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MG MGB Technical - Pinion lash

I was under the car today, looking for the source of the clunk I get when engaging the clutch. One u-joint does have some play, and the splines in the drive shaft have a little as well, so I'll be saving to replace the whole mess.

I'm a little worried over the amount of play in the differential. If you measure the edge of the pinion flange against the axle housing, it'll move about 3/8" back and forth before the wheels turn.

I replaced the copper and fiber washers in the differential a couple of years ago. Is it possible these have disintegrated again in such a short period, or should I assume the play is in the pinion shaft? Haynes has nothing to say about fixing this, the archives didn't have much. Is it truly a job for a professional? If not, what non-professional measuring tools are required to get the lash correct?

As always, thanks to all in advance.

Matt K.
Matt Kulka

Setting the back lash involves shimming the diff carrier which is easy enough - you just need a dial gauge to measure the backlash. Getting the shims is a different matter!

But it is a waste of time unless the pinion bearings are in good shape - play here means fitting new bearings and setting the pinion height. This really is a job for the professional and an exchange reconditioned axle is the way to go.
Chris Betson

There are a number of clearances in a diff assembly, and they all can add up to give quite a lot of free play.

Pinion to crown wheel
pinion pin to pinion gear
pinion gear to differential gear
differential gear to half shaft
half shaft to hub etc.

Crown wheel and pinion backlash as Chris says is difficult to adjust without proper equiptment and practice. Any excessive backlash will usually be accompanied by a gear whine, either on run or over run. Cause is usually related to bearing failure or wear.

Cheers

Ian f



Ian Fraser

Chris and Ian, thank you both for your replies.

I do have some noise on overrun. I had assumed it was my un-rebuilt transmission. As you pointed out, Ian, it makes much more sense that it's the rear axle crying out. It would be nice to think it's only that the differential needs shimming, as Chris notes.

How do you determine the condition of your pinion bearing?

And in case I have to go that route, does anyone know of a source for rebuilt axles in North or South Carolina?

Thanks again,

Matt K.
Matt Kulka

There are two bearings either end of the pinion shaft, separated by a crushable spacer.

You need to undo the flange nut and withdraw the bearings to inspect them - unless the wear / rust is so bad as to allow you to feel the wear without dismantling.

Chris Betson


I meant to ask earlier - is it a banjo diff, or a tube type diff?

Ian f
Ian Fraser

Matt, You may want to call Von Peterson at Vons Austin-Healey Restorations, at 803-548-4590. I don't know if he does axle assemblies in house, if not he may be able to supply you with a lead. He is in Ft. Mill, SC. Another possible source is Bob Thompson in Statesville, he sometimes posts here. His shop phone is 704-873-9548. Email; wOmgb@conninc.com. I have not had repairs done at either place, but Von turns out first class AH restorations and Bob has been working on MG's for over 25 years. Regards, Clifton
Clifton Gordon

Ian, this is a '74 roadster with the tube axle.

Clifton, thanks for the leads. I must be getting old not to have thought of Bob Thompson. I read his posts here all the time.

Chris, "...undo the flange nut." So after checking the bearings, you replace the crushable spacer. But rathen than torquing to some spec, there's a method of setting a preload - using a spring scale on the flange or something like that? My Haynes manual makes no mention of it, so that's where I'm in the dark. Is the procedure documented anywhere - Web Page, Bentley manual...?

Matt Kulka

Matt

Replacing the collapsible spacer is a complicated procedure requiring some special tools. But where you need only to replace the oil seal for example, you can refit the original spacer without those tools. The secret is to mark the flange, the nut and the pinion shaft before dis-assembly with a centre punch, very carefully.
Then count the turns as you remove the nut exactly.
Do 'whatever' and then replace to the exact number of turns to reposition the punch marks in line. This will reset the pinion as it was before. So if you have whine before, you will probably have whine after.
I've done a couple this way for the oil seal. The first one was tight. The second (a V8) surprisingly was finger tight, so that's how it went back - with locktite on the nylock. It was fine.

Dave
Dave Wellings

Hi Matt

Whats your email address - I can send pages from worshop manual if you need them

Ian F
Ian Fraser

The main thing to remember here is : if the diff. isn't whining , singing or howling , it's fine leave it alone . manufacturers tend to allow much more freeplay than I was tought acceptable , but the diffs. seems to last indefinately when kept full of lean fresh oil .

Incorrect mesh pattern is what causes early failure and that will make noise , always .

-Nate
Nate

Ian,

Forgive the political incorrectness, but please send the pages to Flyingmidget@earthlink.net. We had the Earthlink people on the phone and had to think of an email name on the spot. My petite 15-year old daughter - whose friends have nicknamed her "midget" - came up with it. Having the inadvertant MG connection, it worked for me.

Nate,

There's a clunk between overrun and underrun. And when I decelerate from highway speed, there is a bit of a howl. I'm sure this is more than the factory let slide. As I said before, I thought it was the transmission singing a death knell, but having found that much play at the pinion, I've changed my opinion.

Given the howl and amount of play, I know I may have damaged the crown and pinion, and may be due for a reconditioned axle. But I really enjoy resurrecting assemblies on this car, so I'd like to have a go at the pinion lash if I can.


Continued thanks to all,

Matt K.
Matt Kulka

Certainly any howling is cause for _immediate_ remedy . you all may recall the terrible 1970's cars , especially Ford products that had appx. 3/4" of driveshaft backlash in them , when you started it up on a cold morniong and the engine was racing on the choke , it would make a big 'CLANG!' as if you'd hit the ring gear (crownwheel) with a ballpien hammer .
"normal" said the factory rep. 'bulls*t' said I .

None of the LBC's I work on or in my club have the dreaded ' clunk ' so I haven't yet heard it .

With proper mesh , a damaged ring & pinion will sing a bit but still work just fine for hundreds of thousands of miles .

-Nate
Nate

Matt,
I have a w/w rear axle from a '71 B I parted out a few years ago stashed away. As I recall it appeared to go in good shape. If you want one to play with first I'll let it go cheap to cleat up some space! I don't know where in NC you are, I'm in Charlotte. Let me know.
Mark
mark

This thread was discussed between 04/11/2002 and 07/11/2002

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