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MG Midget and Sprite Technical - Problem With New Diff

For some reason my late 74 1500 midget had a 4.2 Diff so journeys on Motorways/Dual Carriageways were not much fun with a high revving engine.

I bought a secondhand 3-7 Diff, which may be a 3-5 if I have interpreted the markings correctly as per photo attached.

I understand that when disconnecting the diff from the prop you mark both items to re fit the same way, but with a new diff its not possible?

After fitting the new diff and filling with Halfords 80-90 Diff oil after a few miles and when up to 45 mph the car started to vibrate quite badly and became very noisy.
After getting home presuming the props the cause I have undone the prop to diff and turned the diff round re connected but there is still vibration above 45 mph.

As two holes are approx 5mm apart and the other two 5.5mm approx apart there are only two ways of fitting prop to diff unless I am missing something obvious.

Anyone got any ideas on how to sort as I could return the diff to seller but would prefer to sort and not go through all the hassle of swapping diffs again.

Thanks Tim




Tim Lynam

11-41 is 3.7. Did you check the diff by hand and wash it out before fitting? How did the bearings feel? Did the backlash seem normal?
David Smith

11/41 is a 3.7

I've never bothered marking prop flanges and I've never had a problem. Also, as you say, if the diff is from another car, marking won't hel anyway.

If the vibration wasn't there before your changed the diff, it sounds like there is a problem with the replacement. If you're grab hold of the diff flange, can you move it up and down or side to side?
Dave O'Neill 2

Worth checking for excessive play in the diff gears as those later final drives used phenolic thrust washers for the sun gears and I've seen a few where they had worn away to nothing leaving excessive clearance although I don't know if that would cause a balance issue. The propshaft shouldn't be an issue as they are balanced as a unit and should go on either way fine, worth checking the UJs while out though.
David Billington

Thanks for confirming it is a 3.7 diff.

Its the first time I have swapped diffs so unfortunately did not know what to check for, the old saying you learn from your mistakes.

The U joints were replaced about a year ago, so should be able to count those out, I can check for play at the diff flange and if the prop shaft can be ruled out after fitting both ways, looks like I may have a dodgy diff.

Tim Lynam

Another Vote for dodge the diffy

Prop
1 Paper

Whats the thoughts on the tightening of the diff retaining nuts to the correct torque which is stated as 65 ft lbs in the manual?

The nuts on the old one were pretty easily undone and I would have thought nowhere near 65 ft lbs, is this not far too much, or could the under tightening be the cause of the excessive vibration at over 45mph.
Tim Lynam

Had this last year when I converted my Midget from steels to wires. Severe vibration over 45 mph which you could never drive through.

After swapping wire wheels front to back, rotating prop shaft, replacing UJs, replacing half shafts, it was only when I swapped out the diff that it went away. The diff looked fine but obviously wasn't.

Rob
MG Moneypit

Tim,

The torque figure of 65 lbft is for the Diff/Gear carrier retaining cap nuts.

If you are talking about the diff into the banjo casing i.e fitting the diff into the casing I don't think there is a torque figure quoted - they just need to be tightened normally for the bolt size - 'snugged up' - 65 will very likely snap them - its far to much !

R
richard b

The nuts that Richard refers to (65lbs/ft) are the ones that are visible in your photo.
Dave O'Neill 2

How fast is the vibration? Would you guess the same as prop shaft speed? .... or slower? Does the vibration/noise change much between power and coasting?

If there is a problem with the spider gears (sun & planetary) I would think there would be differences in the vibration/noise when driving straight, curving left and curving right?

If you need replacement spider gears and can't find a good set locally, let me know because I have a box full of sets to select from! I don't think they are available from Moss etc. I am in Australia but I have previously posted a couple of sets to the UK.

Good luck solving the problem.

Bob Schapel
R L Schapel

Thanks for the clarification and comments, I do not want to mess about with the internal workings of the diff, does anyone knows a company in the Midlands that services and repairs diffs?
Tim Lynam

Dave O'Neill or David Smith would probably know one (and more).
Nigel Atkins

I only know Hardy's down south, but googling 'axle rebuild service in midlands' brings up quite a few possibles with a recommendation off Pistonheads for one of them...
David Smith

Me too.
Dave O'Neill 2

I almost put a link to Hardy's and I had a quick search for any in the Midlands but didn't recognise any of the names.

Thanks for replying David and Dave even if you did both let me down. :)

http://www.hardyengineering.co.uk/
Nigel Atkins

I've used Central Axle Services in Birmingham in the last year to rebuild the diff on my '72 Midget and can recommend them.

