Diff question- How bad is too bad?

[Gary Schulz]

As I finish the reassembly of my gearbox (I finally got good baulk rings so that is perfect now); I decided to take apart the differential to have a closer look at it. Of course one thing leads to another and now I see what appears to be a lot of wear and other issues going on...
The first thing I noticed when taking it apart was the bronze bushing that is pressed into the crown wheel stayed with the output shaft. It is loose inside the bore of the crown wheel so it was just floating around between the output shaft and crown wheel. Is this just a case where I need to press in new bushings? The surfaces of both pieces look decent.

20181028_075009_resized.jpg

Next up was a closer look at the pinions/shaft. I notice that the pinions are not even remotely close to a precise fit on the shaft and seem to just float with huge clearances. I also can see some galling and scuffing marks on the shaft as well as a pretty nasty inside surface on the pinions. Never saw a pinion shaft with the cross-hatch grooves but I guess that was either knurled to improve the fit or was done for improved lubrication maybe? Any thoughts on how close the pinions should fit to the shaft?

20181028_075056_resized.jpg

Lastly I inspected the cage and can see wear at the point where the pinion thrust washers mate against the inside of the cage. Is this normal and tolerable wear?

20181028_074855_resized.jpg

Maybe I should have just left it together. Every time I have a close look at something it seems to cost me a lot of money!

Thanks

Gary

[Peter Laidler]

I'm just repeating my not inconsiderable MGB and GT and V8 diff experience here Gary (that used to plague the megga torquey V8 diff until it ......anyway it went bang big time). It looks to me like the steel embedded in the copper thrusts (photo2) looks like the case hardening that's broken away from the hardened teeth of the pinion/planetwheels. Once it's there, it digs in and acts like a bloody lathe tool chomping its way around and creates havoc. Once the breakdown of the hardening starts, it's a slippery slope and I think that's what happened between the planet wheel and the centre pinion.

I may get a severe arse kicking here but this was the MGB and V8 answer to part of the problem. If the scoring on the diff cage and underside of the planet wheels was what you'd call acceptable (very subjective I know) then use new copper thrusts. If it's slightly worn (easy to measure) it is simplicity itself to press 2x slightly thicker copper planet thrusts given that you've got a) both sides of the press tool, b) the pattern and c) the material spec of the original to work from.

As for the worn diff pinion and planet bearing surfaces. Anyone.......... Just a later thought but I have a funny feeling in my bones that the pinion pin is a standard imperial hardened ground stock size

I appreciate that you can just go and buy new - at a price

At the time we investigated this big problem for the MGCC and MGOC we had full engineering and testing facilities. Same meat here - but different gravy so to speak

[Spider]

You can fit a bush back in to the crown wheel, but as yours was floating, you will need to be sure that the bore in the CW hasn't worn. After pressing in, it will need to be sized, I usually do them with a Reamer, they are an odd size, the shaft being 1.050", but if you use an adjustable reamer, then size it off that and give it no more than 0.001" clearance.

Your Pin and Planet Gears are toast. Getting good pins these days is an issue. I don't know of any from the UK that I'd suggest, there maybe some, however, we do have a local supplier that does have proper hard pins, they need to be upwards of 61 Rc to get any life from them. You may need to go through a few gears to find good ones, as these also seem a bit all over the shop for sizing.

The Diff Housing you can re-dress, but if you are only doing the one, then it may not be worth your time to make the tooling. I'd load up a picture, but where I am right now the internet speed is so slow, you'll have the gearbox finished before it loads,,,,,

After dressing, there is later type thrust available (the type that have the 'ears' that stop them spinning) that are thicker. Or, just get a late type Diff housing that already takes these trusts.

[BRI MK1]

Just fit a new cross pin diff there not that expensive

[LuisM]

on this topic, has anyone bronze bushed the planet gears ?
would this be a reliable option for reusing gears where the pin bore has wear ?

Cheers
luis

[Peter Laidler]

Yes, planet gears can be bushed, certainly on the MGB ones but if the hardening of the teeth has already broken down and it's breaking away, don't bother. I think that the pin is a standard size.

I hope Gary can come back with more photos and comments

[Spider]

on this topic, has anyone bronze bushed the planet gears ?
would this be a reliable option for reusing gears where the pin bore has wear ?

Cheers
luis

I looked at this long ago and I do see there are some available these days.

I didn't go through with it as when you look at the 'thin end' of the planet gears, there's very very little material between the root of the teeth and the bore for the pin. These gears in stock form split quite easily from there, so boring them to fit a bush, would only make this worse.

[dklawson]

I have yet to install it but... years ago I bought a diff rebuild kit (MiniSpares C-BTA167) that included the bushed planet gears and a hardened pin. The new pin replaced the hatch/knurl with small flats. The kit also came with new thrusts. The thrusts were for the later type diff that has keying slots to prevent the thrusts from rotating.

