Gears & Axles - How hot should my rear be?

I have a 9 inch now with a detroit locker and 4.11 gears

I feel like the rear may be getting too hot. I really don'nt know how hot it should be. After about a 45 min drive at 75 MPH on 25.75 inch tires The case is hot enough that after about ten seconds it begins to burn my hand as I hold it. It feels pretty darn hot.

Any opinions would be great. Thanks!

Road racers sometimes use finned covers so I reckon
the pumpkin can get hot. And with 4.10s, you may be
spinning the chunk at 80MPH, the way those guys do
at 125 - thrashing all that heavy oil and shedding heat.

I wouldn't worry about it until you get at least to
boiling water type temperatures. You could always solder
some handmade cooling fins to the back of the 9....

My 4.10 rear diff runs just as hot as yours, even after 3.5k miles on the rebuild. My stock 3.42 diff runs at ambient temperature no matter what. My car sits out the winter so this year I filled it with 75w140 to help keep the oil thicker under the higher temps.

9" Ford's naturally run a little hot. The extra power they consume is turned to heat. Has to do with the low pinion height Ford used to keep the driveshaft hump down.

Al 95 Z28

I figured they had more friction due to that pinion location. I take it I don't need to worry about it too much. The third member is out of a Nascar car and I got fluid from them also. I know its setup correctly, but I also know they run coolers so thats why I wanted to ask. Thanks for the replies I will just keep an eye on it

Nascar cars do have rear-end coolers. In the old days they used an old shock absorber with AN fittings above and below the shock piston. Mounted the shock to the frame and the rear-end and when driving on the track every time the car hit a bump or the suspension compressed the shock acted like a pump. I always thought how ingenious the guy was who thought of using a worn out shock to circulate diff fluid through the coooler.

the old saying is... if you spray it with black paint that is cheap... if it turns white the rear got to hot...

grandfather use to tell me this from time to time... man built quite a few rears in his life...

Nascar cars do have rear-end coolers. In the old days they used an old shock absorber with AN fittings above and below the shock piston. Mounted the shock to the frame and the rear-end and when driving on the track every time the car hit a bump or the suspension compressed the shock acted like a pump. I always thought how ingenious the guy was who thought of using a worn out shock to circulate diff fluid through the coooler.

Looked at Home Depot's car some years back. Its 9" just had a sheet metal piece that allowed the ring gear to pump a little oil through a regular looking heater box with a stock looking blower. It was on the floor inside the car.

Bet a NASCAR rear ends get real hot running that much RPM and horsepower. Tow a heavy trailer up a hill with a pickup and try to touch the center of the rear axle. You'll be surprised at how hot they get.

Al 95 Z28

I definitely think I got a tad scared when I touched it and it was warm. Im still very cautious with the new rear. The locker gives me a lot of false reads in normal driving. Tends to do some sideways walks in and out of gear but I am getting used to it. As for the heat I will keep an eye on it. If worse comes to worst Ill just get a Nascar cooler from the team. Thanks for the replys