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MG Midget and Sprite Technical - Is my diff repairable or new diff?

The diff was the last unknown on my car, but I I remembered it was getting a bit noisy last time I drove it, and so I was somewhat prepared for what I’d find when I removed it.

It doesn’t look to pretty. The pinion is quite worn and the small pin that hold the pinion pin in place has sheared off. The meant the pinion pin could float about ¼” back and forth. One of the sun gears has a lot of end float and wobbles on the shaft. I suspect one too many burnouts as a lad has done the damage.

Gear wobble https://vimeo.com/58286011

So what do people think? Is it repairable or am I looking for a new diff?



Greg H

Sun gear gap


Greg H

the most important bit is the amount of wear on the pinion (and possibly crownwheel too) teeth, from the pic I'd say it'll work OK but *might* be a bit noisy. It's worth a go - you need 2 new cup washers for the planet gears, a new cross pin and locking pin. The thrust washers for the sun gears could be worn but I think they are NLA anyway. I'd leave them alone. Even when new or rebuilt there's a fair bit of slop in the sun & planet gears when bone dry - the oil seems to take up most of it.
David Smith

Thanks David. How do I get the coss pin out? As you probably know the housing has been centre popped to keep it in. Do I have to drill it out?

Is the centre the same as some other english car that may be in Aus.e.g. morris minor or major?
Greg H

You haven't said what ratio it is. For example if it's a 4.22 it might be cheaper to buy a decent secondhand item and keep yours for spares. If it's a 3.7 you'd want to rebuild it.

The Wolseley and Riley 1500s have the same diff but using in a 3.7 ratio from memory. I think in Oz you do call them the Austin Lancer and Morris Major but I'd need a photo to be certain.
Daniel Stapleton

yes the lock pin comes out the same way it went in. Use a 1/8 " drill to remove the peening, and drive the pin out with a long 1/8 " pin punch from the other end. Then the big crosspin falls out easily. And yes again, it's all the same as any Austin/Morris A series from about 1955 to 1980! So A30 A35 A40 Morris Minor and 1000, Riley 1.5 Wolseley 1500 etc.
David Smith

I counted it at 38/9 So it's a 4.22. A taller diff like a 3.9 might be nicer.

This is what's called a Morris Major in Aus. All those cars listest are hard to fnd these days.


Greg H

4.22 is Morris Minor 1000 from about mid-sixties to end of production, and Spridgets up to 1968. The 3.9 is Spridgets from '68 to '77.
David Smith

Mine's a 67 spec. The Aus cars up to 70 were all 67 spec. Is a 3.9 better?
Greg H

slightly down on acceleration, better in the cruise...
David Smith

Remember if you change your diff your speedo will read incorrectly!
G Williams (Graeme)

It's in miles so it's never been correct, it'll just be a different incorrect. ;)
Greg H

So I'm chasing up parts to fix my diff. I can get the thrust washers, planetary gears and the cross pin, but I can't find a supplier for the cross pin locking pin. 6K631. I belive it's 5/32 x 1".

Anyone know where I can get one from?
Greg H

Wow, it's showing as NLA.

I still have a few NOS.

midgetbitz at gmail dot com
Dave O'Neill2

When I asked some time ago, the opinion was to use "roll pins "
Dan Cusworth

Hi Dave I sent you an email a couple of days ago but it may have ended up in your junk mail box. Thanks for offer although I managed to get one from morrisminorspares.co.uk.

A roll pin may be harder than the solid pin but there's less of it and so I wonder how it would go.
Greg H

Roll pins are used in the B series tube axle so are basically OK. They have been known to break, but only after excessive wheelspin cause the spider gears to seize onto the cross shaft. But then excessive wheelspin in Midgets does that too with equally disastrous results (I can assure you!!)
So, roll pin is OK unless you get wheel spin lots, in which case your whole diff is a risk anyway (so you should fit an LSD to control it and get the power down too ...)
Paul Walbran

Couldn't you make a locking pin from a hardened nail of the right size?

Or use a roll pin and push a nail into that to reinforce it. That technique is employed in the Ford Type9 gearbox by BGH geartech.






Lawrence Slater

I could have just got a piece of 5/32" silver steel or spring steel and cut it to length, but when I found a supplier that sells them, and at only £0.75 why bother. I mean why would you stuff about spending your own precious time on something when you could just buy a new one? ;)

I have to wait for the other diff bits to get here from the UK anyway.
Greg H

I’m having some problems getting the Pinion Pin out of my diff. The small pin was broken and so the Pinion Pin will spin around in circles. I spun the Pinion Pin around so that the small pin lined up and tried to hit it out from behind with a 1/8” pin but that didn’t work. So I drilled the pin out from the front where it had been staked over. About 3/16” drill bit down to the Pinion Pin. Small pin is 5/32” so not drilled to the full diameter. So now the pin shouldn’t foul anywhere.

I then tried to hit the Pinion Pin out from one end but it only goes about 3/16” then jams. Not sure but I think it may be jamming on the sun gears. So I still can’t get it out.

The yellow arrow points to the position of the small pin.

Anyone got any good ideas on how to get it out?



