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MG Midget and Sprite Technical - Differential Backlash Adjustment

1966 MKII midget. I am bothered by what feels like excessive backlash in the drive train. The U-joints are new and tight and I have installed a rebuilt 5-speed gearbox. I have never looked at the diff and thought this might be a good time to do so.

I removed the carrier assembly from the axle housing. I measured the backlash on the crown gear to be .012". The workshop manual tells me, "The correct figure for the backlash... is etched on the face of the crown wheel concerned and must be adhered to strictly."

The backlash is indicated to be .008". The "must be adhered to strictly" is the part that worries me. Will .004" make that much difference in the backlash that it can be felt in the drive train?

I think the bearings need to be removed to install the shims. If so, there are special tools that I don't have. If that is the case, can the bearings be removed and installed with tools commonly found at auto parts suppliers?

What I am trying to decide is should I put the diff back into spec or should I leave well enough alone and reassemble it as is?

Thanks!

Paul Noeth

Hi Paul,

If the diff isn't noisy, I would leave the backlash between CW & pinion alone. If it's not broke don't fix it! They have been happily working and wearing together for however many years and 1000s of miles. And besides, it's a PITA job! ;-)

But one thing to check and can have a big effect is the thrust washers behind the sun and planet gears in the differential. These can cause some right slop and clunking, especially noticeable coming onto and off the power.

Cheers,
Malc.
Malcolm

I've rebuilt two diffs and I don't recall needing special tools for the crownwheel bearings. One tip - the manual says to put shims inside the bearing, but it's easier to put them outside.
Les Rose

Thank you for the responses. Malcolm, the sound that bothers me is the clunking when on and off the gas at low speeds. The trust washers behind the spider gears may contribute to this? Thank you for that information. I can feel the play in the spider gears, but I cannot measure it. So I may be looking at replacing the thrust washers as well as eliminating the backlash on the crown/pinion gears to get rid of the clunking?

If I understand things, once the pinion gear is in place and the crown gear is installed, any variation from the backlash distance etched into the crown gear (.008) is corrected by adjusting or replacing the shims on both sides with those of the proper thickness, thus moving the crown wheel a slight amount left or right until the proper backlash is achieved?

Your advice to put it back together the way it is if the diff is not making noise is wise, but I would like to eliminate the clunk if I can.

Les, your advice to put the shims on the outside makes a lot of sense. I have yet to remove the bearing caps. I will pull those off, marking the position of each cap, and see where to go from there.
Paul Noeth

Paul, you shouldn't need to mark the caps, as one of them is normally stamped, as is its mate on the housing.

I think that wear in the thrust washers can induce quite a lot of play. That's where I would be looking first, rather than disturbing the crownwheel.


Dave O'Neill 2

Backlash between crownwheel and pinion does typically increase a little over time. The important thing is the cwp tooth contact pattern rather than absolute backlash. You are likely to find that reducing backlash to original will make the diff whine a bit.

Besides, I have yet to discover a drive train clonk arising from cwp backlash, it has always been excessive play in the gears in the cage. Check for worn thrusts, spider/sun gears, the shaft or even the cage itself.
Oversize thrusts are available as a cheap and effective cure for the noise, even though the resulting side/spider contact is not perfect.
Paul Walbran

Does the car have wire wheels--are the splines in good nick.
William Revit

Dave, you are right, the caps are marked. The crown wheel side cap is marked with a stamped 1. The other is not marked. I would not have noticed it if you hadn't called it to my attention.

Paul, the CWP wear pattern looks very good from what I can see on the gears. Is there a special paint that is used when checking the contact pattern?

William, the splines are good on the wire wheels and hubs.

I can feel a lot of play in the spider gear assembly. I am not sure I can measure that but by feel the play is greater than in the crown/pinion backlash.

I need to inspect the pinion gear. Now that the gear assembly is out of the carrier, the preload of 11-13 inch/pounds seems to be way off. And the gear does not rotate smoothly. It has some variation in turning resistance as the gear is rotated. I need to remove the universal flange, disassemble, and see what is going on.

Thanks all for the advice. I shall order the thrust washers, and other parts needed. I am looking forward to getting the car back on the road.