Cheers,
Dave

http://www.centralaxles.com
Dave Thrussell

Thanks for the suggestions, the B'ham option is not too far from work so a great suggestion and if that fails the Surrey company is only a few quid away by carrier.

Not too chuffed at the thought of removing the diff again but I suppose getting a 3.7 diff serviced and running properly should be well worth the upgrade from a 4.2.
Tim Lynam

Good luck Tim

You might as well pop in a Ford Mk9 five speed gearbox while you are In the modifying mood! (Or buy an MGB GT with an overdrive gearbox for the motorway...)

Cheers
Mike
M Wood

As an aside, how much backlash between crown and pinion is acceptable? I've acquired a 3.9 to put in the 1275 Frog which "looks" ok but has slight backlash.
Bill Bretherton

Backlash in the diff can often be repaired by simply renewing the 'olives' on the cross shaft. Very easy to do whilst it is out of the car.
GuyW

Thanks Guy, I'll look into that.
Bill Bretherton

"Backlash in the diff can often be repaired by simply renewing the 'olives' on the cross shaft."

That will take up the 'slop' between the wheels, but it won't alter the backlash between crown wheel and pinion.
Dave O'Neill 2

True Dave, it depends where the backlash is being found/ measured. If at the wheel rim, then renewing the olives may well be sufficient and its very easy to do. But if it is checked between the crown wheel and the pinion and there is lash, then that is a different matter and it will need setting up properly. I always say do the simple and the low cost things first before diving in with the more complex stuff. You may be lucky, but that is an optimist's point of view, not an engineer's!
GuyW

Ah, but the question was "...how much backlash between crown and pinion is acceptable?"

There should be some. I would have to check the manual to see how much, though.
Dave O'Neill 2

Indeed Dave, but l still felt it was potentially useful ancillary information whilst the diff was out of the car. Worth the simple check involved.
GuyW

Indeed.
Also enables checking of cross pin and retaining pin - I've had one 'fall out'. It can ruin your whole day!
Dave O'Neill 2

From the WSM :
The pinion to crown wheel backlash should be between 0.005" and 0.011" measured with a dial gauge. The actual lash when set up new is stamped on the rim of the crown wheel. Lash is adjusted by moving the shims behind the main bearing caps from one side to the other, so the total number of shims remains the same. Each shim is 0.002".

Run out on the crown wheel should be checked first, also with a magnetic base dial gauge, as tight spots will be caused if it doesn't run true. Max run out at the rim is 0.002".

This can all be done without disturbing the pinion bearing and it's pre-load setting.
GuyW

Bob Schapel in Australia asked if the vibration/noise was much different between power & coasting?

Just been out for a short run and there is no vibration on coasting but as soon as the foot is back on the power the vibration is very fast and unable to carry on at above 45 to 50 mph, foot off the throttle and vibrations goes immediately even at the problem speed of above 45mph. Hopefully this will mean something to you guys that understand the issues with props and diffs, does a propshaft keep in balance with one diff but may the prop require balancing when attached to a different diff.

That may sound a daft question, but just hoping it may be the propshaft requiring balancing rather than removing of the diff?

There was no vibration at all with the old diff, just replaced.

Tim Lynam

Hi Tim,

I have a theory which fits your symptoms. It might not turn out to be correct but here it is ..........

If there is end float in the pinion bearings (possibly caused by worn or damaged bearings) the pinion would push forwards under power load (due to the spiral cut of the hypoid gears). While the pinion is forward, the front bearing is "loose" because it is a taper bearing. That allows the front of the pinion shaft, flange and prop shaft to go off-centre and out of balance.

When coasting, the spiral nature of the gears pulls the pinion shaft back so the front taper bearing, flange and prop-shaft is centred and puts the prop-shaft back into balance.

I would think that the diff would be noisy under power and coasting. If it is noisier under throttle load, the rear bearing is the main culprit. If noisier coasting, the front bearing is the main culprit. Perhaps the cage (holding rollers in place) has disintegrated? If bearings are the cause, you might be able to feel slight RADIAL play in the flange. (Rotational play/backlash is normal.)

A real "long shot" .... Have you checked the pinion nut is tight? You can do this without removing the diff. Technically, 135 ft/lb is the nominal torque but that all depends on whether the pinion spacer is solid with shims or a collapsible type. If it is loose and it is the cause of pinion end float, only tighten it until there is no end float and BARELY DETECTABLE resistance in the rotation of the flange. (Really should be all reset by a specialist.)

Changing the diff should not affect prop-shaft balance. I use various (Spridget/Morris) ratios in my race TC and have no balance issues. I am not a diff "expert" but have worked on nearly 50 Spridget/Morris diffs. Most were conversions to fit them to MGTCs. Hence my carton of spider gears, as they were jettisoned in the conversion process.

