Gearbox noise - throwout bearing?

[dizzycap]

My 901 gearbox/clutch has started to make a horrible sort of grinding noise just as I raise the clutch pedal. Must be the throwout bearing?

It is hard to avoid it when starting to reverse. I really have to eeaaasse off or this horrible noise starts. Worst case in these standstill situations I can’t even make it stop by just pressing down on the clutch pedal again. I have to yank it out of gear to make it go quiet.

Also in first gear it is pretty sensitive in the same mannner, but not quite as bad as reverse.

So no issues selecting the gear, but once I let the pedal up the noise starts. Once it has passed no issues coasting or anything.

And this happens on all gears. I can put second third, fourth, fifth gear in without problem, but ever so often I get a short crunch just as I start to let the pedal up.

I was thinking the synchros, but surely they can’t all have gone from good to bad in a few weeks.

Also I can provoke/investigate the noise by pressing and releasing the pedal whilst staying in the same gear. So same gear, still get the noise just as I release the pedal. That would rule out synchros, I think?

So… throwout bearing must be the culprit?

Bad pilot bearing? But that should make a noise more constantly?

Should mention I run Swepco, but have ordered mineral EP90. Don’t think symptoms points to oil though.

[Bruce M]

Throwout bearing is more likely to be noisy when the clutch pedal is down & it’s spinning. Would also be noisy when you are in neutral.

I’d be suspicious of the bearings. As the gears have angled teeth, when you lift the clutch and the forces increase, the gears push sideways towards the bearings at each end of the shafts, as well as pushing the shafts apart.

[dizzycap]

Thanks! Yeah, you have convinced me. Could well be the internal bearings.

If it was the throwout bearing I think I should be able to provoke the noise also on every occasion including in neutral, not just when in gear.

The problem is just getting worse, so car now garaged and will get the gearbox out and - unless I find something obvious and immediatebly fixable - I’ll swap in the old/original (simplified diff) 4-speed while I send the 5-speed of on repair.

So we will hopefully soon find out what was making all that noise

[Bruce M]

If you run the spare gearbox for a while, remove the output flange centre bolts (one at a time!!!) and check the pip on the end for wear. Replace if unsure.
Given you know what the simplified diff is, I suspect you know to do this but worth typing the warning I feel.

[dizzycap]

Anybody in the market for a 912? Some sorting out required

Kidding, but this is weird and frustrating.

Pulled out the noisy 5-speed, swapped in the original 4-speed. Swapped in a brand new throwout bearing too.

All good initially, no gear lever adjustments or anything needed. Reversed up out of my basement car park garage a bunch of times. Test drives. Nothing. Engine shudders a bit just when releasing the clutch but I had that down as potentially needing to adjust clutch cable length and/or bowden tube.

Driven a couple of hours including some spirited drives.

Today the same noise is back. Reverse is - again - worst. First gear only sometimes, and 2nd through 4th haven’t started making noise. Yet.

What am I doing wrong? I am grasping at straws now.

I do not have the speedo gear cable connected. Is that a bad thing?

Something in the clutch disk or flywheel?
The gland nut? Could that be sticking to the incoming shaft?

Could it be that two gears (reverse and first?) get connected simultaneously? How could I have so bad luck that both my gearboxes develop the same error though.

Open to every suggestion now

[dizzycap]

I think I might be an idiot.

My engine was potentially built for a 356 but is now in a 912. I am thinking there is maybe a difference in the pilot sleeve/ pilot bearing size.

So I Googled failing pilot bearing symptoms…

“A worn or seized pilot bearing (or bushing) in a Porsche 356 causes a squealing, whirring, or rattling noise when the clutch pedal is depressed or released, difficult shifting into first or reverse…”

Out with the engine. Again.