i can rock my piston in the jug is that right?

 

Pistons are smaller at the top than the bottom so some movement is possible. Pistons are measured at the lowest point on the skirt and that number is subtracted from the measurement you get on the bore. Ususlly the smaller the bore the less clearance allowed. Were there abnormal noises coming from the engine while it was running?

 

There has to be some clearance in there but being able to "rock it"? Doesn't sound to good. Take the rings off the piston and slide them into the jug, then check the gap in the rings with a feeler gauge. If it is ok and the rings seem to be in good shape, you need to mic the jug and the piston. Different pistons have different clearances, but your local dealer or parts place should be able to help you with how much clearance is allowable. (if you can't find it there, ask on here and I'm sure someone will help)

GOOD LUCK!!

 

What is the year and model we are talking about?

 

a yamaha 350 moto4

 

i took it ti the shop and they said it needed bored it was bored .50 allready there takeing it to 1.0 not a bad price $170 for piston kit & bore . im going to lap in my valves and put in some seals hopefuly it will be good togo