Greg H

I'm sure you will need to remove the crown wheel from the carrier.

I don't think it is the small gears - if the locking pin has been removed the it should slide easily out.
Dan Cusworth

The pin looks like it will clears the crown wheel so unless there is another reason why I need to remove it I think it can stay on.

The locking pin has not been not removed but since it's broken the pin spins so I don't think it should be stopping it.

Holes are a bit slogged in the housing and the sun gears spin on the pin so like you say it should slide out.
Greg H

How hard are you hitting it?
Dave O'Neill2

Quite hard with a drift and it goes clunk clunk like it's home on something but it'll happily float back into the centre.
Greg H

I'm trying to remember if the pin has a flat where the planet gear sits. If it does, the step could be catching on a thrust washer.
Dave O'Neill2

Yes is does, so it could be that. I'll continue on.


Greg H

Yes, just use a bigger hammer!
Dave O'Neill2

I'm using a FBH. As soon as the part of the pin that was in the casing moves through to the planetary gear it jams. As they are both hardened nothing gives. It's like the hole in the gear is smaller. Perhaps build up either in the inside of the gear or the shaft has a ridge where it meets the unworn section. They all turn freely when centred and running on the worn part of the shaft but not the unworn.

Avoidance mode, perhaps I'll paint the housing.
Greg H

Any ideas on how to get the pin out or knowledge of what's stopping it?

Bashing just isn't working. I'd like to cut the middle of the pin out and hit the halves inwards but surely there;s an easier way. I could put a press on the pin but I think that'd bust the housing or do some damage to the holes.

Anyone else had a similar problem?
Greg H

the pin just slides out, no hammer is required

either you have to remove the ring gear ("crown wheel") because it is in the way, or you still have some of the 5/16" pin in the retaining hole that is preventing it from sliding



Norm

Norm Kerr

Norm, the crown wheel has now been removed. The locking pin had already sheared and so the Pinion Pin happily spins in the holes and will slide out a 1/4" then jam. This is also the case if the Pinion Pin it's rotated 90 degrees. I also drilled out the locking pin from where it's staked all the way down to the Pinion Pin. Visible in the above photo. I can see the Pinion Pin spinning through the hole where it's been drilled and both sides look clean. When it’s rotated at 90 degrees the locking pin holes in the housing would be aligned to a clean part of the Pinion Pin and so this is way I don’t think the locking pin is what’s fouling it, and think perhaps there’s a ridge on the Pinion Pin.
Greg H

The sun gears both float on the shaft but problem is that when the shaft is moved out the gear with blue arrow will not stay still and move with the shaft until it jams the sun gear on the driver gears. At that point it all loks up and when the shaft is hit it's jammed solid. I'm confident of the diagnosis but not how to fix it.:(


Greg H

Sounds like the shaft has become spread or damaged on the outer end so that it won't slide through the planet gear. Can you move it out the other way at all? You may get it far enough out to inspect it and if necessary de-bur the end of the shaft.
Guy Weller

No I can only get the end out about 3/16", not far enough to see likewise if pushed the otherway because it's covered by the gear.

The shaft may have been spinding for a while because the hole in the housing is a little worn. Could be a build up of metal from the housing embedded on the shaft.
Greg H

If the hole in the housing is worn then the cage should be replaced. You'll still need to get that shaft out so you can transfer the innards over though.
[It sounds like it's trying to tell you it wants a slippy diff fitted! :-)]

If you can't find a cage locally, we have surplus ones here (having fitted a few LSD's) so we could do a very cheap one, say $20. (Though probably as much again for postage)
Paul Walbran

At least you're lucky the pin didn't come out while you were driving.

It makes a big mess - don't ask me how I know!
Dave O'Neill2

Dave,

I had the pin came out too but never heard a thing, I had lifted off and when I reapplied the throttle no drive. I diagnosed a diff or half shaft failure and got recovered by the RAC. Once removed the only damage was one tooth on the pinion missing a small section but big enough to scrap it.
David Billington

Ditto! We already had the LSD to fit but thought we'd get away with one more event before doing so. Wrong! New CW&P needed too. Excessive wheelspin has a nasty habit of making a planet gear seize to the shaft and shear the retaining pin ...
B series has the same problem.
Paul Walbran

Thanks Paul. I'll have a go at getting it apart on the weekend and reassess when it's in bits.
Greg H

By the sound of it, someone with a handy press would be a good idea.
Paul Walbran

I can press it as a last resort but I didn't want to chance more damage to the cage just in case it's salavagble.
Greg H

I ended up pressing the pin out. I pressed it out the side that it was jamming on so that it didn't load up the other gears. It took 10 tonne because that's the force that was needed to split the sun gear. It wouldn't budge until then.

A leason it why you shouldn't do circle work and burnouts in a Midget. The pin and copper shims are well worn and look like they've been very hot.

The new large shims that go on the axle gear weren't copper but fibre which surprised me, but I assume they'll still work ok.


Greg H

"A leason it why you shouldn't do circle work and burnouts in a Midget" ... definitely time to buy a slippy diff then, not only cheaper but much more fun doing said things!
Paul Walbran

This thread was discussed between 27/01/2013 and 23/02/2013

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