It doesn't matter too much about the outside diameter as long as the mating surface is proud of the casing (which they are). If its too big then the bolt head will tend to cut into it when tightening it.

I get all my bits and bobs like that from the local motor factor, or local fastener shop. You should be able to find one easy enough near where you live. In fact I think even Halfrauds stock them?

If going on line I'd try here or on ebay. BTW 9/16UNF is the same as 14mm aluminium sump drain washer, they are often quite easy to source.

However, all the washers are only about 25-50p each from a Yamaha dealer!

__________________