6. Clean the top of the old piston until you can read the stamped number (see Figure 47). Compare the measurement of your old piston with the diameter of a new piston of that number as detailed in Figure 53.  If your old piston is smaller than a new one, calculate whether a new piston would come up to the required clearance without re-boring the cylinder.  If the clearance is still too much, re-bore the cylinder to the next oversize and install the oversize piston.