I am no differential expert. However, you could add the slots for the later type thrusts and use your existing differential housing. As stated above, you will need new planet gears and pin. The MS kit is 55 GBP so it's not a bad way to get the parts you need to rebuild your diff.

[Spider]

The Diff Housing you can re-dress, but if you are only doing the one, then it may not be worth your time to make the tooling. I'd load up a picture, but where I am right now the internet speed is so slow, you'll have the gearbox finished before it loads,,,,,

Photo,,,,

Just fit a new cross pin diff there not that expensive

The Planet Gears in the common types are largely unsupported and where they are supported is at 90 degrees to where they are loaded, so they break just as easy, but for different reasons

[LuisM]

I didn't go through with it as when you look at the 'thin end' of the planet gears, there's very very little material between the root of the teeth and the bore for the pin. These gears in stock form split quite easily from there, so boring them to fit a bush, would only make this worse.

humm , makes sense.. thanks Chris!

[Gary Schulz]

Thanks for all the inputs and observations. I agree that one could possibly bush the planet gears but I also completely agree that there is very little material between the root of the teeth and the bore. I can see that failing pretty readily especially starting at the "small end".

If I go the route of the replacement pin and gears then I need to machine the cage to take those new thrust washer tabs? Not sure how to machine that accurately. Looks like it might be tricky to fixture it. Then if the bore in the crown wheel is screwed up and the new bush still falls out I will be back at ground zero. Anybody know the dimension of the crown wheel bore so I can check it?

Gary

[dklawson]

While I agree that the thin section created by adding the bushings will make the planet gears vulnerable, you have to ask why MiniSpares would continue to offer bushed gears if they experienced high failure rates. I guess after my next gearbox work I will become a Guinea Pig.

[Peter Laidler]

I can see exactly where you're coming from Doug. We seem to forget that tooth root strength, especially in straight cut gears that we have in the planets is linear - along its whole length - with a large contact area dependent on and throughout that whole length. If that makes sense! And to be fair, that is a LOT of strength. But you've got to weigh up whether the aggro of boring and bushing is greater than the availability of the parts. Simple if you've got the know-how, practical and theoretical knowledge - and the kit - as Spider, Gary and Doug have.

The cut-outs for the new style thrusts isn't a real problem Gary. It doesn't need pin-point accuracy after all......, it's just a one-hit milling cut to a depth set by the knib on the thrust

Personally speaking, I would bore and bush - and make a new pinion shaft to suit

PS Gary..... You seem to be expressing doubts about your mech ability. But looking at what you've already done here and elsewhere, I personally don't think that you've anything to worry about!

[dklawson]

PS Gary..... You seem to be expressing doubts about your mech ability. But looking at what you've already done here and elsewhere, I personally don't think that you've anything to worry about!

+1
I fully agree.

[LuisM]

ressurecting this topic

any feedback on Diff Pins quality for a standard diff ?

http://minispares.com/search/classic/C-BTA166.aspx

would this suit a standard 998 gearbox + engine ?

Chris,
you mention you can get quality ones there in AUS, any idea on current cost ? anyway it might get expensive with postage and customs fees.

Thanks
Luis

[Spider]

Hi Luis,

I had getting Diff Pins and Layshafts from a Local Manufacturer / Retailer here, MG Heritage, however, he's pretty much closed up shop now
He's on ebay selling off the last odds and sods. I don't know if he has any left ?

The Molly coated Pin from Mini Spares might be OK. I've not tried one.

Certainly, the bigger the output of the engine, the more wear it's going to put on these parts, however, it doesn't nearly increase as much as one might expect due to that. The cars all weigh around the same and they all go around probably similar numbers of corners, all putting wear in to these parts.

Part of the problem here too is one of lubrication. If you have a look at the Pin in Gary's photo one page back, the Galling there has largely come about as a result of lack of oil. The main body of these diffs is quite thick and when you look at them, the windows in them aren't all that big. As it spins, it appears to me, that at a bit of speed, oil can't flow in to the center fast enough and is being centrifuged out. When Stationary, the Oil Level come up about 1/2 way on these, but when running, it drops a fair way, due to the Oil that's up in the engine. I can't see any of this helping here.

On a few of these Diffs (when I used to bother with them), I tried some with channels and drillings in them so that as it spun, the oil would (hopefully) get scooped up and in to the center. I did pull a couple of these down after some use, and while not a cure, they were better.

[LuisM]

Hello Chris,

thanks! all pins i had removed from gearboxes look similar to Gary's one... there is probably a lubrication issue as you point.

I have bought from MG Heritage in the past ( quality layshafts and ACL small bore head gaskets ) , those layshafts were good! I had them tested here and their hardness was similar to a NOS one . I'll check them on ebay

Thanks
Luis