I recently had a 7.25 rear go bad. In a hurry I went to the local salvage yard and got a replacement that had probably been in the yard awhile. When I test drove it I found it hot to the touch after only about 5 miles or so. I drove it a little more and it just kept getting hotter. I thought it might be a bad pinion bearing or something. I then swapped in another 7.25 that I recently acquired (thanks Dart270) and it is doing the same thing. Hot after 5 miles, very hot the longer you go. Now I wonder if my luck is just this bad or is it something I'm doing wrong. I put in fresh Valvoline 80-90 weight gear oil each time. The vent is clear. Granted I never had a reason to check the temp of the rear before so don't know how hot they might be in normal condition, but this thing is hot enough you can't hold your hand on it. It is not hot out by the wheel bearings, only the center. These are not sure-grip units. Thanks for any help.

A rear end does get hot to the touch but "how hot is hot" is the question.
I have never measured the temps but would guess that they run between 120 and 180 degrees.
If the grease on the outside starts to smoke, it is running too hot!!
DD

Here is an update. I taped a temp sensor to the outside of the rear. I went about an hour on the interstate at 65 to 70 on a 80 degree day. The temp reached 187 and showed no sign of leveling off. It would just slowly creep up a degree as miles went by. especialy going uphill. Don't know if this is a problem but it is making me paranoid as I have a 1000 mile trip planned in 2 weeks. Never had a reason to check the rear before so maybe this is normal? What is too hot, that is the question.

Rear ends get hot, there's lots of friction involved. Unless it's making a noise or fumes are emminating from it or something, stop worrying about it and drive! (just my opinion, mind you)...

"DW"

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If it ain't broke, fix it!

The race car guys know all about this, NASCAR and other high speed cars have to run rear end pumps and coolers to keep the gears alive.
For a Pass car rear, my bet is the max temp. would be the point where the oil you are using starts to break down, most gear oils are good to 300 to 500 degree range.
DD