Sorry for resurrecting an ancient topic, but we delayed buying the spark arrestor because we didn't know which size to get, and didn't feel like going into a battle-to-the-death with my current silencer mounting. This thing is famous for leaking oil and gunk from the hose clamps we've used if it's not done perfectly, and for the first time since 1982, we had it done perfectly. But, now I think we've pretty much hung the bike up for the year and got around to measuring the size of the stinger.
Now, the outside diameter of the end of the pipe is 1-3/8 inches, and the stock silencer was exactly that diameter as well. It was held on by a nice strong piece of hose, clamped down to the balls. However, I'm not sure if the FMF one I've been looking at (the TurbineCore II Universal that Opferman posted) works in this way. From the picture it kind of looks like it is meant to slide into the stinger itself, and I measured that inside diameter at 1-1/4 inches (the larger of the two sizes FMF makes). I don't want to go ahead and buy the thing, only to find out that I was wrong and I needed to buy a 1-3/8 size spark arrestor (which is not a size offered by FMF). So if anyone could just clarify for me how the thing works or what that 1-1/4 number refers to, that would be much appreciated.