Driveshaft Doughnut Failure

Failed inboard left hand drive shaft “doughnut”

Instead of universal joints (hook or cardan joints) in the BMW 700 (and many others) used a rubber “doughnut” – named for fairly obvious reasons.

Rubber does not last forever and this one gave up the struggle as I was leaving the Torquay Motor Show in Feb 2020.

Perhaps unsurprisingly I was not the only “classic car” to fail and there was a 4 hour wait for a flat bed recovery truck. But RACV got me a taxi home so I could wait there rather than at the roadside.

I am unsure of the cause of the failure. Looking back through the records that I have I can see that 2 couplings were purchased in 2009 and another in 2018. To me, replacing anything less than all 4 is ridiculous. But, on that basis I presume the one that failed was older than 2009.

Another consideration is the fact that all the bolts were loose which would have allowed the rubber to flex in ways it was not intended to. The Nylock nuts had clearly been re-used and were no longer locking. Plus the bolts were a much finer thread that the design calls for which would inhibit their locking capability even when torqued up.

Anyway, 4 new doughnuts, one new set of bolts and all new nuts plus a mental note to spanner check periodically.

Possibly the doughnut was on the way out, or the bolts loose or whatever, or maybe an illusion, but it does seem less prone to transmission wind up now.

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