Hey guys and gals.

I want to do an oil change on the real axle of my 73 Custom Cruiser with a 455 and THM400 but I'm really struggling to work out which axle it is and which oil and gasket to order. Being based in England and having to order parts from across to pond, delivery takes a while and can be very costly (if even possible) to return parts if I get them wrong.
I've looked around the forums and it seems that there should be and ID number stamped on one of the shafts but it looks like it may have been covered in a protective paint (or more likely just 50 years of oil leaks and road muck).

It has 12 bolts in the cover plate (if that's the correct term) and two horizontal ribs on the side of the body but other than that I'm struggling to find anything that'll help identify it.

Also it has a small metal tab attached by one of the bolt holes that says "Use Lim Slip Diff Lube Only". I read something somewhere about a factory option for some kind of LSD upgrade although I think it it had a funny name. I jacked both wheels off the ground and span them, the results were puzzling. I span the passenger side forward and the driver side span forward a little and then stopped. Then span the passenger side backwards and the drivers side moved forwards a tiny bit then stopped. Tried again, nothing.
Span driver side forward = passenger side forward a bit
Driver side backwards = nothing
Driver side forwards again = nothing
Passenger side forwards = driver side forwards a bit.
I know basically bugger all about axles, especially old American ones, but that's not how the LSD in my more modern Japanese sports car works (it locks almost immediately in either direction). Is this one functioning correctly? My assumption is that maybe it'll only lock up when they really start spinning.
If I do have an LSD, what oil needs to go in it?

Any help would be appreciated and if you want any more info I'll do my best to provide.

 

Your car has the 9.375" Type O axle that was used with anti-spin on these cars. The correct gasket is P/N 393212. FelPro RDS 55009, Mahle P32993, and Victor Reinz 711487600 are all available replacements for the OEM gasket.





The correct gear oil for the differential is 75W-90 or 80W-90 with limited slip friction modifier added. Red Line is one supplier that should be available in the UK. Some of the synthetic differential oils already have this modifier in them and don't need the additive. Once you drain the old oil and reinstall the cover, remove the fill plug in the side of the differential case and fill with the friction modifier and new oil until the level reaches the bottom of the fill plug hole. That's all there is to it.

 

I'd like to make a general suggestion regarding old axle housings. Make an attempt to remove the fill plug before you remove the rear cover.

Sometimes the fill plugs are really tight. If you can't get the plug out, you can't fill it with fluid while the housing in installed in the chassis.

 

Fair point. I've found that an impact wrench gets them out reliably. Just be sure to clean out the driver recess and fully seat the wrench before using it.

 

Thank you so much guys, very useful information and some good tips there!

 

I believe the standard gear set was 2.93 with the 3.23's optional.

 

That is correct.