Hello. I'm putting together a couple of the XO-1 differentials for my Rustler- what is the single washer for (on part 6882x)? is it simply a shim? or does it serve some other purpose?

I ask because for the life of me, I can't get them smooth, or if I get them smooth, it'll bind one spot, on both. I'm aware of some of the typical issues (eg, over-tightening the ring gear screws, I-bar upside down...in fact, even the tabs have a specific orientation and that caused problems).

If I remove the washer, it's better, but don't want to if it's a bad idea, indicating something else is wrong.

Thanks for any insight.