This is going to seem like a pretty stupid question I'm sure but here goes...
What is the small O-Ring in the following picture for? I assume it is for the gearbox as it came in my gearbox kit...

I searched and came up with the following thread which seems to be talking about the same O Ring. When I stripped the Gearbox originally it didn't have an O-ring behind the output shaft cover, does that mean I shouldn't put it in? Or is it better to have it on than not?
http://www.theminiforum.co.uk/forums/topic/215033-detent-spring-sleeve-ball-bearing-o-ring/
Thanks heaps!











