rear end fluid fill question
 

ok i have a rear end that i want to swap out with my stock one. it came off a newer mustang and has a better gear ratio. my question is, if i drain it where the hell do i fill it back up at? i believe it has a drain plug on the bottom of it so i don't think i need to remove the cover. either way that isn't what i need to know is this fitting on top the fill plug? it has a kind of spring loaded cap on it that i cant figure out how it comes off. is that just a pressure overflow? do i need to unscrew the base of it and top it off till it starts comming out? and if it does have a drain plug on the bottom is it necessary to remove the cover for any reason?

I have no clue what that plug thing is, but I don't mess with it or anything. My little brother and I just did my Rear End Fluid, if yours is the same as mine. The plug is on top behind the cover, it would be on the right hand side of the drive line side.

Thats your axle vent tube, you do not fill it from there, don't mess with it. Some open track guys have an issue with it in high lateral G situations with fluid coming out and getting on the exhaust and such, so they run a flexible tube up wards to help keep it from coming out.

The picture that bikrsk8rkid posted is what you need. You drain the fluid by taking the differential cover off. Use RTV sealant to seal it back up. Use a 3/8th socket extention to unscrew the fill plug which is located on the top driver's side of the differential facing the front of the car.

hmm. i need to look at it again. the fill plug that is in the pic looks like what i thought was the drain plug. I was thinking it was on the bottom half instead of the top. i was probably looking at it upside down. is it a good idea to go ahead and just change it before i put the rear on? thinking about going ahead and painting it black since it is off already. should i drain it and paint it, then fill it when it is on so it will be slightly lighter or is it really not that much weight difference? anything else need to be lubed or greased when i take the cover off to drain it?

If you want to paint it, do it now before it's on the car. Drain it, don't fill it back up until it's on the car. The reason I say that is becuase you may need to remove an axle and you have to open up the diff to do that, waste of fluid. Weight difference is insignificant. Only about two quarts of fluid.

what would cause me to have to remove and axle?

IDK, if you decide you need to replace an axle seal, take off the brake to axle backing plates. Chances are you won't need to.

oh, i don't think i will be fooling with anything like that. it dosn't have any fluid staining around anything except that little axle vent tube. i was looking at the drain plug and in the picture above it is in the upper half of the housing. this one is in the lower half the way it looks. is the fluid only suppose to be half full in there? filling to the hole will leave the upper portion of the cherry or whatever you want to call it empty. i started painting it today but ran out of paint so should finish it up tomorrow

It shouldn't be at the bottom, there is aftermark Diff's that will have drain plugs at the bottom. But the fill plug should be at the top to fill with oil. You fill it until you feel it through the hole and/or starts coming out the fill hole.

Not exactly sure, but take a some pictures?

it is a stock 99-04 rear end from a v6 here is how it will be sitting on the car unless it sits different from the 94-98. this is the only plug that i can open

it just seems kind of low to me to be a fill point. seems it should be higher up

Yes, the fluid only fills up to around half.

Yeah that's it, but doesn't look like mine at all. Maybe it's the years, but also could be considering yours and that one is a V6 and mine is the GT? Who knows, but that's the fill plug.

maybe, 8in and 7.5 may be slightly different

It's very possible.
I don't even know what I have. haha