Bob Schapel
R L Schapel

Tim, I can recommend CAR Transmissions in Creswell (just north of Mansfield) if you need to get your diff looked at. It's a small, family setup and used to working on older stuff.
John Payne

Nice day out if you take in some sight seeing Tim.
Dave Squire

Thanks John, I had not heard of the company at Creswell and less than half an hour away, a bonus.

Always good to deal with small family run businesses and a picnic at Creswell Craggs Dave, sounds like a good afternoon out, only bad bit is getting that diff out again!
Tim Lynam

Bob, Thanks for your thoughts on the cause of the problem, as the diff appears to be the cause I'm best getting it to an expert who can fix the problem.
Tim Lynam

As several helpful comments were given as to possible reasons of a newly fitted second hand diff that vibrated badly above 45 mph, if it helps anyone in the future there's a happy ending.

After removing it and taking it to Central Axles in B'ham who stripped it and initially could not find the problem, the cause was found to be a badly worn drive flange.

To make matters worse they now seem to be unavailable but after swapping the drive flange from the old diff on to the new one and renewing the bearings all is now good and no vibration and a nice quiet diff.

Tim





Tim Lynam

That's good news.

Coincidentally, I was going to visit a customer recently and spotted Central Axle Services as I drove along Montgomery Street. They seemed to be very busy, with lots of cars on ramps.
Dave O'Neill 2

Tim, worn where exactly? Was it the splines, or had the bolt holes gone oval, or summat else? (so we know what to look out for & check...)
David Smith

They seem to be very busy, but its good to find a company that will take the trouble helping with the awkward jobs which may well be a pain to them but a major problem to a classic owner.

The bolt holes were fine, but the flange was worn where it mates up inside the diff, there appeared excessive play at the drive flange.
Tim Lynam

Interesting
I wonder if someone had fitted an incorrect flange
Some of the Japanese diffs run a very similar flange with the same bolt pattern but very slightly larger studs and a metric spline that is a tiddle larger in diameter as well
Just guessing----but maybe
It was second hand after all
willy
William Revit

I've come to this thread late in the piece but will comment as it is very familiar!

While there was clearly a fault with the flange in this case, Bob Schapel's post above is very relevant to this symptom.
Our supercharged K Midget had exactly that symptom, felt for all the world like a prop about to explode. Three propshaft overhauls later (each overhaul & balance done despite no obvious faults) we were at our wits end.

Someone (Arie I think) on the K series pages mentioned a related problem that turned out to be diff. We pulled the diff out and checked it over, found that the collapsible spacer had completely collapsed, allowing the pinion to slacken off a little. This in turn introduced freeplay to the pinion bearings.
Interstingly enough there had been no CW&P whine at all.
We tossed away the dead spacer, made up a solid one, shimmed up the pre-load got a good tooth contact pattern and refitted. Problem completely gone.

We also had a spacer collapse in our 1440 A series Midget, though not to that extent and not with the same vibration.

Then much more recently I had the same symptoms with my MGB. Early car with banjo diff which has solid spacer so in my mind that ruled out a repeat. So it was check out the propshaft: 3 x rebuilds later even though it did not appear to need it (special stage rallying on rough gravel roads is a bit tough on the drive train so I wouldn't have been surprised) and the problem was still there.

A couple of weeks ago the B broke a halfshaft, so the diff came out and the slippy dismembered to extract the remnants.
And in the course of this work, I found the pinion bearing preload completely gone and tuned into a small amount of end float (later traced to a 10 thou shim compressed to 3 thou). Set everything up again, refitted the diff and halfshafts and road tested ... vibration gone.

Tim, I note you renewed the bearings - if the pinion bearing were renewed then the preload would have been reset so there may have been more than one cause. It would be interesting to know whether the person who did the job found the preload or lack of prior to dismantling.
Paul Walbran

Paul, when I took the diff for repair I mentioned that I thought the bearings had gone due to there being far more play in the new second hand diff at the flange that bolts to the prop, as against the one it was replacing, but I know nothing about diffs!

On stripping the diff I was told that the bearings were fine, but I assume it was decided just to replace them whilst it was all apart which made sense to me and the only part at fault was the drive flange, but as Willy mentioned it may have been the wrong one previously fitted, unfortunately I purchased off Ebay and not from within the Midget owners who are usually pretty reliable, big mistake by me, but you learn:)
Tim Lynam

This thread was discussed between 20/08/2017 and 10/10/2017

MG Midget and Sprite Technical index

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