MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Diff rebuild with 3.55 ratio and LSD

Hi Folks,

An update for those that missed all this on FB (MASC page), I'm sure some of you will be interested.

I have been pondering a diff rebuild and upgrade for some time, and pulling the old diff, it seems not a moment too soon. I had two pinion teeth rattling round in the pumpkin, with another two badly chipped and yet another with a hairline crack running the length of it! When I disassembled the diff, the pinion nut and whole pinion assembly seemed quite loose, so something has gone awry here! I can't remember if I have been in here before to replace the oil seal...? *Shrugs* I didn't think I had, but it might be self inflicted.

Anyway, the replacement is a 3.55 ratio CW & P ordered from Bastuck in Germany. I believe they supply Moss and going direct saved over £100 on the Moss prices. Although ordering is a little tricky.

I have added an upgraded helical type limited slip differential too.

It's been an enjoyable build, quite simple really. Some points to note:

- The pinion bearings are eff-ing tight. I have not seen anything like it before! Especially getting the old bearing off to rescue the pinion spacer. I lost interest and cut it off!

- I shimmed the diff on the outside face to make adjustment easy (saves removing the bearings to adjust). I found a UK bearing supplier that has the right size shims to do this.

- New crush spacers are pretty soft. Don't use a torque wrench, just nip it up bit by bit and check the bearing drag each time. I made a large peg spanner to hold the pinion whilst I did up the nut.

- Sudocrem (nappy rash cream) makes a good gear marking compound!

Pinion pre-load checked off nicely at 11 - 12 in-lb. Backlash bang on 7 thou. Tooth pattern looks great. Beautiful!

Sadly it's probably going to be spring by the time I get to give it a test run.

Cheers,
Malc


Malcolm

Finally built up

Malcolm

Tooth pattern (with Sudocrem!)

Cheers,
Malc.

Malcolm

Malc,
thanks for thinking of us too young for FB yet and reporting here. I don't understand it but still interesting and a nice style of writing and interesting photos.

However, I feel I must must mention, you could also use Bio Oil, and other products available, for your nappy rash, big bottles are available for adult use.;)

Bio Oil wouldn't be as good for marking the teeth though, but perhaps a good additive to the no doubt very cheapest LSD oil you'll be purchasing.

Carry on. :)
Nigel Atkins

Nice job Malcolm,
I thought you'd enjoy doing that
7 thou is quite a heap of backlash though, normally we'd be looking at 4-5 but in saying that your marks look around the same as the lapping marks on the adjacent teeth so all good I guess
Personally, If it were mine and was setting it up purely off that marking I'd move the crownwheel a couple of thou. closer which would bring the backlash in a bit and move the mark slightly towards the toe a bit- specially as it's an alloy housing
But- if the spec is 7 thou. then that is it I guess
Is that what was marked on it
It'd be interesting to put a texta line accross the top and bottom of the lapping marks on a tooth and compare that to your Sudo. mark
Cheers
willy
William Revit

Interesting to see your broken teeth. I noticed when I pulled a 3.7 diff after the diff cross shaft came out and broke off a pinion tooth that there were visible cracks running from the root of each pinion tooth to the centre so failure of some sort was likely inevitable. The edge of the pinion teeth at the nose end is razor sharp so acts as a stress raiser so I think if I had one apart in the future I would use a stone and add a slight radius to that edge and polish it to reduce the potential stress raiser and the likelihood for cracks to form in the future.
David Billington

Willy, the CW is hand marked 7-20. I assumed these must relate to the backlash and/or shimming settings?

With the shims I have for the outside of the bearings I can only move it in 0.1mm (4 thou) increments and more annoyingly, with the selection I bought, 0.1mm closer is the one combination I can't do!

(not sure why the image has rotated upside down - sorry!)

Malcolm

One of those i-can't-phones or other mobile device(?).


Nigel Atkins

Thanks Nigel. I could have done it myself but was in a rush to go out.

Thanks also for your kind words regards my writing. It's not something I work on, it's just how it comes out. I did notice when re-reading that my tenses are a bit all over the place! I shall work on that in the future... or do I mean in the past?! :-D
Malcolm

Malc,
I wouldn't notice mistakes like that, my wife would but without the slightest interest in the subject, and that's a point about communication.

Sloppy photos are unforgivable though!
Nigel Atkins

All good Malc.
Just me being me--i'll get over it, I'm used to setting up big horsepower racers and get a bit carried away--sorry
7-20 would normally be 7thou backlash and 20 being the gearset matching number, This(20) would be on the pinion as well--so the pinion would have either '0' for a zero pinion or up to plus or minus 5 for pinion height and then the same 20 set number
You have done well young man, keep up the good work
And yes 4thou would be too big a step to take ,it would move the mark way down to the end of the tooth and would be noisy on light cruisy throttle
Cheers
willy
William Revit

Have you dispensed with the spring washers for the bearing cap nuts, or weren't they there before?
Dave O'Neill 2

Eagle eyes Dave! I added spring washers before final torque up.

Maybe interestingly, if you look at pic 1 (broken teeth) this is my original 3.9 ratio housing, the studs are a touch shorter (only 1 or 2 threads showing) and didn't have spring washers. The new diff is built into a 4.2 housing with slightly longer studs (about 5 threads showing in pic 2) for spring washers.

Thanks for your input and validation Willy. Much appreciated.

Malc
Malcolm

A little question Malcolm if I may--
The Bastuck c/w and pinion set--Did the pinion have the normal 0 or +-5 marking on it or have they gone to the new total depth measurement system-
William Revit

Hi Willy,

The pinion is marked with a zero inside a square. Picture isn't that clear.

Malcolm

Ah, very good
The reason i asked was that i got a v8 conversion c/w and pinion set from a UK supplier here a couple of months back and it was marked using the other total height method--must be from a different manufacturer i guess,
Thanks
willy
William Revit

This thread was discussed between 26/11/2020 and 29/11/2020

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now