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

 

It's a little more scientific than that... Do you have a stock piston? You need to measure your bore diameter, and piston OD and note the difference. The factory service manual will denote the service limits on your piston. While your at it, measure you end gap on the rings...

If you have a forged aftermarket piston (Wiseco), it greatly depends on what the engine builder set it up initially. Forged pistons tend to 'grow' more than cast, so they are set up differently in the beginning.