'40 roadmaster differential access

[Drakeule]

I noticed 2 bolts on the forward part of the differential, and a plug on the rear stamped plate. I assume that you remove the lower bolt to drain, and refill from the top one. So what is that plug on the rear plate for?

 

Also, you are supposed to use a 90 wt. GL-4; could you use 90 wt. lower half outboard oil? It's GL-4. Or is the best lube to use 85W-90 (GL-4.5) ?

 

Input appreciated. Thanks,

CD

[EmTee]

This is what I used in my '38 Century:

[kgreen]

Are these the forward bolts that you mention?

 There are actually three of them.  You don't need to play with them if your only looking to change the oil.

[Drakeule]

Ahhh!!! There's another one on the other side! I didn't bother to look. So, use the lowest bolt to drain and the one on the back plate to fill?

Leave those two "Frankenstein" bolts in the above picture alone....

[Bloo]

It may not have a drain. The lowest bolt on the rear pan can act as a drain. It will drain very slowly. The plug on the pan is the fill plug.

 

The bolts in @kgreen's picture should not be disturbed. They hold the pinion bearing in place.

 

EDIT: you posted while I was typing. Yes.

 

Edited June 1, 2022 by Bloo (see edit history)

[Jim Nelson]

As Green shows you,  those two (three) bolts forward are bolts you do not want to loosen  up.   Look in your service manual and you will see that those three bolts hold the pinion bearing / shaft in place.   That is not a maintenance item,  only when you tear down to rebuild it.  The bolt in the back plate  that is about 2" up from the bottom,  is the 'fill port'.    To drain the rear you first get a new gasket and then you remove the 11 or so bolts around that stamped steel cover.    Its a bit messy so have a container that will hold over a gallon.    Yes you don't put a gallon back when refilling,  its so you don't have a 'full' container and spill it while doing it.   I had 40 series Buicks so you might have a 'bottom drain' bolt.   My old brain just knows what I need so I don't think about it.   If you have a 'new' Buick' and you have not drained the rear to check condition of the oil,  its something you need to do - maybe every 20,000 miles. :-))     I live in the South and I use 85w140  standard rear end oil.   GL 4 is great.   I use GL 1 in both my rear and tranny.    You find it at (I did) Tractor Supply in 2 gallon containers.  Its 'Ford Tractor'  90W gear lube.    I have over drive in my Buick and it requires GL-1.     Its better than Buick had up and thru the 50's.    Yes the modern lube's are very good but I need the GL-1   so I use it in my O.D.  and my tranny and the modern GL 4 in my rear end. JMHO.