Go to WTS MISC in the marketplace. RocketTomatoe (spelled right?) had a couple of sales of O-rings, for a few bucks you got a couple of o-rings in each of about 15 or 20 sizes. Had two different types available. Pulled out one just this weekend - put it one, too tight, went down a size and worked. So you don't necessarily need to know the size as long as you have a bunch of sizes....