411 driveline 'clonk'

[Bottomboy]

Hi
I am looking at a 411, which has a hefty clonk from the driveline when engaging reverse. I was thinking excess backlash between crownwheel and pinion, but what might be the cause of that?

Any thoughts would be very much appreciated before I start taking things apart.

Thanks.

[Barrie]

First step is to consider the more likely wear of universal joints on the propshaft.
Get the car up as high as poss (jack with axle stands or on a hoist) and grasp the propshaft firmly and twist backwards and forwards. There should be no discernible movement. If there is it's relatively cheap to replace them. Subsequently you may find there is still a clonk but the UJ's would have needed replacement in any case. If it persists then take it to a gearbox specialist to see if there's any play in the GB, otherwise your then left with the axle as the culprit.
Usually it's the UJ's

[John Rolfs]

The clunk is probably the cross pin for the differential which sits in the cast iron crown wheel carrier, rather than excessive play between the teeth of the crown wheel and the pinion gear. The holes in the cast iron diff carrier wear oval. I rebuilt my axle after losing a crown wheel tooth but did not fix this and so still have this clunk. It does not mean any impending failure.

If you take the axle apart, you can have phosphor bronze inserts made to match the steel diff pin and drill out out the worn oval holes in the casting to fit them.

Other things to check. Your prop shaft universal joints rotate OK when the rear end is lifted. If you drain the axle oil make sure there are no bright flecks of metal to indicate bearing or tooth failure.

[Bottomboy]

Thank you both. That is really helpful. I now have something to think about!!