I've got around 600 miles on my new third and it still gets really hot after driving a high speed (55mph) for a while. Is this normal?...is it hurting anything? How long should this go on before the gears finally break in all the way and the rearend runs at a normal temp? I babied it the first few hundred miles and the next few hundred miles I got it up to highway speed.

I think you might reduce the heat if you go to a synthetic gear oil....probably best to use the thicker 80W-140 stuff.

Also, have any info on the install. brand of gear? pinion preload?

Zuk,
They are the gears that you helped me through the setup. Yukon v6 4.88's. The pinion preload was 9 in/lbs. I just figured they should be broken in by now. The housing is still very hot to touch. You can leave your hand on it but it's very hot. I put some valvoline synthentic blend stuff in there that I had laying around the shop after about 200 miles. I'm planning to put redline in it once it's completely broke in. I guess the thicker oil can't hurt? Does it normally take this long for the temps to break the gears in to where the run at a somewhat normal temp?

jeesh 1000 miles i put redline in after like 500 or so... prolly less.

Did you notice a significant difference in the running temp of the diff. after switching to redline. Which weight did you choose?

no heat at all in my diffys... i remember touching it first couple times driving at it was right up there... but now its barly warm to touch. Im using redline heavy weight shockproof. running hour and a half at 70 and it was just slightly warm to touch.

THIS should also help a bit.

I did what randys breakin guide says. I put a 100 continuous miles on it after the first 500 and it was very hot after this. I'll try the redline but I don't know that that will solve the problem. Maybe my pinion preload is to tight? But it was within specs? Who know maybe this problem will just correct itself with time.

Have you changed the oil yet? When I was breaking in my gears I changed the gear oil at 100, 200, and 500 miles, then switched to Mobil 1 at 1000 miles.

go take it offroad and beat the crap outta it if that doesnt set those bearings in then nothing will.... I was wheeling mine right off the bat.. ZUK said to. anyting goes as long as its not highway.