I'll have to go with proptop on this one, that crankpin looks to have been cracked almost halfway through when you got the engine. You just happened to be the lucky one running it when it let go. Don't blame the seller too much, there is no way he could have known about the crack. I have a couple of OS 2- strokes (a 70 and and a 90) that look just like yours (or even worse)on the inside and they have been running for years with no problem. The 70 Surpass is just now beginning to feel a little scratchy when turned over carefully by hand. I will replace the crankshaft bearings before next flying season.