If you want to make sure your pistons are not hitting your head do this.
Revove your head,get a 1/4" piece of thin solder,run up piston to 1/2" from top of bore,then lay the solder on top of piston at outer edge of piston paralell to wrist pin. Bolt up head not using o rings,1 dome at a time. Slowly turn over piston till it goes over TDC. Then unbolt head & measure thinnest point of solder. This will be you squish.