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MG MGA - Differential opinions?

Hi all,

Haven't been on here in some time, busy raising kids and so forth, but trying to bring my 1500 back from semi-retirement. It's running great due to a miracle, i.e. finally replacing the distributor rotor, after years of every-so-often trying one arcane thing or another to cure a hot-restart problem.

HOWEVER ... it has the dreaded take-up "clunk" in the rear end. Per archived threads here and the shop manual, I did change the side gear thrust washers as so well documented by Barney, Mike Ash, others. Back in the car, and I'm pretty sure it's worse than it was before, though not a big difference. Steel wheels, btw, and everything's tight there. Had the driveshaft out also and U-joints are good.

What I'm wondering is, do the thrust washer and/or shim on the pinion input shaft need replacing? The suppliers offer them, in various thicknesses. What is the effect? Also, when I started taking things apart, that 140lb. nut was NOT on very tight, I almost fell on the floor when I leaned into it expecting resistance. I put it back on with the intended torque, or as close as I could get trying to hold the thing in my bench vise.

Thanks for any illumination,

Dan Bradley

Get Barney to pay a visit on his next pass through your area.
Dominic Clancy

May be quite a while Dominic as Barney is 4500 miles away in Vancouver Island right now :-)


Colyn Firth

Just a few weeks ago he was over 2000 road miles from there in Anchorage, so he might not have to wait so long. Barney does get around.

D Rawlins

Vancouver, Alaska for three weeks, back to Vancouver, what's 8400 miles in 30 days between friends? Was around Vancouver, BC area for 10 days, now in Calgary, AB for 10 days. About to do the 12 hour trek west over the mountains back to BC again, because there are a bunch of clubs we skipped on the way out. By end of month we should be heading east again, ultimately destined to cross Canada en-route to Gander, Newfoundland. When weather begins to deteriorate we would head south. May be in North Carolina area late October.

Meanwhile, .... grab the differential input flange, and see if it wiggles. There should be a slight preload on the input shaft bearings, but commonly not. If the flange can move in any direction other than rotation, the input bearings are loose. You can fix that by removing shims (thinning out the shim stack) behind the front tapered roller bearing. See Workshop Manual.


Thanks, Barney, I'll fiddle with it, may take it out and go through it again. Only saw one shim before, and confess I didn't mike it since that wasn't what I was after. Threw me when I found the nut not very tight, guess that could result from wear internally. I've had the car 20 years, don't know if the diff was ever out before I had it. Then I see the washer under the pinion head referred to as "thrust" (i.e. consumable, right?) in the Moss catalog, various increasing thicknesses available, and thinking, "should I have replaced that while I was in there?"

Of course I don't have any of those "special tools" mentioned in the MG manual, but the Haynes manual talks a little about using a spring scale to check preload, maybe I can manage that. It also lists among possible causes of the clunk: "excessive crownwheel to pinion backlash" (not equipped or competent to check or remediate this), "worn gears" or "worn axle shaft splines" (neither was apparent while I had it out), but also "pinion bearing preload too low" (only found that one shim, so ?, but I'll try) and "loose drive coupling nut" (if this refers to the input shaft nut and not the flange nuts, yes but I tightened it). Was expecting this clunk problem to go away after I changed those other washers, baffled now. I should point out, aside from takeup the car goes down the road great, smooth, no diff noise, no rumble. Oh well, guess I should be happy enough with that!

By the way, although I don't think you and I have met, I believe we have a mutual friend, my neighbor PJ Lenihan.

Dan Bradley

Yeah,I met PJ a few years ago:

Preload on the input bearings is not a lot, 12 lb-in, but you should definitely feel the small amount of drag with your finger tips. If it is loose you can make the shim stack thinner. Last time I did this it had exactly one 0.020" shim, and I didn't have spares handy, so I sanded the shim down to 0.014" to get the specified preload.

So I just tried something obvious, that I'd been meaning to do since I saw it in an archived thread on this same topic, took out the battery cover panel so I could actually watch what was going on. On takeoff the nose of the diff is rotating up/down (depending on forward or reverse) a matter of inches ... is this natural? Don't seem right. And if not, what should prevent it? Never looked at it before.

Dan Bradley

Barney, thanks for the PJ link. By the way, he and some buddies have now built a GT with a 455 Buick nailhead in it. Actually works, he let me drive it around the block. :-)
Dan Bradley

Solved. Sorry for all the drama. Evidently over the years the spring shackle u-bolts had stretched, or the pads crushed, or both. The whole axle/diff housing was rotating under load, hence the clunk when it traveled as far as it could. Tightened all the shackle nuts and it's like new. Just like that rotor I mentioned at the top. Jeez. Never try the simple stuff before spending lots of time and money first. At least now I know what's inside that pumpkin.


Dan Bradley

Dan, I had loose axle U bolts as well. Symptom was a change in car direction as I went on and off the gas. Did you have this as well as the clunk?
Art Pearse

Hi Art, no, it ran straight enough. Maybe mine weren't loose enough to allow the axle housing to travel laterally or anything, just rotate. Watching it with that battery cover off, it was pretty drastic the way the nose of the diff would go up, down, bang, clunk. Tell you what, I never looked at this before or thought about it much, kind of surprises me the friction effect of those bolts is all that keeps this from occurring. Well, I've finally learned my new thing for this year. I also have a '38 Buick, with the torque tube, this gives me some new comprehension of THAT, too.
Dan Bradley

This thread was discussed between 16/08/2017 and 18/08/2017

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