t see it being a problem and have seen worn fingers on both systems.