I had some problems with a leaky dellorto SHA carb, mine's a 16/16 and the shim wouldn't seal up against the carb body. If you're getting a leak around this area these o-rings are the perfect size, they did the trick for me. I've read that its pretty common for larger SHAs to have this problem.
I forget the exact dimensions, but they fit like a glove. The size of the oring matched with the shim pressing up against it makes it pretty hard for one of these to get sucked in to the engine. If you're nervous just glue it to your carb, I'm pretty sure that's what I did. Paypal me like 50 cents or a dollar or something and I'll throw a few in an envelope and mail them to you. Not looking to make a profit.
Just thought I'd put this up for anyone who doesn't want to bulk order them like I had to do, I've got a whole bag of orings I can't do anything with now