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MG MGA - mga rear axle clunk

Need some advice from someone who experienced the same problem with his rear axle. Every time I change gear, when releasing the clutch I hear a very nasty metal to metal clunk coming from the rear end of my MGA car. It must be coming from inside the differential. I'm good at DIY and have lots of experience working on cars. If I take out the diff is it something that I can fix? will there be any special tools needed to do the job? any idea of the expense?

Frank
f camilleri

Frank

Check first that the axle is tight on the car - the U bolts come loose as the rubber pads compress / rot. That causes clunks and errattic handling.

Then check that your noise is really from the differential, not from the universal joints in the propshaft - that typically manifests as a metallic 'ting' when releasing the clutch or accelerating and then gets noisier as the bearings disintegrate

If it is the diff, it's easy to remove (just remove the half shafts, undo the U bolts and the propshaft at the diff, move the axle backwards on the springs, and then you can get the diff out after undoing the nuts that hold ot to the axle case). The reason you have to undo the U bolts is that the handbrake braces in the gearbox tunnel prevent you removing the propshaft and it's easier to remove the U bolts and shift it backwards than get at the bolts for these braces.

The best place to look for instructions on the rebuild of the diff itself is either the workshop manual (free to download on my site) or - naturally - MGAguru
dominic clancy

If you car has wire wheels it could also be the wire wheels or the splined hubs making the clunks
Robert (Bob) Midget Turbo

I disagree with Dominic here, there is no need to move the axle backwards on the springs. Simply leave it in place, remove the half shafts; mark the prop shaft & “pumpkin” coupling, drop the prop shaft and undo the bolts that hold the pumpkin (differential) to the axel housing. There is adequate room to pull the pumpkin without moving the axle.
David werblow

Frank,

Before pulling the differential, also check the silent block busings, rear spring busings, and the rear shock linkages (see pic). These may not be all that likely since the noise occurs on shifting.

Is the differential oil at the proper level?

A more remote possiblity is to check the front differential pinon nut. It needs to be torqued to 140 ft-lbs. It can be accessed by disconnecting the rear prop shaft flange. No need to drop the differential for this quick check.

John


jjb Backman

If it is a flanged gearbox output - late 1500/1600/1622/MGB - it can also be a loose nut retaining the flange on the gbx output. But my money's on the U joints!

FRM
FR Millmore

Dominic, John, David, Bob and FRM, thank you all for your advice. The car was recently restored and all the bushings, paddings, etc. was replaced with the new upgraded polyurethane stuff. The pinion nut on the front of the axle had been properly torqued to 140 ft/lbs. Yes there is enough oil in the diff, it is 90 wt. Everything seems to be tight at the rear end, I must confess though that I did not check the universal joints on the prop shaft, although I have a feeling that these are OK too. I shall be under the car this very morning to check the joints very carefully. I suspect that the damage, if thats what it is, is inside the diff. Could it be shimming adjustment of the crown wheel and pinion thats needed? A friend of mine who owns an MGB has the same problem on his car and he tells me that the problem lies inside the diff. I would like to know how difficult it would be to carry out this shimming adjustment if needed. I have no problem in removing the axle from the car, but in my many years fiddling with cars I have never stripped a differential for repair or adjustment.

Frank
f camilleri

Frank-
Usually the clunk from the diff is worn or broken thrust washers on the diff spider gears. More common on MGB tube axles though. It is not necessary to fiddle with the crown wheel & pinion settings to fix this.
Here's a pictorial on the MGB, MGA is similar once you remove the diff, but it's a taper pin that holds the center shaft in; must come out the right way.
http://chicagolandmgclub.com/photos/b_clunk/

FRM
FR Millmore

Frank,
Interested to read your story.
Sounds a very familiar noise to mine !
If you do decide to strip and replace anything in the diff please keep us posted especially with photos if you can manage it. Best of luck.
David
D C GRAHAME

Thanks for the link FRM. If and when I decide to have a go at the diff it would surely be very useful. I'm not sure I know what you mean by the taper pin and exactly where it is. But of course when the diff is out I'm sure that I will spot it on examination.
David I most certainly will. I will communicate with you via email and give you all the information.

