how do i measure backlash in a rear end?

Answer 1

the previous answer is correct...however if you don't have access to a dial indicator...get a piece of paper the consistency of a cigarette paper and run it through...if it cuts the paper but doesn't tear the paper then you are at your tollerance....I've done so many I do it by feel...once the gears are installed...just feel it...you will feel a little slack but it will be minimal...there must be some play in the gears...but not so much that you can hear it clunk...and make sure your crush ring is all the way crushed or it won't work at all.

Answer 2

Hmmm?
It been awhile, but I thinking you use a feeler gauge. I use a Repair Manual fro an auto-parts store, so, I think this is car specific. I go look at a manual for a general explain as you not mention your model/year/car/truck!

Heck, I not even have to get up, here how a generic explain work;
Checking Set-up
46. Check backlash using dial indicator mounted to machined differential cover surface; dial indicator should be touching outer most part of tooth with the indicator perpendicular to tooth surface.
47. Backlash should be .008 to .012; change shims to adjust backlash, if necessary.
48. Use paint on the teeth to check pattern (primarily when using non-Motorsport gears and/or installing a new differential).

http://home.flash.net/~ivc1/cobra/solgear.html