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Clutch Release Arm Pivot & Pin Breakage

This problem is probably related to the Bell housing stress fracture problem. The clutch release arm retaining pin (12) hold the clutch release arm pivot (9) in its hole in the bell housing (1).

If either the pin (12) or the pivot (9) break, then the release arm (8) and pivot (9) fall inside the bell housing where they whiz around hitting anything and everything in sight. This usually causes chunks to be taken out of the release arm, and can also damage the clutch cover plate (6) as well. The noise is unmistakable - a bit like a load of ball barings in a cement mixer.

In my experience, it is usually the pin (90393171) that breaks off, which is particularly gauling because it only costs 59 pence to replace. However, if this is what happens to you, you will almost certainly have to replace other parts as well. Over the years there have been 3 different pivot designs ( 90393280, 90393831 and 90393866) and 3 different release arms (90393170, 90393833 and 90393865). The latter two pivot and release arms require a new bell housing (90393830) to be used if your car still has the original (90393286)

What I believe happens is this. If  for any reason the pivot pin becomes ever-so-slightly loose in its location in the bell housing , then it will tend to waggle backwards and forwards in the hole as the clutch pedal is depressed. This will place tremendous sheer-stress on the retaining pin which will eventually break off. When this happens there is nothing to keep the pivot seated in its hole, so it falls out, allowing the release arm to rotate freely clobbering anything within reach. Now I'm no mechanical engineer, but I can't see how the redesigned release pivot and release arm are going to help matters.

So what causes the pivot to waggle in its hole. I reckon there are 2 causes. The first is if the bell housing starts to break up, causing weaknesses in the aluminum which permit the pivot to move. See the Bell housing stress fracture problem. The second is if the retaining pin itself isn't screwed in tightly enough, or isn't loctite'ed in position correctly and so works loose. This second cause is most likely to occur within a few months of having your clutch replaced, because they have to dismantle the pivot arrangement to do this. If  it isn't put back together correctly then, this can and will happen. 

The two photos above show what happens to the pin. As you can see, it is long enough to screw right up inside the pivot to within a few mm of the end. However, the first 10mm or so of the hole up the middle of the pivot are not threaded. This means that if the pivot can move, no matter how slightly, that a shear stress is exerted on the first 10mm of the pin, and it will break. The two pivots shown in the second photo are 90393831 and 90393866, so the re-design obviously hasn't fixed the problem.

Since I also had the bell housing stress fracture problem, I decided to get my own pivot made to try and fix the problem. I think the solution is to tap the hole in the bell housing, and to put a thread on the shank of the pivot. This does make assembling the clutch and bell housing more difficult however. I decided to use a left hand thread for this. You have to make a new pivot because there isn't enough meat on the original's to hold a thread. The pivot retaining pin is a normal right hand thread, so I reason that the combination of a left hand thread on the pivot and a right had thread on the pin should lock the arrangement in place. It isn't possible for the pivot to unscrew without the pin tightening up. The only single cause failure mode I can see is if the pin unscrews or breaks off. But even if this does happen, the pivot will be held inplace by it's own thread, for a time at least, unlike the original which would have fallen out immediately. The thread on the pivot means it is less likely to waggle in the hole, thus reducing the likelihood of the pin shearing off. The pin can't unscrew very far, because the gearbox is bolted to the bell housing, which leaves very little space between the head of the pin and the face of the gearbox for it to unscrew.

The photo below shows the two original pivots, together with my new one on the left.

Any comments on the above would be greatly appreciated, particularly from those who consider themselves engineers. I'll keep you all posted as to my success or failure !