Differential Backlash question
 

I know that I showed be aiming for FSM .0036- .0043 although JPM Miata rebuild video has him at .004-.005 .

I’m getting .004 around the ring outside of a .0045 that pops up on occasion as I circle the ring. Is this variation ok? I’m just unclear what kind of tolerance is .

That's fine.

Are you using new gears or used gears? Used gears like to be a little on the loose side of spec.

Really, if it patterns okay, it'll be fine as long as it isn't too tight.

Used gears from a Miata. (4.1:1). Unknown mileage, but no visible wear to my untrained eye. It’s interesting to see what the FSM reads to the Mazda Comp Prep and Service Manual reads .0067 to .0075 for backlash.

i'm not sure what the use case is for the FSM, but the use case for the Comp manual is for like an endurance race car, so full throttle for a day... it would be a little warmer

Every USED set of gears I have seen, the backlash was around .007".

Is this a suggestion to set at .007?

Personally, and I just went through my rear end a second time. go by feel IMO.

you need to make sure the pinion spacing is spot on and tighten the nut to around what the specs are....but you can "feel" when the bearing/grit is over torqued...and we are talking inch pounds....

another hard earned lesson is you need to have tension on it when setting the pattern. i.e stick the axles in when spec'ing it out. and less paint or marking material the better as too much and you can not tell the pattern.

as to backlash. well, a lot of people say set it up tight lol. I did that the 1st time. did not last long. get the pinion spacer and crush bearing (mine is solid--so it was a little tougher) perfect, then move on to the carrier bearing adjustments. set it to spec......not tight like everyone says.

just my .02 but my 1st one fried the pinion and ring gears pretty darn quick and it was set up tight. Once I got my pinion perfect I did not touch it...hell, I even threw a little jb weld on the nut so it would not loosen up as I had it on and off a few times.

This time. PUUUUURFECT. no noise at anytime...and used gears...matching 3.909 from 1980 I did not even check the backlash lol. I knew a pretty good idea of slop it had when taking it apart and it had virtually none when I finished. (I used all the dial indicators etc on my 1st go around) I think a lot of people crank down on those carrier bearing to compensate for slop. do not do this. get the r&p and spacer right FIRST.

hope this helps. it really is an art I think.....