checking differential ring gear backlash before run out?

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Mar 10, 2019
melbourne, Oz
Sorry for ignorance. My manual says to check differential ring gear run out before checking backlash. If not in spec, replace.
But if I read broader than my manual, folks say you shall never achieve correct run out without setting backlash correctly.
Should one try to rectify backlash with new bearings before throwing out the diff ring gear?
This is a stock diff from unknown neglected wreck of 1985 2h h55f, 4:11 I believe, should apply to most diffs though I imagine.
Ring gear runout would be an unusual problem wouldn't it? I'm no diff guru but assuming the differential case is running smoothly & with minimal play in the side bearings you should get an OK reading for ring gear runout. If things feel too loose it could just be minor bearing wear, so adjust the backlash & preload, check that the bearings feel ok & try again.

Ring gear lateral runout is to check if the ring gear is flat. If it's bent you'll get significantly different backlash at different points on a rotation of the diff. So in my mind, you can't set backlash until you know runout is good.

I try to check the runout with the diff mounted so the ring gear teeth are facing the ceiling so you get a more consistent reading as the CW weight is sitting on the bearing. If the bearing is totally trashed, or you have zero backlash (i.e. the pinion is pushing on the ring gear), this will affect your runout readings, but I've never had a ring gear out of spec on a couple of old diffs I've re-bearinged. I don't see how backlash will impact the LATERAL runout reading unless it is zero/negative.

Now, if you have swapped the ring gear (new gear or transferred old gear to a locker etc) you can induce runout if the mating surfaces weren't correctly prepared. If runout is out of spec you may be able to remedy by removing and reseating.

The comments about setting backlash first may be so that you ensure all slop is taken out of the crownwheel bearing (preload). Fair call, but you aren't really setting the backlash (which is the space between the teeth of the CW and pinion)
By the way, I actually have an 80 series front diff sitting in a mount in the vise as I write. I did my pre-rebuild checks yesterday. All in spec except for side gear backlash, which is always difficult to check. I'm gunna risk it.....

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