I have some friends in Moab right now for Blazer Bash. My friend Brent, who drives a 72 K5 on tons, just broke a Reid Racing inner C on his Dana 60
This is the part he broke (pic is of an unbroken one):
Reid Racing and ORD came together and got him a replacement sent out next-day air. He is at moab 4x4 outpost right now where they are welding the new one onto his axle tube.
Here's that the packing list said (emphasis mine):
NO CHARGE FOR REPLACEMENT INNER C
Original invoice #108067
Replacement okay per Stephen Watson & Dennis Reid.
Inner C broke across the tube bore.
First and only known breakage of an inner C.
Preliminary photos indicate that the axle tube
may be oversize, which caused the shrink fit
to be too tight, which led to residual stress
and eventual breakage in that non-load-bearing
area of the inner C.
No greater than 2.5/1000" (.0025") press fit
is recommended. Advised to grind/sand axle tube
to no greater than 3.125" diameter before
pressing the replacement Inner C onto the tube.
D60 Forged Inner C 3.1225 Bore
They measured his axle tubes after the broken C was removed, and they measured right at 3.125, which is Reid's spec. So, nobody really knows why it broke...