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MG MGB Technical - Damaged banjo axle

A friend of mine found that the banjo axle on his trials car was making a strange noise. So he swapped it for a tube axle. He new that I would quite like to fit a banjo axle to my 74 roadster as a weight reduction measure for when I compete in sprints so he let me have the axle on the basis I could pay him something if was able to make use of it.

A couple of years have gone by and my son and I got round to stripping the axle.
1)The oil that came out looked like metalic paint.
2)There were some chips of metal in the bottom.
3)The end of the pinion teeth were burred and the end of the pinion had scratches on it as if it had been rubbing on something.
4)The cross pin in the diff is not proud of the cage at either end but bits of one end look to have broken away.
5)I can't see any damage to the crown wheel.

Given that you can only buy a crown wheel and pinion as a pair and they are daft money, I am thinking of cleaning up the burrs on the pinion, changing the cross pin and putting the axle back together.

Has anyone got any ideas or had this problem before.
David Witham

From your description, the only scenario that I can come up with is that the cross pin drifted out and contacted the end of the pinion gear while running. Why was the pin able to float back and forth?

Not sure, but I think a roll pin is used to pin the cross pin. Did that come out? As I recall, during replacement of the roll pin, John Twist suggests putting a cotter pin through the center of the roll pin to prevent just that.

Other than that, what you suggest should work OK. That is, if you are replacing all the bearings and shims.

Wayne
Wayne Pearson

"cotter pin" - what we would know as a split-pin, a cotter pin being a wedge pulled tightly into position by a nut.
Paul Hunt 2010

Hi,

From memory, when I did my diff and found the cross pin half out of the carrier, it should be locked into position with a solid pin. This solid pin is held in place by slightly burring the edges of the hole with something like a countersink punch. Pther brands of diff do this as well.

Herb
Herb Adler

The pinion shaft, having been damaged, will lead to damage of the crown gear. What you describe will be a temporary fix, at best. RAY
rjm RAY

If there was going to be damage to the crown gear, it would have already happened or at least shown signs of impending doom. By grinding down the burrs to below the gear tooth surface, no damaged area of the pinion teeth will contact the crown gear. I wouldn't say this unless I believed that the damage to the pinion was only on the end of the teeth, not the top land of the gear tooth.

A photo would help, though.

Wayne
Wayne Pearson

Sorry about the lack of picture.

I took the various parts along to the MGCC natter in Warwick last night. Every one had a good look. One of the members is a recently retired design engineer from a major clutch manufacturer in the next town.

He pointed out that there is very little of the damage to the pinion teeth on the force bearing surface of the tooth. It was on the end and on the top of the tooth. He agreed with the clean the burrs off put it back together carefully and see how it does approach.

So I now need to have some fun with engineers blue. I seem to remember there is a difference between engineers blue and mark up blue. So I better get the correct one!
David Witham

David
Yes, don't do what we did years ago rebuilding an differential as a training project. We found out that marking blue does not wear off when the gears mesh :-(.
Cheers
Andy Taylor

Herb, your description is about spot on.

I cleaned out the burr with a drill as per the factory workshop manual. Tried to drive it out with a punch through the reduced size hole at the other end. It would not move. Tried to drill it out from the top. The locking pin is harder than the casting so the drill wondered! Then for some reason tried hitting the cross pin. It moved! I was able to drive it out with the locking pin apparently still in place.

Looking at the cross pin it looks as it the locking pin has been sheared off at both ends as it passes through the cross pin. I think this must be from a prior removal rather than in use, but I don't know.

However, the cross pin has some heavy burrs on it and the bores of the 2 bevel gears that turn on it are badly damaged as well.

The cross pins are £21 new which is not too bad but the 2 bevel gears are £88 each which is rather alot. So I am thinking of finding a machine shop to bore them out and sleeve them back. I dare say it would not take long for a machine shop to hit the same price.
David Witham

David exactly which gears are they? I may have some!!

Colin
Colin Parkinson

Colin, on the diagram in the attached link they are item 26 part number ATB7123. There are 2 of them.

http://www.ukmgparts.co.uk/browse.aspx?CID=a06aab03-5ebc-49d9-a937-a6d8adfce6c1&SID=9d349504-67fe-4945-a275-d5db28a41105

Thanks,

David
David Witham

In case it helps the equivilent part in the MGA diff has the same part number.
David Witham

Hi David,

Your description of the inner surfaces of the gears and the cross pin is the same as mine were. I too thought that a new, slightly larger pin and the gears bored out would do. However, the diff specialist told me that these surfaces were all case hardened and would first need to be annealed, then bored and re-hardened, quoted about $200 each to do. I eventually got a replacement set from the guy I bought the car from.

I think that the set pin through the cross pin does shear under use. It might have to do with a dry diff and the inner surface of the gears binding to the cross pin and thus shearing the set pin.

Herb
Herb Adler

The planet gears only turn when the diff is required to compensate for different wheel speeds, normally for a short time through a turn. That it was a trials car means it spent a bunch of time with one wheel spinning - same thing happens on cars driven harshly in snow. The gears seize on the cross shaft from lack of lubrication, as the oil supply is iffy and gets centrifuged out of that area. When the gears seize the shaft turns, shearing the location pin. The cure is to cut oil grooves in the shaft or bores with a small stone on a die grinder, and/or avoid spinning one wheel for any but the briefest moment. Many OE (not MG) shafts have grooves or flats on them for this reason.

FRM
Fletcher R Millmore

Herb, Fletcher, thanks for those comments. I think I understand the way forward a bit better now. As it happens there are some flats on the shaft.
David Witham

David, somewhere in the workshop is the complete centre bit of the diff with the gears you need.

Unfortunately I cannot find it!!!

Will have another search at the weekend

Colin
Colin Parkinson

David, I have sent you a PM

Colin
Colin Parkinson

This thread was discussed between 28/06/2010 and 24/07/2010

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