Frank
f camilleri

It might be worth actually measuring the differential backlash between the diff flange and casing. I think that Barney's site gives acceptable tolerances for backlash.
J H Cole

Clunk in the MGA drive train is more likely to be a bad U-joint in the prop shaft, which is easier to fix than a clunk in the differential. If the U-joints look good, rotate the prop shaft back cand forth to inspect the backlash. Up to 1/8-inch movement at edge of the bolt flange should be no problem. Movement up to 1/4-inch at the flange will cause a clunk as torque reverses direction. This is nearly always due to worn thrust washers at the differential bevel gears, and is moderately easy to fix.

The later MGB tube type rear axle uses phenolic thrust washers for the differential sun gears and bronze spherical thrust washers for the planet gears. The phenolic washers are a problem for early wear, commonly not making 100K miles before developing the nasty clunk. That one is easier to fix with access by removing the rear cover.

Not long ago Moss Motors switched to supplying fiber thrust washers for the sun gears in the MGA and early MGB banjo axles. I will never forgive them for that, and I refuse to buy those parts. I want the good bronze thrust washers back again. Are you listening, Moss?

The MGA and early MGB banjo type rear axle (originally) use bronze thrust washers for both sun and planet gears in the differential. The bronze thrust washers are much more durable. My original parts made it to 335,000 miles until I swapped out the differential for different gear ratio.

For the banjo type axle you need to pull the half shafts out an inch to disengage the inboard splines. Then disconnect rear end of prop shaft, tie it up out of the way, and pull the differential assembly out the front. R&R of the thrust washers is fairly easy once it's on the bench. You have to remove the large bearing caps to lift the ring gear and differential out of the carrier for access the the pinion shaft locking pin. You do not need to touch any of the shims for gear alignment.

The pinion shaft locking pin is a straight steel pin secured in place by peened over edge of the hole. Touch it with a drill tip to remove the burr, then tap out the pin from the opposite end using a small diameter punch. Remove pinion shaft, bevel gears and thrust washers, and replace same in a matter of minutes, all with fingers only. Replace pinion shaft and locking pin, peen the hole over again, reinstall differential into carrier with large bearing caps, and put it back in the car. I have a photo tech page (3 web pages) for the whole process here:
http://chicagolandmgclub.com/photos/a_diff

If you have leaky hub seals, this it the golden opportunity to check the seal surfaces and maybe install speedy-Sleeves. See that process here:
http://mgaguru.com/mgtech/rearaxle/ra101.htm
Barney Gaylord

Frank. Check those UJ's VERY carefully. I had the same symptoms about 3 years ago and I convinced myself it was the diff. Had checked the UJ's but while all was still bolted up. Then took out the diff and found it to be in A1 condition. After much scratching of my head I took the now free propshaft and gave it a twist. The shaft had some play in it and when I checked the gearbox end UJ was moving.
After extracting I could see that some of the pins had dropped out of the UJ and the remaining ones had marked the UJ shafts like a helical gear!!
Changing the UJ's solved the problem.

Neil
Neil Purves

Thank you gentlemen very very much.I cannot express in words my gratitude for your help and advice. A very special word of thanks goes to Mr. Gaylord for his extremely well explained and very detailed article on removing a clunk from the A differential. The pictorial link is also very clear and easy to follow. I can confidently say now that I shall take out the diff and by carefully following Mr. Gaylord's article and pictures I will definitely do the job myself. Obviously the first thing I shall do is follow Neil Purves advice and take out the prop shaft for a careful examination of the U joints. Thanks to all.

Frank
f camilleri

Frank, this is a problem that many of us have or will be going to have at some time so when you sort it out it would be nice to know the outcome.
J H Cole

Frank,
Does your car have wire wheels? I had a wire wheel MGA with worn splines. When ever I would shift I would hear a clunk.

A while back I had a thread looking for a website for help with my '66 T-bird turn signal problem. With help from the T-bird ranch I found the problem was just a dirty turn signal switch. I cleaned it and it took care of the problem. It was an easy fix and I didn't have to buy a new $100 switch.

Thanks for the help guys.
Lyle
Lyle Jacobson

This thread was discussed between 19/10/2009 and 21/10/2009

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