Follow up question - MGB diff repair

[Ian F]

Another question - for anyone who has done this job - renewing the differential thrust washers.

I've just replaced the 4 thrust washers on the differential wheels and the differential pinions (sun and planet wheels in the diff if you prefer that terminology). All went very smoothly thanks to the several articles available via this forum.
My question relates to the relative position of the inner ends of the axle shafts and how far they should protrude through the differential wheels. I didn't notice how things looked before dismantling, but now after reassembly I can see the left hand axle shaft (adjacent to the crown wheel) pokes through quite a bit and almost touches the pinion pin, whereas the right hand axle shaft end is just about flush with the face of the differential wheel. See attached photo.

So --is this difference important and indicating that I've made a mistake somewhere, or should we expect the axle shafts to come through to different degrees?

I did replace the differential wheel thrust washers correctly - both fibre washers being new and circa 36 thou in thickness, so that is not the cause of this difference.

Any advice welcome please,

thanks,

IanF.

[Peter Cresswell]

Hi Ian,
I've been pondering on this for a few days! From your picture it looks as though there is enough clearance between the end of the halfshaft and the diff pin not to cause a problem but if they are touching then the halfshaft will quickly cut some clearance for itself!
Looking at the exploded diagrams that appear in all the manuals, it seems to me that the location of the halfshaft and the distance from the flange on the outer end of the axle casing to the centre pin in the diff are related. I assume you took the halfshafts out of the axle casing, so things to consider in this relationship are:
1. Did you replace the outer bearings whilst doing the work?
2. Are you sure the spacer between the bearing and the outer bearing cap has been replaced?
3. I don't think the halfshafts are handed but did they go back into the axle on the same side as they came out?

This all clutching at straws a bit so in the next day or so I'll take the rear cover off the spare axle I have and see what that looks like. I don't think it has ever been rebuilt, so should be much as it came from the factory but it does have the problem you have just cured on your axle. I doubt I will do it tomorrow as it looks like rain here all day and the axle lives outside!

[Ian F]

Thanks for the input Peter. I have also been having a bit of a ponder about this - as you might expect! There are a few more facts which I probably should have included in the original posting, but initially I was just interested in what other folk might have noticed when doing this job on their MGBs.
I really should have taken careful note of this spacing (and a photo) before I dismantled the diff, but I didn't and am now left to ponder......

I think I may have come up with the conclusion after a bit of thought - as follows:-
1. I didn't remove the left hand half shaft (the one which is close to the pinion pin) whilst doing this job, so it should be in exactly the same position as previously.
2. I didn't alter the location of the diff carrier (of course) and it is only this which determines the position of the pinion pin (?).
3. So unless the diff carrier has somehow moved by itself (when tapping in the pinion pin or replacing the right hand half shaft?) then the relative positions of the end of the left hand half shaft and the pinion pin must be exactly as before, and have always been just as close to each other?

Do you think this analysis is correct?

The clearance I have between the end of the left hand half shaft and the pinion pin is 32 thou (or 0.8mm), the end of the right hand half shaft sits flush with the inner face of the sun wheel (it is in place in the photo, but can't easily be seen).
My son (a mechanical engineer in the oil industry) has done a quick calculation of the expansion of a half shaft given a 50 degree celsius rise in temperature (which I think is about as hot as it might get in operation - circa 60/70 degrees celsius?), and he concluded that the expansion would be only around 0.32mm. This being the case I'm tempted just to put the diff cover back on and drive it down the road!!

I can see that when originally setting up the crown wheel and pinion, the diff carrier will be adjusted laterally to get the required meshing, it just seems to me that in this case the position of the diff carrier is a bit on the extreme outer limit of what you might expect?
Would be really good to see what anyone else has found when looking into the diff on their Bs!

Any further thoughts most welcome,
thanks again,
IanF.

[Peter Cresswell]

Hi ian
I managed to take the cover off my spare axle this afternoon and take some pictures in the glorious sunshine!
[attachment=0]diff.JPG[/attachment]
I believe the axle is the original to the car and has not been overhauled - yet!
Hopefully you will see that the halfshafts are equally spaced from the Diff pin. There is 90thou clearance between the pin and the end of the halfshaft on both sides.
There is a spacer between the inner bearing that are available in different sizes to place the centre of the diff (the centre of the diff pin to the centreline of the pinion) and the gap needs to be measured if the crownwheel assembly is removed to replace the inner bearings. I think the make of the bearings is important and it is essential that the same make as the old ones are fitted as replacements, as this ensures that they are exactly the same dimensions. Otherwise the diff carrier has to be realigned and new spacers fitted of the correct size.
I have found this documented in the MGC manual I have, but not in any MGB manual. If you want a copy of the relevant pages then please PM me with your email address and I copy and send the to you.

[Ian F]

Thanks for the help Peter.

I have used your measurements of axle shaft inner ends to pinion pin (90+90=180 thou) to compare with my axle. One side of mine (LHS) was 32 thou, so I reckoned that anything around 150 thou for the RHS would be acceptable. I've just measured 160 thou, so am reasonably content!

The diff carrier/crown wheel definitely do not move laterally when levered with a screwdriver so I am content that they have not moved, and are as set up initially in the factory (I've owned the B since 1978, it left the factory in 1972).
I can only assume that the initial set up of the crown wheel in relation to the drive pinion has resulted in this asymmetry?

There is just a small amount of free play between crown wheel and drive pinion, and also just a small amount of free play between the sun and planet wheels in the differential (much better than before I replaced the thrust washers) so I am content with the set up so far.

Everything is back together apart from the diff cover. I've started the B and run the axle in first gear and all seems smooth and quiet. By holding each rear wheel in turn it's possible to observe the differential wheels in action. Again all smooth and quiet.

So, diff over will be back on soon, new oil, and off down the road. Unless I have a major failure I expect this to be the last word on this subject!

Thanks again,

IanF

[Peter Cresswell]

Hi Ian,
That all sound like a good plan!
Having taken off the rear cover for the picture (above) I now know what is wrong with my spare axle - the sun and planet gears have a considerable amount of play! So I'll rebuild it now I know what's wrong, rather than the plan to strip off the brakes, take the halfshafts out and weigh in the rest.
The axle became noisy with a lot of vibration chasing a couple of Cooper S' around the Derbyshire lanes on an HRCR tour. As time was tight to get the car running again, I bought a low mileage axle and installed that. Only problem was it came off a GT and the brake back plates are different as the GT has different sized wheel cylinders. The back plate now accepts both GT and Roadster wheel cylinders.
The axle is related to (but not the same) as the Altas axle fitted to rally Escorts, and is considered unbreakable. I've now managed to break two in my time - one on an MGC GT I had in the 1970s and this one on my Roadster!