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MG Midget and Sprite Technical - Differential fun and games

Coming back from the MASC club night last week my midget started making some horrible grinding noises from the transmission.
I nursed the car home and the noises had disappeared after a few minutes. I suspected the gearbox.

On inspection of the car I noticed lots of oil around the fuel tank. The differential had a small pokey out hole in the rear of the casing......

Rear axle off, and the rear differential out. I found two pieces of 1 cm long bits of metal in the bottom of the diff casing.

An inspection of the pinion, crown, sun, and planet gears showed they were intact with nothing missing. The rest of the diff looked ok, and works smoothly. The bearings are all in one piece. Theres no sign of any damage apart from the small hole in the casing.

The two pieces of metal match the splines of the half shafts, but my half shaft splines are un damaged!

So unless I have missed something which would have to be very obvious given the sizes of the pieces, they must have been the remains of a previous half shaft that got broken and the pieces must have been left in there. Im having trouble understanding how they could have been missed. If something breaks in a piece of moving machinery you would check to make sure all the remains come out???

The noise happened on a straight bit of road doing about 50 mph in fourth, so nothing unusual. But somehow these pieces got disturbed and got caught between the crown gear and casing.

Pic shows comparison of of one of the pieces and the end of one of my half shafts



C MADGE

When I had a diff failure when qualifying at Assen, I made sure I got all the bits out of the axle casing with the help of a strong magnet.

Your halfshaft would appear to not be the later, stronger version, as it doesn’t have the groove in the end. It could be from an earlier Spridget, or even an A35.

You should be able to see the part number on the outer end of the shaft. The later version is BTA806. Earlier Spridget is BTA501 and A35 2A****

I would post a photo of the groove, but I’m not on my PC at the moment.
Dave O'Neill 2

Here’s a pic of the ends of my half shafts, one has the groove, one doesn’t.

So is one the original, and the other the stronger version?


C MADGE

The end of the grooved one is indeed BTA806
There isn’t a part number on the end of the other
C MADGE

Chris,
the stronger BTA806 is what the 1500 had, it'd really pay you to get Terry Horler's book as a very, very early Xmas present before you too much more work on your car.

The unmarked one is probably an earlier one but even the BTA806 might not be original to your actual car, both half-shafts might had been installed by a PO.
Nigel Atkins

The one without the groove or part number could be a modern replacement, but I don’t know what material they use, so can’t comment on strength or longevity.
Dave O'Neill 2

Early cars had half shafts graded EN7 strength, with later cars having the uprated EN17. The original ones fitted to my 1970 car had EN17 stamped on the ends, as well as other codes. However my new ones purchased from MGOC were not stamped at all, but I was assured they were EN17 strength, which is logical as I doubt any manufacturer would bother making weaker ones! Peter May's super duper half shafts are graded EN24 I believe, but are of course considerably more expensive. If you have in fact got 2 "odd" half shafts it may be wise to get a couple of matching ones. If buying secondhand, make sure you know which is offside and nearside, as it would not be good from a stress point of view to reverse the way they have been used to turning.
Geoff Mears

I'm impressed that you took the rear axle out, Chris. All those things to undo. Wish I still had that 'get on and do it' attitude.
Nick and Cherry Scoop

Thanks for the replies.

I had the axle off during last winter when I repaired the inner wheel arches. The U bolts, nuts, bushes, shackles etc were all replaced with new. So taking it off again was nice and easy with nothing seized :) Only took a couple of hours.

I'm waiting for a reconditioned diff with a new 3.7 crown wheel and pinion. The original 3.9 was very worn and dull. Lots of backlash too. You could actually see the contact patch from the wear. I suspect it was the original to the car with 98K miles on it.
C MADGE

Interesting point about the wear on the diff. Would 98k-miles (assuming that's a correct mileage) cause a lot of wear or is it more likely to be a combination of mileage, use, abuse and lack of general maintenance/regular servicing? (and of course I'd say lack of oil changes :))

I don't know the answer but others will.

Be interesting to know when you're able to drive it again if you think the change to 3.7 is enough to put off ideas of a 5th gear/overdrive.
Nigel Atkins

"(and of course I'd say lack of oil changes :)) "

Possibly also the use of incorrect oil.
Dave O'Neill 2

Yes or possibly wrong type of sawdust or stockings additive.
Nigel Atkins

I have broken a half shaft in the past and removed the largest end piece before copious rinsing and feeling about inside the axel.I then fitted the Peter May units and placed two very strong magnets on the outside of the case to collect anything magnetic that may have escaped my notice. No problems since, also fitted with a LSD.....slight plug here as it is for sale on Car and Classic if anyone is interested.
The other midget (not for sale) had 'Morse' uprated half shafts (paper lables still in place) which on inspection were twisted slightly so I also replaced these with Peter May units.Since then I have read that it is OK to use twisted half shafts as they have taken up the strain but I am not so sure.Opinions welcome, think I read it on Blue Oval site.
The magnets were flat 10mm discs which could not be easily separated without sliding one across the other....still in place on the diff!
John
J Sloan

Geoff, I didn't realise the shafts are "sided" or do you mean the side they've been run in? Does it really matter? I've a pair of S/H EN17 shafts ready to go in the Frogeye when I rebuild it but I've no idea which side of the donor car they ran in.
Bill Bretherton

Bill,
The usual advice is to make sure they go back in the same, original side as the splines get distorted or stressed and they are more likely to break if used on the wrong sides.
Maybe ts just a myth though.
GuyW

I don't believe it's a myth Guy, I've read that in so many disconnected places there has to be a grain of truth in it. I always mark the shafts when I remove them so if sold on, the buyer can install them the right way round.


