POST-MORTEM DIAGNOSIS
I usually like to take a look at the old parts after any job, just to make sure they wore/broke in an expected fashion. All too often the real cause of a failure is some other part that has yet to go bad. WIth clutches it's usually pretty simple to see the cause - the disk is worn beyond tolerances. But when I took this clutch apart, the clutch lining looked to be in disturbingly good condition.
This page is a collection of photos of the old parts after running them (quickly) through a solvent tank. The BMW spec for worn clutch lining is 1mm to the top of the binding rivets. My clutch had 1.05mm on the pressure plate side, and 1.45mm on the flywheel side - not too bad, and certainly not bad enough to account for the slippage I was experiencing.
As you can see, the pressure plate and flywheel surfaces both show the expected galling and overheat points. The input shaft retainer (or "guide bush" as BMW calls it) was thrashed, as was the bearing surface of the throwout bearing. The clutch release lever is fine (if anyone needs it let me know and we can work something out). Click on each picture to see a larger version.
So, why all the problems? Three issues come to mind. 1) there was significant "dust rust" on the transmission input shaft and clutch disk splines - it's possible the clutch was hanging on the shaft and preventing full engagement (not real likely). 2) The clutch is pretty small for this size motor and weight of car - it's also possible that when the disk gets close to spec tolerance, it just stops working. 3) My car is making soooo much power that this poor clutch just couldn't take it :)
| The clutch disk, showing both sides, and a closeup of a typical rivet head showing amount of friction material remaining. White fibres are from the towel I used to dry the disk. |
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| Flywheel: some fairly serious overheating going on, but nothing disastrous. A "regular" flywheel in this condition could most likely be machined and put back into service. |
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| Pressure Plate: same heat/wear patterns as the pressure plate, as to be expected. The only item of interest is the release fingers - the ones marked with a red 'x' sit lower than the rest. Given the pattern, I suspect these are higher-rate springs and are supposed to ride lower than the rest. I couldn't compare to the new plate due to the nifty retainer mentioned elsewhere. |
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| Release Lever: minimal wear - could easily be re-used as is. |
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| Retainer or Guide Bush: thrashed. I wouldn't even consider putting this back into service. |
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| Throwout bearing: the bearing itself was still in good shape, but the bearing surface (the part that presses against the pressure plate fingers) had a nice fat groove worn into its surface (red arrow). |
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