Unpleasant knock

[chris_seven]

We have an old 3.0 SC with around 150k miles.

It runs quite well and is fitted with Webers and a lightened (RSR Style) flywheel.

At idle there is a knock which vanishes when the clutch is pressed. In this condition the engine is uloaded.

It recently had a new clutch and release bearing.

The engine runs well and makes no other knocking noises, oil pressure is good and oil temperature is OK.

I am not convinced that the knock is associated with the release bearing or the clutch.

When the clutch is operated there is a small axial displacement of the crank. We have measured this on the crank pulley at about 0.25-0.3mm (12 thou).

I realise this is close to the axial wear limit but I cannot persuade myself that there is any reason why the crank would move back and forth at idle and generate a knock.

Any ideas?

Loose flywheel? Bad release bearing?

[Flieger]

My car used to make a knocking noise when in neutral and with the clutch pedal released. A new throwout bearing appears to have solved the problem but the transmission also had a bit of a rebuild so it is difficult to say what the issue and fix were.

[Steve@Rennsport]

Hi Chris,

Based on experience, two things come to mind that have caused such noises in the past:

Worn crank thrust bearings

Worn intermediate shaft bearings

Its not uncommon.

[chris_seven]

When we put it up on the ramp the knock seemed to come from the gearbox.

Changed the clutch cover, fitted a new release bearing and all was good - silence and smooth operation.

30 miles later knock came back.

Originally Posted by Steve@Rennsport

Worn crank thrust bearings

Steve, The thrusts on this car do seem to be loose but how would this translate to a knock which must mean the crank is moving back and forth.

For 30 miles it drove very smoothly and very well.

Very confusing

[BURN-BROS]

Chris,

That endplay can create quite a bit of lash between the intermediate shaft gear and crank gear. Perhaps that is what you are hearing. I have heard the knock at approx 800 rpms and quickly goes away at engine speeds above 1000 rpm. Some Dealer mechanics have told me that the fix for the complaint was to increase idle speed....

[cstreit]

You sure it's not just the clutch friction plate knocking around? Perhaps when you push in the clutch the springs on the clutch plate aren't rattling as much... I tried to imagine how pushing in the clutch would impact the end-play on the crank (ie because you said this made it go away)... Couldn't do it...

[chris_seven]

Chris,

When we press in the clutch you can see the engine pulley move slightly and this is when the noise goes away. I assume this is due to slight axial clearance in the thrust bearing. Pressing n the clutch takes up the clearance.

We spent a little more time listening to the 'knock' with the car on the ramp and it seems to come from the gearbox when in neutral.

At 800rpm the knock is quite pronounced at 1000rpm it is much quieter and at 1200rpm it goes away completely.

The box has around 120k miles but isn't noisy when running in gear and shifts as well as any other 915 I have used.

As the car has been stripped out and all of the sound proofing removed you can hear it inside the car at idle.

If it was a 5 speed ZF box as used in a Rally Escort I wouldn't worry as they always clattered in neutral but I haven't heard this in a 911 before.

I guess the best plan is to put the idle speed up to 1000rpm and see what, if anything, develops.

Thanks for the input.

[cstreit]

Ah...

I would assume then that the centrigual (sp) force of the crank at higher RPM's is overcoming the back and forth knocking of the crank. I replaced the crank in my 3L race engine because it was at the limit too... The rear thrust surface was just inside spec...