Paul Noeth

I have dismantled 2 diffs where the bearing inner races were both rotating on the diff carrier.
Bearings were good so removed carefully using either special puller or a pair of OE spanners of appropriate size as otherwise bearing is highly likely to separate and shedding balls everywhere.
Heavy wear was found on all shims which were therefore removed and disposed.
Bearings were chemically locked directly onto carrier in their original positions with extenal shims being used to set up backlash to figure marked on diff typically 0.008" and checked using suitable clock gauge.
S G KEIL

SG you have answered my question before I even asked it. All of the pullers I have access to grab only the outside race of the two differential bearings. I thought I might be watching ball bearings rolling all over the floor should I attempt to remove them in this manner. You confirmed that. Thanks.

There are two small indentations in the differential cage 180 degrees apart that that appear to allow puller jaws behind the inner race. The hook or foot on the end of the jaw will need to be at least 3/4" long and about 1/4" wide to fit into the indentations.

I am off looking for a loaner or a cheap bearing puller that I can use or modify to pull these bearing from the inner race.

Thanks.
Paul Noeth

I think it was on one of the previous diff related discussions or possibly another Spridget link, use was shown of using a couple of OE wrenches to pry the inner track from the carrier.
Attached photo shows comparison between originally fitted full shims against worn shims associated with a rotating inner bearing track on the diff carrier.


S G KEIL

SG, Paul, ---

The idea of using open ended spanners was a suggestion by Willy, when discussing my diffs, when I wondered how to get those bearings off.

See my thread --- title, "Diff rebuild 2023. Chapter 2." It's in the 2023 technical archive.

I found I needed to use a part of my clamshell bearing puller to create a platform to lever on. Then when the bearing had lifted sufficiently, I could get screwdrivers into the recesses and lever on the inner races to lift them the rest of the way off. They weren't that tight, which is also as Willy said.

Don't use shims on the outside because they are too thin and distort. Using this lever method makes it very easy to use inner shims. I actually made my own from shim sheet. I just cut them using scissors.








anamnesis

Snip snip.





anamnesis

2 originals and a home made. Doesn't matter that it looks rough.

It flattens out under the inner race, and the external eneveness doesn't matter either, as it isn't under the bearing anyway.

Yes it's fine. The diff is in my Sprite and doing well.


anamnesis

Actually, remembering back, the spanners will work on the long side of the Diff, without needing the clam puller as a base.






anamnesis

Thus.

I used a couple of 13/16 spanners, but anything close will do, as long as they don't apply pressure to the outer race. I needed a bit of extra leverage and used rings.





anamnesis

Also, you could make a puller to do the job by altering a clam puller. Just weld on the feet.

Thus.

This was done by the chap at ESM Morris Minor workshop in Ticehurst.

Very good it is too, but since spanners work, I never made one.





anamnesis

Here's Willy's post.
Posted 29 March 2023 at 02:12:59 UK time

William Revit, Tasmania

"An.
these side bearings aren't usually very tight on these carriers--You could try, on the short side, poke the end of the jaws of your largest pair of open ended spanners in under the bearing and lever down on the outer ends, they'll probably move quite easily, you only have to get it up a little bit, just enough to get your new wizz/bang puller in under there (flat side facing the bearing) If that works ok ,for the long end use your puller as a base , just have it sitting on the housing, done up enough to get in a place to lever off with your open enders then use the puller again--actually on that longer side if you've got a decent flat ended punch you can access the cutouts ok and just give the inner part of the bearing a tap or two and it'll probably move enough to get your puller in there

with your 2thou shim you're going to struggle to get that on the outside on it's own it'll crumple up while you're trying to slide the carrier in, really if you're shimming on the outside you need the lot out there so it's a bit more rigid to work with, just adding 2thou is probably best on the inside of the bearing as per normal --".

anamnesis

As Willy said, a thin outer shim crumples when you try to assemble.


anamnesis

Photo shows the diff shims that I made using a lathe for both input shaft (for use in conjunction with a solid spacer) and outside the outer bearing race on the carrier.
Mounted on board provides ready identification and access when working.
A small chamfer, made using an engineers hand scraper, enables ready shim fitment to the bearing housing/cap.
I also made a tool to enable lateral displacement of the carrier to ensure optimum seating of the bearings whilst setting up backlash.


