No, it doesn't get trashed: it just go in, and prevent the ball to move to far inside, and the target of this is to reduce the possible movement of the dog bones toward the inside of the outdiff, avoiding them to pop out...
Don't worry, this is very often used in many kits for this same reason, and the o-ring will survive (it won't even move from the outdiff when you take teh dog bone out, unless you use a small screw driver to get the o-ring off)...
And don't worry about ruining my thread, I document things so they can serve others, and if any one comes with the same question, he'll find the answer reading the thread
