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MG Midget and Sprite Technical - breather assembly

The diff is leaking a bit at the flange to the propshaft, how do I remove the breather assembly to see if it is stil working after 53 years? (frogeye 1959)

Flip
Flip Brühl 948 frog 59

the plastic cap pulls off from the tube, and the tube unscrews from the axle casing.
David Smith

There is a helpful little hole in the tube through which you can insert a pin to help get a grip on it while unscrewing.
Paul Walbran

Flip. I think it's more likely that the pinion seal is leaking. After 53 years it will have gone hard. I did mine about a month ago (Mk 3) and it's easy enough. Just remember when you have installed the new seal to tighten the nut to exactly the correct torque, otherwise you may alter the pinion preload and the axle will whine.

Bernie.
b higginson

oops, watch out, Regarding replacing the pinion seal: if you re-torque a used crush spacer, then you will have increased the crush of the bearing (as the torque won't be reached until the crush spacer's been crushed some more than before).

The best way to do it is to remove the differential, disassemble it on the bench and use a new crush spacer (this is not that hard to do).

The next best way to do it is to do it in place, but then confirm your bearing pre-load result with a 12 in-lb torque wrench (hint, this is a very small, special torque wrench, pretty much only used for setting up differential rebuilds) so you don't go too far with the pinion nut. By the way, when the proper pre-load torque is reached, the difference between "too little" and "too much" is about 5 or 10 degrees of nut rotation with a new crush spacer, a very small margin. Less than one flat. With a used crush spacer it will be almost nothing.

The next best way, or the least trustworthy way, but at least better than nothing, is to mark the nut face, and the mating yoke position before your loosen the nut, and then put it back together and turn the nut back the exact same number of turns to that same spot when you are done. You might get it "back to where it was", if you do this. The odds are that it will be too tight or too loose.

By the way, if you over torque the pinion nut, everything will seem fine for a few thousand miles, but as the pinion bearings are crushed by the over torque, you'll gradually get more and more wine from the diff as those bearings break up (BTDT).


Norm
Norm Kerr

I would never seek to question Norm's knowledge or skill, as I, and I'm sure others have gleaned plenty of great tips and advice from him. I consider him to be one of the more knowledgeable contributors on this BBS.
However, when I replaced my pinion seal I did it as per a 1960s BMC factory workshop manual which I assume to be for the use of the then BMC franchised garage mechanics to use when carrying out repairs.
In the rear axle section it goes through all the procedures involved in setting the preload, setting the backlash etc., but in the part concerned with replacing the pinion seal, it only mentions to make sure that the pinion doesn't rotate during the operation and to take care to re-torque to 140lb/ft. Nothing else. I also have a Haynes manual and the text is similar to the BMC one with a bit more information for the home mechanic about how to crack the pinion nut and how to stop the pinion rotating without the use of special tools.

Your most 'umble servant.

Bernie.

b higginson

I cleaned and replaced the breather assembly (photo). It was clogged a litte bit. I did rebuild the 3.9 diff 3 years ago, setting the backlash, shimming, new spacers bearings etc. I have replaced the oil seal polished the universal joint flange and torqued as in the workshop manual. It whined.....I have replaced the seal on 2 other frogg 4.22 diffs as well. I used to mark the nut (and using the bevel pinion flange wrench made by a friend). It did not whine.I guess that the 3.9 diff that I did rebuild was whining before I bought it: the bearings where gone.

Flip

Flip Brühl 948 frog 59

Hi guys,

The early diffs were built with a solid spacer and shims, instead of the crush spacer. In that case, the torque to 140 lb-ft is straightforward.

But, somewhere along the way (not clear when the change was made) they went to a crush spacer for cost savings.

Studying up about these things (regardless of manufacturer, Ford, GM, or whatever - crush spacers are used almost universally today), and reading guides published by folks who specialize in racing and building souped up muscle cars (people who routinely swap diff gearing around), the warning against trying to re-use a crush spacer was always included in the literature. That, combined with my own experience with rapidly ruined pinion bearings (about 10k miles) after re-torquing a used crush spacer to 140, got my attention.

Then, when rebuilding mine with new bearings, using the 12 in-lb torque wrench to monitor preload I was really surprised at just how small the margin was, even with a new crush spacer.

Probably life would be easier if we swapped it out for a solid spacer and shims, but that takes much more time to set up properly (Brown and Gammons list those parts in their catalog, but most vendors only sell the crush spacer). I went through 3 crush spacers on my build, messing around with this, to learn more about it. Since the part is cheap, and with these things in mind, I am now advocating to replace it, rather than risk it.


The risk is that more expensive parts, ones otherwise fine before starting, can be quickly worn out if reusing one of those crush spacers doesn't pay off for you.

Replacing the pinion bearings can quickly exceed $100 in parts. The crush spacer is barely $20.

Oh! and I just thought of another good reason to do this on the workbench: while the diff is apart to replace the pinion seal (and crush spacer), one can also replace the planet gear thrust washers, which are relatively cheap, easy to replace and help to restore "good as new" performance to a tired differential. In the MGB these thrust washers can be replaced in situ, but our diff must be removed first to get at them.

See? Its a "win-win"! ":o)
Norm
Norm Kerr

This thread was discussed between 03/05/2012 and 06/05/2012

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