The wises thing to do is to get some "silly putty" from the toy store, and stick it in there. Then measure it, but it is not hard to measure with a normal vernier caliper. you have seen this attachment?

http://www.peachparts.com/shopforum/3265356-post8.html

I have one that I need to operate-on that slips after 3rd gear, but I think the rear teflon washer is damaged.

Martin