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Thread: Drive-train grief.

  1. #1
    Join Date
    Nov 2014
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    Canberra
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    Drive-train grief.

    Tallulah is a 1971-ish Series 3. About 25 years ago she had an engine overhaul, and an overhauled gearbox was also put in. Since then, she hasn't done a lot of miles, and I've cared for her over the past 5 years or so with occasional benevolent guidance from her previous, deadly competent owner.

    After about a year of gentle driving, the gearbox started making some weird noises, and the gear lever behaved more like it was stirring a cog-pudding instead of usefully controlling the gears.

    The previous owner can 'do' gearboxes in his sleep, whereas I have done everything but gearboxes, considering them as a black art. I got the box out. When the box was opened and the main shaft tapped out, instead of coming out in one piece, everything separated all over the floor, and the 'dreaded snap-ring' (see here, time 7:15), was not one piece, but ten or twenty separate pieces. To say the 'goddess of gearboxes' was surprised would be a spectacular understatement.

    I learned/helped to overhaul the box. The first/second synchro assembly showed signs of wear and was replaced, but the 3&4 synchro assembly looked good.

    Tallulah worked fine for about 6 months after that, and then she started to drop out of first and third gear when slowing down. I got into the habit of pushing on the gear lever which sometimes stopped it happening, but clearly something was wrong in the box and it would have to be opened soon.

    I live at the bottom of a hill, and one day a couple of months ago, the engine was suddenly no longer connected to the drive wheels. Fortunately I could roll down the hill and back home (how lucky was that!) Weirdly, we could find nothing wrong with the gearbox: no crunchy-munchy noises, every gear engaged nicely, but there was less drive-train noise when the clutch pedal was depressed and a little more noise when the pedal was released with the engine running.

    We felt it was time to look at the clutch, and also fix the gearbox problems while we were at it. The clutch was interesting by itself, but the wear in the gearbox was even more unusual. The brass teeth on the third/fourth synchro assembly still didn't look worn, but the three perpendicular sliders had been bent as if some had grabbed each end of the assembly and simply twisted it by 2-3 degrees. One of the bronze plain bearings on the mainshaft had grabbed its host cog, so the wrong side of the bearing was rotating. Something, but is wasn't obvious what, had given this gearbox a very hard time indeed.

    We took the clutch plate out. The friction surface looked about a third to a half worn, One of the six shock-absorbing springs normally in the clutch plate had fallen out and it was free to rattle around in the clutch housing. The oddest part was that you could put a finger in the splined hole in the middle of the clutch plate that normally accepts the gearbox input shaft, and it was free to rotate in the clutch plate! (You can see this here.) This is obviously where the drive train had completely failed.

    Once everything had been reassembled, I opened up the failed clutch plate to find out how this failure could happen: no-one had heard of this kind of problem before. The photo shows how the outside splines on the 'splined hole' that mates with the clutch plate assembly had fretted and milled away the splines, leaving it free to rotate within the plate assembly.

    It must have taken some very odd forces or vibrations to make this spline-milling action happen at all. The loose shock-absorbing spring that had been rattling around in the clutch housing had a lot of superficial damage, indicating it had been there for a while: its separation wasn't a recent event.

    The once-again overhauled gearbox is working well again, and the gears engage more smoothly than I can ever remember. I am hoping that the accelerated wear to the gearbox is a consequence of the forces from the out-of-balance clutch plate directly attached to the input shaft, but time will tell.

    I guess that if there is anything I've learnt from this episode is that I could have looked around the gearbox a bit more when it first failed unexpectedly, to see if there was another problem that had led the to the original damage. Instead, I had assumed that, 'ah well, it's an old gearbox, you can't expect everything to be too reliable'.

    I'm praying now that this gearbox does remain reliable! it's had so much overhauling that it's a bit like my grandfather's axe!

    I wonder if anyone else has had a similar problem, or has ideas about what started all this?


    Thanks, Mike.
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