So I just took apart an AP1 diff with 50k miles that was in perfect operating condition. The diff came out of a "hellaflushed" car that only saw a commute to work and car shows... the owner was never interested in racing it. Granted he bought the car used and who knows what the previous owner did.

The fluid was clean, and the contact pattern was in the proper spot with 0.005." of backlash. I took it apart because I wanted to install 4.77 gears in this housing. So out of curiosity I inspected the stock 4.10 pinion and to my surprise it had a very very minuscule crack near the stem. It is approximately 1/4 inch long, and very superficial at this time.

I'm surprised to see this. All these years the talk has been about the case being weak, or the fact that hard launching will destroy the gears by breaking teeth instantly. In this case, hard driving didn't appear to be the factor as the unit had been operating perfectly before being taken apart. Is this a case of bad metallurgy? opinions? Feel free to correct whatever misconceptions I may have.

Picture to follow (typed this at the shop)

 

Both the pinion and the bearing caps are weak. My pinion sheared a toothe which crashed into the ring gear, and the torsion had nowhere to go but up. Crack. Your car was launched at some point.

 

the last one i took apart looked like it was 10 mins from exploding. EVERY tooth on the pinion had a crack down the center.

 

Sounds like the person who you got the diff from, either isn't telling you the truth, or he is the owner of someone that abused it.

Just commuting and car showing wont give a 1/4 crack.