Why am I getting a hole (twice) in the top of my pistons?

What engine do you have?

If the hole is about in the center of the top of the piston, the octane of your fuel is usually too low for the compression ratio and or your ignition timing setting.  Holes can also be caused when you are running a pipe or muffler that has too much restriction.  To much restriction can be caused by a poorly designed pipe or a muffler that has malfunctioned or the spark arrestor that is carboned up.

The exhaust temperature is too high if the edge of the piston on the exhaust port side is burned away.  High exhaust temperature is the result of ignition timing that is too retarded or the engine is not getting enough fuel at the throttle setting that was being used when the piston failed.