Clutch Noise Diagnosis, please.

[jeff_b]

When I bought my M42 car, it wasn't running, and had been run up on a curb by the PO's little brother.  His mechanic claimed it has a rod knock, probably a rod bearing.  I got it running, and the truth is, it does in fact have a knocking originating from the base of the engine/bellhousing, but it goes nearly completely away when the clutch is pressed in.  

As I'm new to M42 5-speeds (I rock the M20/autotragic daily driver), could this be nothing more than loose clutch bolts, or a broken release spring (if that's even what it's called)??  :confused:

Any help is much appreciated.

[DesktopDave]

I'm hoping it's a clutch release bearing, also called a throw-out bearing or TOB.  There is also a little plastic pivot on the release arm that can disintegrate after a while.  The parts are all cheap, it's the putting them in that's expensive.

When you press the clutch in, it makes the clutch fork force the flywheel & clutch disk apart.  That loads up the TOB and (hopefully) in your case, causes it to stop knocking.  Much better than a bad rod bearing.

Do you ever get oil pressure lights?  A bad crank bearing can do that sometimes.

[jeff_b]

Thanks for the reply.  You described the symptoms exactly.  I've never seen the oil pressure light come up, but the thing seeped oil so badly, I only ran it until the temp got about halfway.  It leaked coolant too.  Right now I'm tearing down far enough to do the oil filter housing/mess under the intake/air cond delete/electric fan conversion all at the same time, and just thinking about what is next for this thing.  Can I pull the tranny without pulling the motor, or is it easier to lift them out in tandem?

[doitover]

I have the same issue. Doing a little googling after seeing your post it looks like the consensus is.

If the noise happens when the clutch is pressed and stops when it is released, it is the throwout bearing.

If, like in my case, the noise is there in neutral with the clutch engaged and stops when the clutch is pressed, it is the input shaft bearing. That was what I was thinking, I had a transmission replaced under warranty for the same reason on a Corvette way back.

[jeff_b]

Ouch.  Input shaft bearings sound labor-intensive....

[doitover]

Yeah, apparently nearly undoable as well. Most people seem to swap another used transmission in. They seem to be abundant and for not a lot of money.

Metric Mechanics does a rebuild, pretty pricey though.

[jeff_b]

Metric Mechanic prices would break my target budget for this car!

[doitover]

amortize it over 20 years.

[dinu.negrean]

If it is a rod bearing that you can hear knocking, you are in trouble.
http://www.flickr.com/photos/47779682@N05/sets/72157623686817180/

The differece between a rod knock or piston bolt knock and a clutch release mechanism is that the rod and piston knock is perfectly regular and will increase once the revs go up; while the cluch release knock is a bit erratic.

But if your knock stops when you press the clutch pedal, I would not suspect the rod bearings. BUT I would definetly check them, as in fact I have done two days ago with the engine I have in the car right now, after having seen what it has done to my old engine.

[jeff_b]

Thanks for the responses everyone.  The noise lessens a lot when the clutch pedal is pushed in, but it doesn't fully go away.  This little motor may have some lower end noise too, but the loudest noise must be tranny related.  Hopefully, once I get the car back together and it's not leaking coolant and oil so much, I'll be able to diagnose further.

dinu.:  how did you check your rod bearings?  Did you crack the rod caps loose and do a visual inspection, or will a feeler gauge check reveal a problem?

[dinu.negrean]

I have taken off every single rod bearing, because I was also planning to replace them, but the previous owner must have replaced them recently. They were all brand new, installed new bolts anyway, they are stretch bolts, not reusable.