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differential case - hex nut size??



was going to change my rear differential lube, but don't want to get started until i know what size the hex nut is on both the fill and drain plugs. i was going to use mobil 1 synthetic lube, but was also wondering if it needs the limited slip additive or not. any help for the lube change would be appreciated.


Well-known member
Jun 17, 2002
North Hollywood, Ca.
Miss my '62 & '80 4- Sp. Vettes
dif slip additive a must

That hex you are talking about is simply your 3/8" socket wrench tip. Use an extended socket shaft and it will make it easier to work with. I experimented with the white plumber's tape (teflon) and wrapped the hex screw threads for better sealing. The rear end stays cleaner and won't weep gear oil out. Be careful not to over tighten the hex too much!
You have mini-clutch packs inside the Locking Differential Unit. This takes special oils. There is no advantage to move away from the recommended oils provided by the manufacture. In fact, the factory oils are specially formulated for the Saginaw, Borg-Waner and Dana rear ends. Why take the chance?
The problem using the wrong oil or no additive at all to the LDU clutch plates, will gum up and not release the clutches when turning a corner. This causes clutch chatter and accelerates tire wear.
I strongly recommend using 80w-90 GL-5 (GM Part No. 1052271) and adding the limited-slip diff. additive (GM Part No. 1052358). Oil capacity for your rear end is approx 4.5 pints, +/- 0.2 pints.


Site Administrator
Staff member
Sep 16, 2000
New Hampshire
1990 Corvette ZR-1
Actually that's not entirely correct. The fluid part numbers have changed several times over the last couple of years. Consult your dealer first because they will have the most up-to-date part numbers.

Also, I highly suggest consulting your service manual regarding the proper fluid to be used for this application.

I believe that the socket needed is a 10mm socket, not a 3/8.

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