I use a dial indictor on the outer most edge of the piston on the cam side and use a narrow thin screw driver blade on the motor mount between the piston top and cylinder wall to rock the piston over to get the low side measurement on that side, then reverse the piston to rock towards the motor mount side to get the high side. Do the same on the motor mount side to get the highest reading on both side of the piston tops and use a minimum of .035 piston to head on steel rod motors, more clearance for aluminum rods per the rod makers suggestions