I was wondering when to use to a ring gear spacer? 3series carrier with a 2series gear,or a 2series carrier with a 3series gear? thanks

 

3 series gear on 2 series carrier...

Think about gears for a minute. The tooth count ratio isn't the only thing that changes with the gear ratio; the ratio of diameters of the ring and the pinion has to change too. But, the ring is the same diameter in a given model of rear end, no matter what the ratio is; so the pinion gear size changes. Specifically, it gets smaller as the numerical ratio increases. So, the ring has to get thicker. At some point in the series of ratios, they change the carrier so that the flange the ring gear mounts on is closer to the pinion, and continue on. In these rears that happens at 3.23; 3.08 is the highest on the 2 series carrier, 3.23 is the lowest on the 3 series. So a 3.23 gear is quite thin.

You can't put a 2 series gear on a 3 series carrier, because a pinion for any 2 series gear is so big that there wouldn't be any room left between the ring gear flange and the pinion for the ring gear to fit into.