David Smith

Agree Dave-- i like the 'grain of truth'
Axles seem to be able to twist up a little and last ok but if you swap them over ping they go in no time
Bill, if you have a real good close up look at your shaft splines you should be able to see which side of the splines have been doing the work -refit the same way
William Revit

I believe it to be true, but I just wasn't wanting to be dogmatic about something that I hadn't first hand half shaft breakage experience of. I have always managed to identify and get them back fitted on the original sides.
GuyW

Ok thanks all, I'll attempt to identify which side is which.
Bill Bretherton

Bill, if it helps, this is the offside half shaft from my car (I broke the nearside one). I decided not to re-use it.


Geoff Mears

I love a good half shaft breakage thread because it gives me the unabashed opportunity to post pictures of one of the most interesting bits of modification engineering I have done on my car! Don't you just love the satin finish of shot peened steel? :-)

But no one can see my half shafts :-(

Malc.


Malcolm Le Chevalier

Geoff - it looks as if someone has used that shaft both ways. Or maybe you do a huge amount of reversing.
Nick and Cherry Scoop

Malcolm, are those Growlerized?
GuyW

Looks like they are the Growlerized ones:
https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=201606221135141254



Having read up about 1/2 shaft breakages and the unknown condition of my half shafts I'm concerned I'll be breaking one at some point.

The cost of a diff with new crown and pinion is significant, I don't want to damage it by breaking a half shaft. So maybe its worth replacing them.

Magic Midget list up-rated 1/2 shafts at 165 each - has anyone used these ?
C MADGE

Yep, they are indeed :-) It is the only way to remove the inherent design flaw in the cut spline design.

People spend big money on "uprated" shafts that still have the cut spline design. The anecdotal evidence says the uprated material solves the breakage problem as nobody has had one break...

Of course nobody has, they are new parts that only see 1000 miles use a year. Show me the uprated shafts in 50 years time after 100,000+ miles use.

*Climbs off high horse* ;-)

Cheers,
Malcolm
Malcolm Le Chevalier

“Geoff - it looks as if someone has used that shaft both ways.”

My thoughts, too.
Dave O'Neill 2

Chris,
I'd suggest you contact Kim Dear of Magic Midget and telling him of your requirements as Kim is a very straight-up person, which unfortunately seems quite rare in the car business generally.
Nigel Atkins

Nick & Dave. I have to admit that when I did my re-shell back in the mid 90's I completely dismantled the donor car before starting the rebuild. I know for a fact that I didn't mark them up at the time, because I didn't know the facts then! I can't examine the half shaft as it went to the tip with other stuff, so cannot see whether the score marks are on both sides of the spline, but not shown on the picture, which might indicate a different point of view. However, I'm sure you are both right and I must have put them in the wrong way! As Nick knows, I've developed a habit of assembling things the wrong way round (clutch fork)!!!
Geoff Mears

Malcolm,

Uprated half shaft breakages have been mentioned before here on the BB so not impossible. The best option is to change the design to an involute spline as the stress levels in the spline are lower but that involves changing the sun gears at some extra cost.
David Billington

For anyone that's interested, to conclude this thread, I have fitted the reconditioned 3.7 diff with new crown and pinion and done about 50 miles with it so far.

First impressions are how much quieter the car is. The reduction in noise I think is mainly down to a smoother/quieter running diff rather than the 200 rpm reduction. My old diff was so worn with about 15 thou backlash in the crown and pinion and even more in the sun and planet gears. The new diff was nice and tight and 'feels' like its running smoother.

The 200 rpm rev drop is a noticeable but small change.

Sat on the motorway this morning at 3500 rpm and a shade over 60 was a much more pleasant experience.

C MADGE

Hi Guys, Sorry to come in at the "conclusion". But I have a suggestion which might help.

Whenever I have a diff apart, I use a small Dremel grinder on the sun-gear splines. I "bugle" out the ends which normally cut into the axle splines. It is not easy to get the right angle but I try. The aim is to gently taper the square end of the spline so when the axle twists, it "winds up" in the "bugle". In that way there is no stress point created where the usual nick forms. Of course this does not apply on axles like Malcolm's where the diameter reduces. Malcolm's have no stress point.

My application is on Spridget and Morris Minor diffs which I convert to fit TCs. This involves fitting the TC spider gears which have only six splines and so are a bit easier to "bugle".
Cheers, Bob Schapel
R L Schapel

This thread was discussed between 15/10/2018 and 06/11/2018

MG Midget and Sprite Technical index

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