how long does the clutch lasts ???

[jakub]

The first clutch I’ve installed had new linings, it lasted about 4000 miles till I’ve personally killed it. No fault from the clutch, just useless driver. The second also had aftermarket linings however this time the workshop warned me that they temporarily run out of material suitable for trucks. Since I was in the hurry I asked to use what they’ve got. This clutch lasted only about 1000 miles. Then I’ve managed to obtain original, asbestos plate. It looks now like it is close to the end of life. It lasted about 8000 miles. By modern standards it is not a lot, than again we are not talking about modern vehicle. As for driving habits, I have no problem understanding double clutch shift and non-syncromesh gearbox, occasionally I have vehicle loaded with willys jeep, or some 2-3 tons of load, also I do sometimes little bit of offroading.
I wonder what is your, dear forum friends, experience in the matter.

[dr deuce]

Is the disc worn out to the rivets or did the pressure plate fail? I have had 2 pressure plates fail, never a disc. The problem is the adjustment. FORGET FREEPLAY!!! It is what it is. Adjust the clutch so it just barely disengages about 3/4 inch before it hits the floor. The amount of "freeplay" you will have after is how much clutch disc is left (thickness). As the clutch disc wears, "freeplay" will become less and less until there is no more.

What happens if you adjust for "freeplay", you will over stress the spring in the pressure plate and cause it to fail. When it fails, it will twist/buckle (both of my failures did this). When this happens, the clutch will no longer dis-engage.

I don't know if I ever put a new disc in with the original 270 set-up.

I have driven my CCKW ~60,000 miles. If you can double-clutch correctly and use the correct gear most of the time, you should have no problems. I have about 20,000 miles now on my Corvette clutch which is just slightly smaller then the original. And before you get all excited about having a "Corvette clutch", you have to have a flat flywheel like I do on my 1965 GMC 302 engine.

BTW, I also carry my GPW in the back and tow the 57mm AT gun (3000lbs) at the same time going to parades etc.

[jakub]

In the meantime I’ve done 200 hundred miles, however on the way home the clutch started to slip really significantly. Fortunately it was happening only when going up the hill on fifth or fourth gear with low revs. Keeping high revs helped to avoid slippage.
I have taken the clutch out, it is only too obvious that the plate is too thin, exactly as Dr Deuce predicted. The rivets are leveled with linings hence slippage. The overall plate thickness is 7 mm as opposed to around 10 mm for new one.

So to answer my own question the original plate lasted about 8000 miles. Then again it is difficult to determine how old was the plate, how it was kept, was it original new or remanufactured some years ago.

[mudflap]

When replacing the clutch, do you service the pressure plate also? Or do you just replace the driven disc? Just wondering if the diaphragm spring could be weak, or perhaps something in the pressure plate is not setup properly.

What do the flywheel and pressure plate surfaces look like? Are they smooth and even, or are they scored ? Is there any grease or oil on either of the surfaces?

TM9-1802, also offers some other good suggestions relative to clutch problems such as checking to make sure the disc is not hanging up on the transmission input splines, and making sure the release bearing is working freely.

Best Regards...

[jakub]

It took much longer than anticipated but the work is done, even though fitting the gearbox with one pair of hands is not easy. The clutch plate has been relined. The GMC has been road and off road tested. In the meantime the culprit behind relatively short life of previous plate has been found. It turned out that plate was slightly bent as a result the spline was out of alignment from plate. The previous clutch has been bought as NOS, perhaps it left factory like that, or it was reconditioned some time in past, this puzzle will not be resolved. Anyhow it pays to take also flywheel and take whole lot (flywheel, plate, pressure plate) to the clutch shop for proper fitting and alignment.