Overfilling diff?

[RJK]

Hey folks, I am in the process of changing my gear oil at diff and trans. when I opened the diff fill plug, probably a 1/2 cup or so of fluid poured out. Either the car was tilted to fill, or a fill point was used i'm not aware of. Is there an advantage to this? or should i fill it in the standard manner, fill at fill plug til it drips, and stop?

[SteveJ]

If you got only 8 ounces of fluid from the differential, that was not overfilled. That was underfilled. Underfilling the differential can cause overheating, damaging bearings and gears.

You should use GL-5 gear oil to refill.

You did not say which differential you have, but here are the capacities from the service manual GI section. The 1.3 and 1.0 are in liters (or litres for our UK & Canadian friends).

[RJK]

The 6-8 ounces I referred to was from the FILL plug. Not the DRAIN plug. When I opened the drain plug, in total, somewhere near 1.5-2 quarts is what the diff held. Every time I've changed diff oil, the method of filling I have read in every WSM I've seen was to fill at the fill plug til it starts to drip from it, seal it up. The previous owner of my car must have tilted the car before filling. I was asking if this was standard operation procedure on Z cars for some reason, or if this was an isolated event. Sorry, I didnt check the actual diff casting, but it's a 1972 240z, surely stock diff from my visual inspection.

[siteunseen]

I filled my '72 until it dripped from the fill hole. They possibly filled it into the vent hole if they couldn't get the fill hole plug loose and out. I've read that before.

Edited May 6, 2023 by siteunseen

[Zed Head]

Oil expands quite a bit with temperature.  Could be a factor.

If the PO raised the back end that would also allow a little more in.

 

[emccallum]

To answer your question....No, I have never heard of anyone advising over filling the rear diff. 😁