S G KEIL

Yes you have to be careful if you put the shims on the outside. But only one of them is very thin, the others are much thicker so I just put the thick ones outermost. I also cut my own. It took a few goes to install them but I had plenty of material. Putting them inside caused too much swearing.
Les Rose

The problem appears to relate to the possible frequent removal and refitting of bearings from the carrier resulting in a loss of interfrence fit where tenths of a thou make a vast difference. Many of our vehicles are now over or approaching 60 years of age.
Whilst it would appear feasible to fit the internal shims and use chemical lock for the bearings it could prove problematic should any subsequent adjustments become necessary. However with external shims the need for bearing removal should only be necessary in the event of actual bearing issues and wwhere heat could probably be used to break chemical bond if necessary.
S G KEIL

There is no problem removing the bearings, even frequently. They are not *that* tight to begin with. Most diff builders fit the shims internally. Most diffs are not rebuilt over and over again, unless your racing. And most do pretty high mileage before needing a rebuild. The actual age of the diff isn't that relevant. So they're 60, so what? I didn't use or need to use any chemical lock, and took mine off several times to get it right. And that was after the pro diff builder had also had them off.

If you have the kit to spread, fine, an external shim is easy to fit. Most diy people won't have such kit or the means to make one, and using a couple of open spanners to remove the bearings proved to be very quick and easy, as I discovered. So internal shimming is quick and easy.

If the inner race of bearings have spun, that's likely because they were knackered and perhaps even seized at some point, not because the diff bearings have been off before. But most bearings just get worn and slack and the inner races don't rotate. New bearings and internally shimmed as per book solves the problem.

I used to think diffs and rebuilding them, was a dark art beyond the understanding of mere mortals like me. But having been forced into diying it, I discovered that it's not complicated at all.

Nothing is when you understand it. And understanding diffs, and rebuilding them, as long as you have explanations such as from Paul and Willy here, proves how relatively simple it really is.

Use external shims if that's a preference. But equally there is no reason not to use internal shims. Having tried to use an external shim for fine adjustment, I found it much easier to follow the book and internally shim, than to use external shims.


anamnesis

Thank you everyone for your feedback and suggestions.

The diff is completely unassembled except for the inside bearing and spacer behind the pinion gear. All of the other bearings and shims will be replaced.

The carrier fit so tight in the housing that I agree there is no way to put a shim between the carrier and bearing. I am not saying that has not worked for others. I will put the shims under the bearings.

I did use the 11/16" open end wrenches to remove the bearings. Long and funny stories about my bearing removal but now I have questions about the thrust washers for the large differential gears. These are the washers that are about 2" in diameter. The thrust washers I took out were brass alloy of some sort (Metal). Moss part number 265-250.

The parts I received are the correct size but they are made from some type of composite material. See photo.

When asked, the supplier responded, "This thrust washer is made of fiber material. Once available in steel, these fiber-type thrust washers are now the only type available. The fiber material provides durability and helps reduce friction between moving parts."

What is your thoughts on this? Is it true that you cannot buy a metal thrust washer or is this kind better than the metal ones.




Paul Noeth

Paul,

Personally if your bronze thrust washers are in good condition I would reuse them. I've taken apart a number of diffs and have steel and bronze thrust washers and all in good order, the couple I took apart which had tufnol thrust washers in them all that was left was the trace of the washer in the undercut on the sun gears all the rest had worn away or disintegrated as they wasn't any other sign of them.
David Billington

I do not want to use the composite thrust washers. The trust washers removed are .003" thinner than the replacements.

I thought about buying shim stock and cutting a .003 brass or steel shim behind the old thrust washers. But these are moving parts, it would not be like putting a shim under a bearing where the race does not move. I am not too keen on the idea.

I am going to try to find a set in bronze. I cannot believe that someone does not have these. Any suggestions?

Thanks.
Paul Noeth

ESM Minor Spares have them

https://www.morrisminorspares.com/rear-suspension-axle-propshaft-c87/rear-axle-propshaft-rear-suspension-c88/washer-thrust-differential-wheel-ata7039-phosphor-bronze-p1238961

Moss Europe list them in various thicknesses, although they don't specify what material they are.


Dave O'Neill 2

This has been a fascinating and informative thread. Useful should I ever want to work on a diff. But so different to the 'fix' I did some 20 years ago to my Sprite diff. It had developed a lot of 'lash' at the wheel rim; the dreaded clunky clunky on power take up and overrun. Largely because of all the apparently complex technical expertise needed to do a proper rebuild of the diff, I copped out of a proper rebuild. All I did was replace the cross shaft and the two planer gear 'olive' washers.

