I've seen a couple used YS engines for sale that had a bent or broken cooling fin, always near the top of the engine (near the rocker cover)-- does this indicate a crash? If so, are these engines that you especially want to stay away from?

Most likely crashed, but not a gaurantee that the engine is bad. I would'nt buy one in that condition because there are too many nice 2nd hand YS engines out there waiting to be bought. I am very leery of buying expensive used engines anyway, even though they might look good on the out side, it's just too easy to damage them with one lean run.

I have 3 YS engines with a total of 20 years use between them. Not a broken fin among them. But then I've never crashed one Would I buy one ? Probably if it was a smokin' good deal.