""""""I picked up a good used 4.2 diff, '""""""
I assume this is for the car your building? and not for stret use ?
At a a minimum I would check the backlash on the gears (get a number), remove the ring gear carrier, and pinion assy and the seal, wash the case, inspect everything, put in a new pinion seal, give the goofy lock plates on the ring gear bolts the flotation test, and use loc-tite or drilled head NAS screws and safety wire on the ring gear. Watch the under head radius if you use NAS and then use beveled i.d. ground flat washers.
On the caps use hardened washers and new ARP or grade 8 nuts with loctite. Get rid of the lock washers. They tend to damage the original nuts when removed. once you do all this you dont have to worry about it.....cause you did it right. not a pleasant sight when the bolts back out.
Whatever you do., do not use gasoline to wash off anything.
You don't want to get any solvents in the pinion seal cavity also. When you remove the seal it will be hard as a brick unless the p.o. has replaced it in this century. you can also clean up the outside of the case real nice and remove any paint. In your spare time you can polish it also...

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