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MG Midget and Sprite Technical - Differential Build Crush Spacer and Bearing Shims

Hi folks,

I am pondering the intricacies of a differential rebuild. I will be effectively starting from scratch as I plan to build up the unit with an ATB diff (Torsen/Quaife/gear driven LSD) and a new crownwheel and pinion (3.55 ratio).

I have a casing that I have stripped of the old bits and is ready to go.

Question 1:

With respect to setting up the pinion. I see that the pinion thrust washers are not available from the usual suspects, so I guess I can only set the pinion depth to whatever it is set at now and then set the CW as best as possible to suit?

This being the case, could I measure up the removed crush spacer and machine a solid spacer to the same length. Would this be advantageous in setting up the pinion bearings? i.e. I can just torque it down and it should be correct?

Question 2:

There was a discussion recently about shimming the diff bearings on the outer face rather than the inner. Whoever it was (sorry, I forget!) were you successful in doing it this way and where did you get your shims? Or did you make them in the end?

Cheers,
Malc.
Malcolm

Malc

You may find this Midet Mania video handy:
https://m.youtube.com/watch?v=H9yKWKV8poc

There should be some experienced folk along shortly to advise.

Cheers
Mike
M Wood

Malc

You may find ten Midet Mania video handy:

There should be some experienced folk along shortly to advise.

Cheers
Mike
M Wood

Don't throw your old pinion out, there should be a number on the end of it to help set up your new pinion height, ---unless your new c/w and pinion has the new numbering system
Some manufacturers now are really helpfull and give you the actual measurement from the washer face to the centreline of the carrier that the pinion ended up at after lapping in, just depends on what you get/ have got
William Revit

Some more pondering, question 3:

When doing up the flange nut, it seems that 140 lb-ft is more an indicative value than an absoloute and the key thing is achieving the correct bearing pre-load. So what about ditching the old style plain nut and lock washer for a plain washer and nylock and ignoring the torque as long as I get the correct pre-load?
Malcolm

Slow afternoon at work, I think I have found all the information I could ever want on pinion bearings, spacers and nuts here:

http://www.mg-cars.net/mg-midget-sprite-technical-bbs/differential-collapsible-spacers-201304041223001355.htm

Good ol' archives.

That Lawrence bloke seems like a smart cookie ;-)

Malc.
Malcolm

Archives and Lawrence - this BBS rocks!

Seen this? https://m.youtube.com/watch?v=Iejr6nIkW2w

Cheers
Mike
M Wood

good video, I'm surprised he didn't check the backlash or contact pattern.

Bob
R.A Davis

I only noticed the excellent oil to be used in the new diff. :)

A word of caution, and it may well not apply to this subject, or others, but I found Norm's MGExp article on 'the famous fwb' to be inaccurate and misleading - and yes I did let Norm know, I joined MGExp just so I could contact him off-board to let him know. He thought the essence of his article was the main thing, to inform about different bearings sold and the history of fitting different bearings, but I disagree, and I've not been able to close my account there either (never got responses from webmaster).

Not everything on the web is correct, even I've made mistakes, yes I know difficult to believe but I have.
Nigel Atkins

I just viewed the video and thought the wear pattern on the CW teeth looked rather odd and as already noted checking the backlash would be a good idea. Also applying loctite and leaving the bolts to be tightened later is not clever as the loctite would likely have cured by the time he got the diff assembly back in the final drive housing,.
David Billington

I always assemble eveything all tightened up then one by one remove each bolt, apply loctite then torque. Takes longer but avoids the loctite going off.
Paul Walbran

I was going to warble on about our collapsible spacer experience and why we swapped to solid, but then I read the link and saw I'd been there before....
Paul Walbran

Malcolm, one thing you raised which hasn't been touched on (though I may have missed it) was the NLA pinion head spacer. Usually a different one will be needed when fitting a new cwp. Unless you are lucky. I was astounded to find instructions with a new cwp saying to use the old one and set up on that.
We had a full set of spacers made up, expensive due to low qty run but worth having. An alternative approach is to use shims in conjunction with the existing spacer, though it might be too thick and need a skim with a grinder on a magnetic table.
Paul Walbran

Malc
Pinion thrust washers are listed by ESM Minors , I have always found the cheapest and best stocked for diff spares.
Ian

Ian Webb 1973 GAN5

"When doing up the flange nut, it seems that 140 lb-ft is more an indicative value than an absoloute and the key thing is achieving the correct bearing pre-load. So what about ditching the old style plain nut and lock washer for a plain washer and nylock and ignoring the torque as long as I get the correct pre-load?"


Yes, its the pre load that is important, the force to get it to the preload is just that, a guide. When I did mine I had trouble finding the preload figure as there seems to be various numbers banded around and it depends on how much of the rear axle assembly you have connected. If you use the crush washer be aware that its very very easy to over tighten once you get close to the preload figure you are chasing. You are working in just a few degree increments. A very long breaker bar helps with control.

Chris Madge

Malcolm
You're spot on with your thoughts on the 140ft/lb
The preload is the aim of the game
Something I do when doing a diff, specially if i think it's coming apart to change shims, is to fit the pinion up without the seal and without the collapsible spacer--just carefully tighten up till the correct drag/preload is achieved
Doing it like this makes it easier to pull apart and do it over- sometimes you can have it out 3-4 times to get a spot on mark---It's not necessary to go to the trouble of making up a solid spacer for setup as long as you are carefull not to overtighten and get excessive bearing wrecking preload
Good fun setting up diffs and a definite feel-good when you get that spot on marking

have fun, you'll enjoy it
willy
William Revit

Also, I'm a bit interested to hear why you are planning to go for a gear type centre as against a clutch/cone type
I had a gear type in my midget hillclimber and it was a pain in the butt, completely useless once a wheel comes off the ground--While both wheels were on the deck it would lock up fine, even maybe too early and would push like a spool but if you jumped a kerb and got one in the air it would just unlock and loose all drive like an open centre does

willy
William Revit

Hi Willy,

It is still a road car first and foremost and I generally do 3000+ miles a year. I want an easy, fit and forget system. That is the primary reason for going ATB.

The last tenths of a second on a hillclimb is secondary to this (and my driving ability is probably still the limiting factor!)

Malc.
Malcolm

All good
Cheers
William Revit

I often wondered why the diff ratios were 3.9, 3.7 and 3.5 until I looked at the different CWP in each.
3.9 = 39/10
3.7 = 41/11
3.5 = 39/11

so is a 3.5 just a 3.9 crown wheel mated to a 3.7 pinion? Is it possible to make a 3.5 from a 3.9 and 3.7 diff?

Just wondering.

Rob



MG Moneypit

Interesting.

But sounds an expensive way to do it!

And the crownwheel and pinions are matched aren't they?
Malcolm

I think not, because the profile of the teeth are different.
David Smith

This thread was discussed between 28/09/2020 and 30/09/2020

MG Midget and Sprite Technical index

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