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MG Midget and Sprite Technical - Diff problems?

Posted originally on the MGA site but as the mechanics are similar to my Frogeye I thought there would be some useful experiences here too.

I drove the car into the garage two days ago and left it, as usual, in gear with the handbrake off.
When I came to move it this morning it was locked up as though I had left the handbrake on.
Jacked up the car and, with both wheels off the ground, found the offside rear wheel was locked (out of gear). However what was slightly disconcerting was that there was a slight rotational rock on the wheel (a few mm). My first thoughts had been a jammed brake cylinder but would there still be this rock? When I applied the handbrake there was a solid lock up.
I removed the wheel and then the nuts holding the drum (wire wheels!) and found I could pull the drum forward slightly and then the shoes gripped the edge of the drum. Unfortunately because I couldn't turn the wheel I couldn't get to the brake adjuster.
Wondering what to do next I went over to the other rear wheel and spun it over. I then found the problem wheel was free.
My worry that there is something happening within the diff because a seized cylinder doesn't seem to fit the bill. I would appreciate everyone's thoughts.

(after thoughts.... wheel bearing with cage breaking up???)
Graeme Williams

Broken brake return spring maybe? Or the lining of the brake shoes have come adrift, seen that a few times and they lock the wheel up like you've jammed the hand brake on
J White


Since the trouble wheel is now free, better pull the brake drum and have a look while you still can. If it locks up again, you may loose the opportunity to do it easily.

CR Huff

IIRC wheeler dealers showed this problem with an Isetta bubble car locking up at the rear. it turned out to be the linings coming loose and jamming the wheel.
Bob Beaumont


I'm with the last poster, gotta be something amiss with one of the brakeshoes.

Had this happen twice with bonded linings, it's a devil of a job to get the drum off, good luck!
SR Smith 1

I was able to get the drum off and couldn't see anything remiss but by that stage I was thinking "diff". I will take a closer look.
Graeme Williams

Drum off again. No foreign objects, broken return springs, delaminating shoes. Still a mystery.
Graeme Williams

When you checked the "good" side did you spin the when both ways? I'd strip the good side to check the problem isn't on that side and just fooling you to think it's the other.
J White

Problem def the driver's side. I'm 99.9% it's not in the brake drum.
Graeme Williams

Car in gear, wheels off the ground when you spin one wheel does the other spin? It should spin in the opposite direction unless you have a LSD.

If they don't turn freely (jammed), or it tries to turn the engine over, then there is something amiss in the diff.
J White

Had the same problem trying to roll my car off trailer at a hillclimb. I am guessing your cross shaft (which the planetary gears run on) has dropped out. It should be held in place with a roll pin or similar ... which has sheared. If this is the case and both wheels are off the ground, the wheels will turn (in opposite directions) but the diff and tailshaft will not turn more than a fraction of a rotation. If so, the cure is to remove diff and fit a new pin to locate the cross shaft. Hopefully that is it because it is not an expensive fix!

Bob Schapel (South Australia)
R L Schapel

The confusing issue is that now it is all ok. It has just happened the once! If that pin had dropped out, Bob, wouldn't that have broken it permanently?

When the car rear-end was in the air and I first checked out the problem it was out of gear. THe free wheel turned in both directions, the other was locked. Under these circumstances I believe the crown wheel and carrier would have been turning and the small diff gears would have been running around the pinion on the end of the locked wheel's half shaft. So everything was turning freely except the offending half shaft and bearing, which makes me wonder whether something had temporarily
got into the bearing to jam it.
Unfortunately, I can't remember trying it with it in gear.
Graeme Williams

That would make me very nervous Graeme. What if it locks up again? Maybe at 70mph on a dual cabbageway?

Providence forbid......I dread to think what might ensue. Sorry for the "downer" but a quick peep at the innards might be in order.

Safety Fast.

Speculating : if the roll pin had sheared or dropped out then the cross shaft might move axially and the end might jam against the crown wheel. But wouldn't that jam both half shafts? Seems to me to rule out a diff problem, and it must have been something at the outer end of the half shaft: bearing brakes or drum.

Greybeard, my worry too.

Guy:can't find anything yet! Would be such a relief if I could.
Graeme Williams

You could have broken teeth in the spider gears. A broken piece might have locked it up but fell out with some jiggling.

It is possible the cross shaft was the problem but has slipped back into place due to your jiggling the wheels (or gravity). However, I would have thought that a loose cross shaft would result in more than a few mm of movement. If the filler is on the rear of the banjo, you could take out the plug and see if the end of the cross shaft can be seen. If so, give it a push with a bent wire to see if it moves. You might be able to see the spiders too. I agree that it is risky to drive until the problem is solved.

When it happened to me, the car had been in previous races, stored in the shed and pushed onto a trailer. Probably the roll pin had sheared in a race but constant motion had kept it in place until the vibration of a trailer trip. My Spridget diff is in a TC race car with a TC banjo, so I am not certain what you can see through the Spridget filler. Torch and mirror or one of those flexible camera probes might help.

I have lots of spare spider gear sets if you need them.

Bob Schapel
R L Schapel

Thanks Bob! You will see from my next post I am preparing to bite the bullet. But I will carefully drain the oil to see what comes out first.
Graeme Williams

This thread was discussed between 27/05/2016 and 01/06/2016

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