I agree with looking in the plug holes with the scope to see if there is anything hitting. I have a hard time thinking that it is only in one cylinder because it keeps locking up at both ends of a half a turn on the crank gear. that would mean it would have a different piston approaching top dead center at each stopping point. I'm probably going to have to buy another cam bolt for $100 and go ahead and pull the timing back off. at least i will be able to tell if it is a piston by turning the crank only or if it is one of the lifters by turning the cam only.