Sunday, July 31, 2016

Clutch pressure plate bearings and the like

Third 748 story, this one a 1997 SP.  The owner was complaining of the clutch losing pressure over time.  Fluid level was ok, colour typically not, no obvious leaks, aftermarket slave.

I removed the slave to check for the rubber bellows (there, but fitted the wrong way around), but found a short section of pushrod stuck in the end of it.  There are two o-rings on the pushrod that goes across the engine, just inside the cases on the slave side.  On the later 3 phase alternator bikes, a pushrod broken at one of the o-ring grooves is a very good indicator of the flywheel nut being loose.  But this was an older single phase alternator bike, and it didn't sound like it had a loose nut to me.

But I thought it might be more likely to have a seized bearing in the pressure plate, which it did.  Totally seized.  This is one of those issues that suddenly started appearing after many years of none at all.  I see quite a few 2000> era bikes with the bearing seized (or at least rough), but not older ones.  Late model bikes have a pin through the pushrod at the LH end that locates in the slave cylinder boss and stops the pushrod spinning, which is a typically involved solution to a seemingly simple issue.

The bearing in the pressure plate is a normal ball bearing.  I always figured it was used somewhat poorly, given it's not specifically a thrust bearing but someone who knows bearings told me once that it was probably just fine.  Maybe, like the 2002 ish onwards front wheel bearings, it's a bearing quality issue.  Although on this bike the pressure plate was aftermarket, so a non Ducati bearing.  Possibly worse.

As the pushrod had been spinning, it was quite stuck in the slave piston and had probably damaged the slave bore.  The easiest solution was to replace the pushrod with the later, 2001> longer part and fit a new aftermarket slave, in this case an Oberon as they are both well priced and, from my experience, well made.

It also had the usual wear at the centre of the hub, where the big steel washer slowly eats its way back into the hub.  I see a lot of this, and if the wear is not great you can add another washer to take up the clearance.  Clearance here allows the hub outer section - the splined aluminium piece - to move in and out on the cush drive rubbers that sit over the steel inner section.  This translates as excessive freeplay at the lever, as the movement of the hub moves the pack and pressure plate with the hub without lifting the pressure plate off the pack.  As an aside, I have seen (and indeed fitted) up to 4 washers here to take up the wear when the budget doesn't extend to a new hub.  It's quite amazing how thin the hub can get and not fail.

I found a photo of the one that needed 4 washers.  You can also see the wear on the pushrod - the stepped section runs in the needle roller bearing just inside the end of the input shaft.  It's not meant to have a step.

No comments: