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Help diagnosing gearbox knock

nerosdad

nerosdad

New member
Hello folks

I recently restored my 911 SC (83 model), I stripped the gearbox and ended up replacing the main shaft (as it was damaged after years of sitting with a broken clutch).

Everything assembled just fine and driving the car is smooth (as smooth expected for a 915). However, when the car is warm and I press the clutch pedal (even just a fraction) there is a knocking noise. Logically as the noise is only present when you apply pressure on the clutch pedal I assume it could be the throw-out bearing but wanted to get your input.
When the clutch is engaged and I'm driving or when the car is in neutral I hear no knocking (hot or cold), its only audible when you dress the pedal (from top of pedal to all the way down)

As a little bit more information I did not replace guide tube but instead had it cleaned and plated

Please let me know your thoughts

Thank you

Dave
 
Hi Dave, Difficult to know exactly what's going on without hearing it. Knocks I have had have been from the Omega spring hitting the heat exchanger. It's easy to disturb the routing in the tunnel where the clutch cable goes. This is usually a clinking sound . I assume you put a complete clutch kit on? Not sure if a worn clutch fork could cause an issue. The clutch spring in the footwell has also clunked before I put lots of grease on it. I assume the gear change is good.
 
Hi Rich, thank you for replying.

I looked at the Omega spring, it seems as though this touches the (or nearly touches) the exchanger when the clutch is engaged - pedal up so I dont think its that, The gear selection is smooth on all gears. Its a brand new clutch kit yes, the only thing I didn't replace was the guide tube.

It's a proper mechanical knock. I would have thought its the clutch arm on the pressure pate if it was only at the bottom of the pedal but this is from top to bottom and onl when the the engine is hot.

Thanks again



 
Hi Dave,
My worry is that you have not sorted the problem which broke the old clutch. Perhaps posting a video of the clutch noise could give a clue.
 
You have not said anything about the feel of the clutch pedal action.
Is is smooth and no restriction etc?
I have changed the clutch on my 915 box about 15 times (competition car) and have never touched the guide tube for the bearing etc and have never had a noise.
The omega spring is within a mm of the SSi exchanger but misses it.

I would remove the spring arm assembly and start again but also while the cable is slack check the fork connection at the pedal and maybe replace it. Grease/oil all the pedal assembly.
Ensure the cable is correctly seated to the box and re-set the levers to the manual specs.
You could delete the spring and see if that works as an experiment.

Finally, sorry if this is daft, but you do have the pedal limiter set correctly if you have the foot board installed?
The noise is not the pedal hitting the pedal travel stop?
 
Hi there, it's not daft at all - thank you all for replying.

Clutch pedal is smooth, pedal stop is set correctly and in this case I think not related as I get the noise from the top of the pedal all the way down. Omega spring is clear of the exchanger by a millimetre or so, but again as you apply the clutch the Omega spring moves away from the exchanger.

I have a small amount of slack when I remove the spring arm (the lugs seem parallel in the bearing grooved section).

It makes the same noise regardless of stationary or moving, in Neutral or in gear (when hot) just apply small amount of clutch pedal it knocks.

If the car is at a standstill, and in gear with the clutch pedal down (main shaft is not turning) I get the knock - I presume this rules out gearbox bearings ??

Could the Spigot bearing or throw-out bearing make a knock only when hot ? ...

* To answer what happened originally to the gearbox, the diaphragm spring was broken in several places, someone must have tried to drive the car without a clutch and this lead to the reverse idler gear being broken (corner of a tooth missing). The car sat for ~30 years and the main shaft got bent, The assumption being the damaged clutch had the input shaft sitting just off centre and its suffered metal memory.

The gearbox was rebuilt by myself with a replacement main-shaft and reverse idler. I replaced one of the main shaft bearings but everything else looked serviceable to me ** But please see my comment about stationary, in gear clutch down - knock - I believe this logically rules out gearbox internals ** I'm interested to see if you guys concur

If its not Spigot or TB Its potentially engine / thrust bearing etc (???) - but the very slightest touch of the clutch pedal creates the noise so I drawn to this area first.

The engine was rebuilt, everything checked for tolerance, new bearings etc as you'd expect, also new IMS.


Thank you


 
Is the knock a once only action when pressure is placed on the pedal, or does it repeat with engine speed.
No engine running must have the knock a single event.

can it be a cable outer seating down when pressure is applied?
this was why I suggested disconnecting the cable and see if the noise happens etc, change one thing at a time while trialling of course.
 
It's only there with the engine running, it's only when it's hot and it's audible from the moment you apply a small amount of pressure on the clutch pedal (top to bottom).

It sounds like a bad throw out bearing or similar but it is brand new and I'm curious why only when its hot.

Again, its not there once you let the clutch engage (pedal up), its not audible any other time apart from clutch depressed (hot)

Thanks

Dave
 
If I had the issue I would get the engine hot. Then jack up the rear of the car and put it on heavy duty axles stands , take the wheels of and get some else to press the clutch. Then see if it was obvious where the noise was coming from.
 
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