EDIT: that's not the fill plug replaced, it's a vent plug, doing its thing. Nothing to see here except a newb learning as he goes... :) And now very much hoping the fill plug is not stuck, because oil is already out...

I'm trying to change the differential fluid on my 1970 car. A PO seems to have removed the fill plug and replaced it with a plastic thing with a pop off top. (thanks for using up another 1.5 inches right there, makes getting a hose in there pretty fun). I tried removing the entire plug: it twists, but I don't know it's screwed in. It's not actually screwing outwards. Is this a common type of plug?

The main problem is this: when I fill, it pretty much immediately overflows from that tube. I'm not 100% sure that NO oil is going in, but it can't be much. I've tried flowing it very slowly. I don't have tube tight in it any more either, just holding it over it, so air bubbles can get out.

This thing isn't a valve of some kind is it? Anyone have experience with these? Can I safely remove it without plastic bits falling into the differential? do I even want to replace this?

Edited September 7Sep 7 by Warty

Can you not locate the fill plug? It's a little above the drain plug at the red arrow.

You should be able to get an open end wrench on it. I'm not sure if it's 17mm or 19mm.

There is the chance someone installed the later fill plug. Those might take a 1/2 square drive. The problem there is the cross member could be in the way of accessing it. You should be able to fit a short extension from a ratcheting wrench into the square plug to remove it.

Thanks Steve those are the photos I should have looked at before starting. I looked in the FSM but didn’t see the actual plugs.

Honestly it was so stupid. When I was under there the cross member hid the fill plug from me and my eyes went right to the vent pipe. The really dumb thing though is I took pictures before I touched anything and if I had just looked at the pictures I could have seen the fill plug. It’s just angles and the cross member. Anyway no harm no foul I guess it’s done. Harder than it needed to be but on my next Z….

Edited September 7Sep 7 by Warty
Fix typo

Those vent caps are still available at Nissan. If you destroyed yours you might want to order another replacement. They are a press fit.

On 9/7/2026 at 1:22 PM, Yarb said:

Those vent caps are still available at Nissan. If you destroyed yours you might want to order another replacement. They are a press fit.

Luckily I didn’t try to hard to get it out so it seems to be fine. Cool that Nissan still has them. The way I found that was a vent tube was by scanning images on google and came across a similar looking one for a Chevy. Light bulb finally went off at that point.

If you had filled the diff to the vent hole instead of the fill hole you would have had quite a mess on your hands.

Well I did try. What might I have done there damage wise? If you just meant gear oil would go everywhere outside the diff then that did indeed happen.

Mostly just oil, blown out seals. I don't think anything mechanical would get damaged. I have never heard of anyone filling the diff up and driving it. No first hand reports.

I think it would take something 5 hours to fill from there. If it even could be done. But I get what you mean now… you would eventually fill past the max level where the actual fill plug is.

I had a customer 1 time that changed his oil on his Mazda this was at a dealership that he was employed as a ' car wash boy' for used car department and he used the overhead hose to fill the oil. He learned the hard way that you don't fill up the oil like you do your gas. He started his car and bam liquid locked the engine and bent a rod enough to rod was hitting block on the stroke.. one learned lesson the some people shouldn't play!