I bought a beat up 85 200x about a year ago, with hopes of restoring it. Well, its been sitting since until yesterday. It fired right up, and I let someone take it for a ride. It ran fine, until it blew a head gasket. I took the motor out and apart and found that the head bolts were improperly under-torque, causing the head gasket to blow. Now to my question. The piston is not original. I think it is a high compression piston. The only markings I found was that it is a 50 mm over piston. The top is pretty tall, with recesses for the valves. Is there any way to tell if it is a high compression piston?

 

That's .50mm over, which is .020". Check and see if there is either a "4394" or "4395" on the top of the piston. If it is a Wiseco piston, the 4394 was 10-1 and the 4395 was 12-1. If it is a 4395 (12-1), make sure you run at least 102 octane fuel or better, or you will have troubles. If you can't find a number on it, measure from the top of the piston down to the step on top of outside of the piston. I can compare to another that I have.

 

Thanks for the reply. Looks like I'll have to get some high octane gas! I put the motor together and reinstalled it in the frame. That thing hauls for only a 200 motor.