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Have I Shimmed This Diff Wrong?


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#1 dsgoody

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Posted 23 April 2018 - 09:01 PM

I have two new end plates that I've fitted to my diff to cure a terrible oil leak. Followed what I though was the correct procedure; fitted and fully torqued the drivers side and pushed the diff as far toward that plate as possible. Then shimmed up the passenger side to give me an average gap of 13 thou (9 thou for the gaskets and 4 for the preload). Fitted and fully torqued the cover.

 

However; after fitting one pot joint and driveshaft back in it's making an awful noise if spun. Is this normal? https://photos.app.g...wvo0J1PSAOH4oE3 <<Video

 

 

Somebody told me I should have loosened the diff cover; can I do this with it in the car and what about the RTV?



#2 hhhh

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Posted 24 April 2018 - 03:24 AM

The diff cover should be just nipped up when you do the shimming because the assembly has to be able to move slightly to get it positioned correctly: hard against the flywheel side cover. The best way to do this is purposely set it up too far in that direction and then torque up that cover against, which pushes into place. 4 thou is a bit heavy preload isn't it? I thought 1 or 2 was the target. Gasket should be about 7 thou compressed, so your preload could be 6.

 

I think you can get away with doing it in the car, but it won't be fun reaching the top diff cover bolts. I think your gasket and RTV should survive loosening.


Edited by hhhh, 24 April 2018 - 03:35 AM.


#3 johnR

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Posted 24 April 2018 - 08:56 AM

As I found when doing the same job recently the pre-load depends on the type of bearing used - I think I'm right in saying a lot of the ones currently available are 1-2 thou



#4 Cooperman

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Posted 24 April 2018 - 09:41 AM

Before 'nipping-up' the diff cover nuts the diff should be pushed as far as it will go to the right. Then tighten the nuts and measure the gap on the other side and fit shims as appropriate.

 

Now this is just a personal opinion, but I have always thought how massive and over-engineered the ball bearings are in the diff housing. They are huge compared with what more modern cars use for this. In my mind I am sure that if no shims were fitted and the housing just tightened up the diff could not move and there would be no major problem. In fact I have never heard of those bearings failing in use. I do always do the correct shimming, but if you don't get it absolutely right it shouldn't wreck the diff.



#5 hhhh

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Posted 24 April 2018 - 01:24 PM

I think the shimming is more about preventing torque steer than overloading the bearings. I agree these bearings are huge and probably hard to destroy.



#6 dsgoody

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Posted 24 April 2018 - 01:26 PM

Thanks for the help.

 

So long story short; I need to take the passenger side cover off and loosen the main cover bolts.

 

Providing I just remove some torque and not actually loosen them completely off the RTV should be ok?



#7 Spider

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Posted 24 April 2018 - 07:42 PM

As others have said, the bearings will be nipped in by the rear cap.

 

If doing (as you are) and in-car swap of the covers, I'd suggest shimming up each side separately. Any pre-load that's needed will be there already, so don't add any more. I find shimming in car like this easier and faster to do by trial and error than by measuring. You'll only what sufficient to ensure the side covers fit up flat with a gasket and no more.

 


 

I think the shimming is more about preventing torque steer than overloading the bearings. I agree these bearings are huge and probably hard to destroy.

 

Shimming the diff has zero to do with Torque Steer. The diff is mounted between the two diff bearings and the shims fit up against the mounting race of the bearings, therefore an shim added to either side will have an equal effect to both bearings. In any case, the output shafts (that connect to the wheels via drive shafts etc) are not directly connected to the diff hemisphere or it's bearings, by via a planet / output gear arrangement that are totally independent of the hemisphere's bearings, so what ever is being done to the diff bearings has no effect what so ever on the output shafts.

 

The need to pre-load these bearings may not appear obvious. It's twofold;-

 

     i)  To ensure that the bearings are loaded all the way around the races of them and not spot loaded.

 

     ii)  To remove any movement ('shake') from the bearings and hold the helix angle between the Pinion and the Crownwheel in correct alignment, otherwise the diff would tend to skew on power and the alternate way on over-run. This will result in these gears meshing only on  the very ends (tips) of their teeth and not across the whole tooth face, resulting in very short life of these gears.

 

I agree that in isolation, these bearings are large, however, it appears they are this size, not because a big bearing is needed for any load they have, but to ensure enough surface area in the gearbox case and diff cap, the alloys that these are made from are soft and it wouldn't take much to have a smaller bearing bury itself in the alloy of the casing. This is noticeable on the diffs that were fitted up with the thin bearings that have around 2/3's the contact face.



#8 johnR

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Posted 24 April 2018 - 09:29 PM

Of course this leaves the question what was the awful noise dsgoody heard as it's unlikely to have been due to the bearing pre-load?



#9 hhhh

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Posted 25 April 2018 - 12:03 AM

I got the torque steer idea from a faded memory of reading "How to Modify Your Mini". What it actually says is "Take care with this shimming because if done haphazardly, it can cause the car to pull to one side or the other."  After reading Moke's post and thinking about it, I can't see how it possibly could.



#10 Spider

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Posted 25 April 2018 - 12:12 AM

I got the torque steer idea from a faded memory of reading "How to Modify Your Mini". What it actually says is "Take care with this shimming because if done haphazardly, it can cause the car to pull to one side or the other."  After reading Moke's post and thinking about it, I can't see how it possibly could.

 

Your memory is good :proud:



#11 hhhh

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Posted 25 April 2018 - 01:08 AM

It wouldn't be the first mistake in a Vizard book. The yellow bible repeats a mistake from the blue bible of blaming the under-side of a rocker fouling a valve spring on milling the head and block with the remedy being shimming the pillar. This is just plain wrong.






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