we just finished putting a strange 12 bolt in my dads 2000 z28. we took it out for part of the break in period. we drove 50 miles let it cool off. drove it another 50 and took it home. i got under the car to feel the rear end and it was hot enough that you couldnt touch it for more than a second. is this normal?? it is just scary that it is getting this hot

 

that's normal.... the friction of the gears is what warms with gear lube/case up.

 

so hot that you couldnt touch it??? that's crazy...i'd have someone check the clearance of the ring to the pinion. it'd be worth the 50 bucks over the cost of a real rebuild. do send it to a reputable place though, not just anyone. if it's hot enough to burn you, then it's likely hot enough to burn up the clutches in the posi

 

They will get pretty hot, higher loads produce more heat as well, but to define too hot to touch is not specific enough. For example, 200deg is OK, but you cannot hold your hand on it at this temp. Try to touch your 195deg radiator hose for a good example.

To everyone, I suggest a different break-in period. I feel 50mi. is too much on the first run.

1. I normally let the car run in gear with the wheels off the ground for 5min. or so before setting it on the ground. This will let the rear rotate & help to break in the bearings without load.

2. Then take it for a 5 mi. ride driving at normal speeds. Afterwards, let it cool.

3. Take another 20 mi. ride & let cool again.

Feel free to repeat this extra times.... it will not hurt.

4. Then a longer ride at highway speed, maybe 50mi. & let cool.
I generally like to get at least 100mi. on them before any serious usage.

This is all intended to allow everything to wear in without overheating. The contact areas create friction & will get much hotter than the actual housing. If it gets too hot it will alter the heat treating of the gears & soften them.

 

its 100% completely normal. If you start hearing a bad whine..then get nervous.