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MG MGB Technical - Getting that clunk again

I fitted an anti clunk kit to the diff a couple of years ago but it’s coming back again, I see that a thicker set is available but am a bit concerned it might be to much. Approximately how much free play should there be on the prop shaft, and is that the best way to gauge if the new anti clunk kit is doing the job. Thanks Trev
Trevor Harvey

Forgot to say it’s a 71 roadster.
Trevor Harvey

Being creaky and wanting to avoid crawling under the car, I measure it at the perphery of the wheel. With the other wheel still on the ground, in top gear (and obviously handbrake of and wheels chocked) I get the following scale for total system free play:
12-15mm as new
Up to 25mm no action required
30mm thrusts getting significantly worn
40mm one or more thrusts gone
50mm the cage and/or side& spiders gears wearing
75mm prepare for a big bill
Paul Walbran

Thanks Paul, that’s useful information.
Trevor Harvey

Paul checked the play as you described, got a mesurement of 32/33 mm or 1 1/4 inches. An anti clunk kit was installed 2 years ago but that only marginally improved things. The clunk is not bad but annoying so if a thicker shim set is installed I should aim at about 12/15 mm of free play. So it looks like I shall be spending some time under the old girl and at 80 years of age I should know better.
Trevor Harvey

Paul - when you say 'periphery of the wheel' would I be correct in assuming that is at the tread surface of the tyre and not the wheel rim?
paulh4

Paul4 that’s where I took the measurements that I got,just short of the tyre tread.
Trevor Harvey

Paul, yes - at the tread.
Trevor, it sounds like there is wear in the cage, though it could be in the gears and/or cross shaft.

Usually cage wear is under the cupped thrusts, if bad it can be shimmed using a second thrust with the OD reduced to fit in the wear recess. Not as good as a replacement cage but very much cheaper and simpler. We have also seen wear under the side/sun gears. If the surface remains flat oversize shims will usually do the job, but we have found the odd one in which it is no longer so and needs another cage.

If the gears are worn they are expensive to renew, if budget is limited close your eyes, use the oversize spacers and give up wheelspin!

The shaft has been a quality issue, with new ones recently loose in their hole in the cage and measuring around 2 thou undersize. That's enough to cause a clunk on its own, not to mention the extra stress placed on the roll pin, a known failure point. We have had to resort to getting them made locally to get the right dims, and being limited volume run they cost. But no choice really.

If all of the above are worn, fitting an LSD starts to look economical! But at 32-33 mm I doubt if it is anywhere near that bad.
Paul Walbran

I had a similar issue on my 74 roadster recently. I fixed it by tightening the wire wheel hub nut properly. I probably turned them about 90 degrees and the annoying clunk of 20 years disappeared.
Gordo

Thanks Paul. The roadster axle was rebuilt by someone else in 1998 has done about 50k since then and has 38mm at the tyre, slight clonks. The V8 has done 250k, half that in my ownership, no rebuild I'm aware of and has 57mm at the tyre, no clonks I'm aware of (possibly being a GT). Having failed to get the pin that carries the small gears out on the old roadster axle I've not been in any hurry to attempt fitting an anti-clonk kit on either.

Wheels on hub splines is another cause, although more of a click IME, particularly the rears. That can worsen as the splines are hammered first one side then the other in acceleration and braking and can eventually strip. In a roadster at least you can usually differentiate between that and diff clonk.

paulh4

Eighteen months or so ago I had a rear end clunk on my 1964 mk1 Roadster. Accelerating, braking even speed humps produced a heavy clunk. I tried all the usual things associated with the noise, springs, joints, knockons, splines etc etc. Convinced the Banjo diff was the problem and although there was minimal play in it I decided to re shim it anyway. With the diff on the bench everything was checked and found to be to spec so was returned to the car. On reversing from the garage 'clunk'. In frustration and being sure I had screwed up the diff assembly procedure a second removal and strip down began. On inspection; nothing obvious. The only solution was let the experts at a local business sort the thing out. Their work found nothing amiss and even commented on its fine condition. Time again to refit. Sorry if this is a bit like a Christie novel but the last page is coming up. One of the final assembly jobs is to refit the propshaft. Maybe you've spotted the culprit already? The shaft slider was seized as solid as a rock and had to be dismanteld using excessive force. Despite having had the attention of a grease gun at fairly regular intervals new lube had not being getting through a solid hardened grease wall. Could it really be the problem? Refit....silence!

It my be worth a check Trevor

Chris Woodfield

Having dealt with both pro-shafts at various times a new V8 one came with three grease nipples so all joints are checked annually, the roadster only had one on the sliding joint, UJs replaced 24 years ago. Two years ago started out on a three-day run and noticed vibration accelerating through 40-50, so stayed below or above wherever possible. First thought was prop-shaft and sure enough the rear joint was wobbling about all over the place. I was taken-aback at the wear, two of the joints looked like they had never had grease from new. Definitely a need to 'twist and shake' annually, although this new shaft also has three nipples now.


paulh4

Following Paul W advice I have checked to see if something else could be causing the clunk ie loose dampers, spring months etc. and all seems tight and OK. What should I be looking for if there’s a problem with the prop shaft. I have looked at the UJs they are non grease nippled variety but how do you know if they are suspect. Thanks
Trevor Harvey

Use opposition twisting back and fore, i.e. grasp the gearbox side with one hand and the shaft side with the other and twist as if you were trying to 'unscrew' it, same with the other end, as well as shaking up and down. Check the sliding joint in the same way.

The other test of the diff is to mark the edge of the diff flange and the case and see how much back and fore movement you have with the wheels braked, not in gear, and that eliminates prop-shaft and gearbox and shouldn't get any at hubs and wheels. One opinion has less than 8mm is fine, more than 10mm needs attention.
paulh4

I have just got round to fitting the oversized shims to the roadster, I bought a set of plus 5 thou and plus 13 thou from moss, quite surprised that they are made of a brass looking material. I decided to fit the plus 5 set and see what effect they had on the play measured at the wheel. Where I was getting a measurement of 32 mm of play it is now 20 mm, I would have liked to have seen a bit lower figure but decided to get it all back together and road test. After a short run up and down the road it appears to have done the job no clunk but won’t really know until it’s had a more thorough run.
Trevor Harvey

This thread was discussed between 12/04/2023 and 06/05/2023

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