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100 differential - swapping

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PSJ

PSJ

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Hi all! I'm helping my dad switch from a 4.11 to a 3.9 rear end in his '53 100. He also is swapping to to a different pair of axles, as a shop once told him his current axle's splines are worn (We'll see when we pull them out).

Questions: anything to watch out for? It looks fairly simple - he has the entire pumpkin, which he's going to take to a differential shop to get cleaned and inspected prior to installation. Hopefully, it's a temporary install until the next run of Lempert 3.54 gearsets.
Also - the used axles he acquired are slightly different from eachother. One has a geometric (hexagonal?) shaped flange (at the brake drum end), the other is circular. I'll do a more detailed examination when I can, but the difference makes me suspicious. He got the parts essentially free,, from a friendly club member.

Thanks in advance for your help!

Phil

PS: Just realized that, for all you know, I could be twelve years old! My Dad is almost 80, and I am 49.
 
At some point in time, the outer shape of the flanges on rear axles went from being round to having flats. This is not a problem. The axles are identical other than that particular feature.
As for the splines being worn - not a chance!!!
However, after saying all of this. Is the rear end in question from a BN1 or a BN2. All of the above only pertains to a BN2, which is the same rear end housing and axles up to and including the BJ8 rear end (with different gear sets of course).
I know nothing about the BN1 rear end as this was a different unit altogether.
 
BN4 has round axles, the Austin/MOWOG sedan diff I got for the 3.54 head (same axles as Healey but wider) has the pentagon shaped flanges and is dated 1967.

BN1 diff up to the end of 1954 is not the same as later ones according to the Restoration Guide. It says the propshaft and brakes are different too so unless the car has already been changed over to a hypoid type diff you may find a few incompatibilities.

If your shop is feeling a lot of backlash in the rear axle it is more likely to be a worn cross-pin/spider gears than worn axle splines. If the axle splines do actually show some wear once you get them out then your diff housing is probably bent.

While you are in there, change the hub seals and check the wheel bearings too unless they've been done recently. There are some other threads in the archive about diff overhaul and special tools required.
 
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