That picture Stephen put up isn't too bad. I used this link to a monitor cleaner that cleans the inside surface well.

http://ezfolk.com/screenclean.swf

Thanks Ron, that cleared it right up!

Stephen,

That might just be the worst picture I've ever seen.

I know but I was just trying to show the space between the plug and the link. I'd have to pull the car out of the garage and put it on 'stands to get a better pic.

If you can get the wrench on the bolt, try tapping the wrench with a hammer. Not to hard but the force will loosen the bolt so that you can turn it the rest of the way out.

Stephen your diff filler plug is perhaps a gearbox filler plug, which isn't a bad thing because I fit them to my diffs also to get better access. Most old datsun diffs I've seen use 2 plugs that require the 1/2" drive ratchet, that was fine when they designed the 510 but a pain in the arse with zeds.

No, actually that's the plug that was in it when I bought it from the P.O., who bought it new, and he's the type that wouldn't even know what to put in a differential much less where it goes. My 810, which doesn't have that link going across the differential, has two square recess plugs. But, I do believe that the setup on the Z is original.

checked the 10 diffs I've got in the shed and only the r200s have the square head all the r180s are recessed. I only know the 2 240z with 3.9 R180s I've owned had the problem of a recessed plug, but who knows with the passage of time

I changed the diff fluid in my 240 this morning, for the first time since I've owned the car. I got the rear end up in the air, and found two 'square head' plugs that a 17mm open-end handled (but didn't fit perfectly).

Like John said, it's always a good idea to get the fill plug out before you try the drain plug....luckily, both of mine came right out, no problem.

Was it common for owners to swap in plugs with the square end? I expected to find the recessed type plug and was very happily surprised when I found the square end plugs.

Well, like I said on the first page, 21/32" is the actual size but 17mm is close. I do believe that the R200 came with the trannsmission type plug the the fill hole and the square recess for the drain. The square recess is actually easier to work with as long as you have access.