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MG MGA - diff

I have just found an mga diff which is advertised as having "very acceptable backlash" can anyone tell me what this means and also if there is any way to tell just by examining the diff if there are any problems,thank you Vin.
Vin Rafter

FWIW. My differential seemed fine from the outside. Everything rotated smoothly. When I took it apart (been idle for 40 years) it looked like it was full of orange sand. The gears had started to rust and were pitted. I would say without taking it apart it would be a gamble but if the price it right....


York, Maine


In an engineering context backlash refers to the clearance or lost motion between two mating faces, in this case the curved mating faces of the teeth on the crownwheel and pinion. One doesn't want them to mesh so tightly that there is a risk of binding nor should they be so loose that there is a significant impact when they engage. Correct backlash is somewhere in between and the procedure for setting it up is described in fine detail in the factory workshop manual.

When you examine the mating faces, look for chips where metal is missing and for obvious signs of wear. A properly set up diff will ensure that the teeth mesh together in the centre of the mating faces, so look for signs of wear off-centre, suggesting that the diff has been run with bad set up. Look also for discolouration that points to overheating. Sometimes the B-series axle drips its oil out past the hub oil seals, the oil level in the diff drops and it runs too dry.

Assuming you are buying the whole diff and not just the crown wheel and pinion, look out for cracks in the carrier casting.

The bearings can be replaced, so wear here is less of a concern.

I have a diff in my garage awaiting rebuild by a professional. It looks fine but whines like a banshee in use but I'm pretty sure that this is a bearing problem because the cw & p are pristine.

Malcolm Eades

Vin, backlash in a complete rear assembly may be and usually is clearance in the spider and axle gears due to worn bushings, not a big deal, but it could also be excessive wear between the ring gear and pinion. Really no way to check without removing the diff assembly from the case and checking visually. That said, unless the rear was terribly abused or used for racing they are usually very reliable pieces and finding one that is truely worn beyond saving is unusual.
Reasonable wear in the spider gears would result in about 1/8" or so movement in the pinion flange before you felt the gears engage. Backlash in the ring gear to pinion gear mesh is down in the range of a few thousandths of an inch and is best measured with a dial indicator on the ring gear.
Bill Young

Clearance between ring and pinion gear teeth is paper thin, and the gears are quite robust. Excessive backlash at the input flange in situ is usually a result of worn thrust washers behind the differential bevel gears. Up to 1/8 inch at the input flange is acceptable. When it gets to 1/4 inch there may be a clunk noise upon reversing torque, telling you it is time to replace the washers. Those four thrust washers are cheap, easy enough to replace. Get the original type brass parts if possible. The flat fiber washers recently supplied for the side gears are crap.

If the unit whines when driven, first check to assure that the input pinion nut (on the input flange) is tight. If so, then you need to remove the assembly from the main axle housing to check condition of the large ball bearings (which should have a slight preload and zero running clearance). Replacing worn ball bearings may eliminate the whine, but this process also requires readjusting shims to properly preload the bearings while aligning the ring gear (not for the feint of heart).
Barney Gaylord

The flat fiber washers recently supplied for the side gears are crap.

Technical term Barney??


Colin Parkinson


I notice in the advert that the production date is post the MGA period, being dated October 1962. You may want to check with the vendor the number of splines in the sun gears as it may not be set up for the MGA.

Not a show stopper though as Barney tells you how to swap them over. I did the procedure. It looks daunting at first sight but is in fact quite straight forward.

Steve Gyles

This thread was discussed between 26/11/2010 and 27/11/2010

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