I guess you couldn't say it was a proper repair, but it pretty well eliminated the lash as measured at the wheel rim, getting it down to less than 1/2" when it had been more like 2"! And the fix has lasted; it's still good after another 40,000 miles or so.
GuyW

The story continues. I have run into a litany of challenges, mostly around removing and installing bearings. And problems that I created because I am an idiot, like installing the pinion oil seal and then seeing the crush washer still sitting on the bench.

Currently the spider gears are installed with new trust washers and pinion axle. Everything feels tight as opposed to previously clanking back and forth. They move freely and I cannot feel any play in the gears.

So I am assembling the crown gear to the differential cage. I am tightening the bolts in a star pattern and increasing the torque by 10 ft-lbs as I go. The target value is 60 ft-lbs. I get to 50 ft-lbs and the torque wrench feels a bit odd. I set it to 55 ft-lbs and it does not click as expected. I am not nuts about inexpensive clickers, I always question their accuracy/repeatability. So I get out my beam-style torque wrench. It is old but reliable.

I watch the scale come up to almost 55 ft-lbs and the bolt keeps turning but the torque is not increasing. I turned it about 90 degrees and stopped. This seemed wrong. I removed the bolt and it was stretching. See pic. These are new bolts purchased from our most famous 4-letter supplier. All six bolts showed some sign of stretch.

I did not want to use the original 58-year old bolts but I reinstalled them anyway. All came up to 60 ft-lbs without anything seeming abnormal. The big question in my mind is should I use the old bolts? Have they been stretched beyond their point of elasticity during the re-install? I don't know.

My fear is that sometime in the future I will be tooling along at 70-MPH (5-speed trans) and one of the bolt heads will break off. The differential will be toast and the 20 bucks that I did not spend on new bolts will be a hundredth of the repair cost.

The bolts are M10 x 1.0 x 20cm. I went to the ARP website and they don't make that size. In looking for new bolts I don't know what specifications, type of steel or grade to look for. I only know that the new bolts I have are no good.

Should I re-use the original bolts.

Thanks.




Paul Noeth

I think you'll be fine with the originals as I can't recall seeing anything about using new although it would be nice if you could get quality new screws. Likely the ones you got were made in asia, I was supplied some 3/16" BSF recently made there with the wrong pitch. The thread is 3/8" UNF not M10 x 1 but I can see them being mistaken as the metric is quite close. The new 3.9 I have on the shelf has W grade fasteners, see image.


David Billington

As David said, the old screws will be fine.
Dave O'Neill 2

Paul
#### should be ashamed of themselves supplying them bolts.--and metric as well--shame.
Could you put up a pic of the head of the bolt showing it's id markings please so that others can be aware of the crappy bolts being supplied.
If your old bolts look ok and tensioned up nicely they should be good---the originals are decent bolts.

willy
William Revit

My mistake on the metric. I have a M10 x 1.0 die and the bolt screwed into it. BSF - I am making a guess that stands for British Standard Fine. That never occurred to me.

They are Atlas bolts. See pic. Not sure where they were manufactured. They were supplied from their US operation. I have noticed that they have some different components on the other side of the pond. I don't know why but I have had to order from the UK when the item was not available from here.

These bolts also have a W marked on them, but they are not the same.

Paul




Paul Noeth

Paul,

The bolts are UNF, my BSF comment was about another fastener that was made incorrectly and BSF is British Standard Fine as you say.
David Billington

I also ordered other parts that were not up to snuff. See pics.

The new lock-tabs were rusty when I received them. They folded up when I tightened the new bolts. I used the old lock tabs when I put in the old bolts.

One of the differential bearings arrived missing a ball. These were made in South Korea and were still sealed when I received them. I am waiting for a replacement bearing before I can move on.

I have noticed that no one mentions our favorite 4-letter supplier by name. Is there a prohibition on the forum outlawing the practice? Or is it just to keep from any legal entanglement with ####?

Thanks again for your feedback.

Paul







Paul Noeth

UNF, BSF, Metric, Grades. I need to learn more about fasteners.

Paul
Paul Noeth

You mean MOSS? If Moss supplied the bolts, I hope you will take it up with them.

I reused my old bolts. No problems.

I also reused some of my old locktabs, and made a couple of my own rather than buy what I suspected would be crap lock tabs; You've confirmed that was the right thing to do.
anamnesis

I sometimes re-use locktabs, and anneal them so they won't crack when bent again.
Les Rose

This thread was discussed between 23/02/2024 and 01/04/2024

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.