Get your old piston and get your new piston next to each other. If the distance from the wrist pin hole to the top of the piston isn't identical, you probably got the wrong one by accident.
What exactly does a "ping" sound like to you? Can you see any marks on the piston crown?
What does it feel like when you turn the engine over by hand? Try that with the spark plug in, but again with the spark plug out. Sometimes the plug is what the piston hits.
Make sure you aren't running an extra long spark plug too, good idea to check what it looks like in the head from the inside whole you've got it off.