Hi the Pixy IS OVERSHIMMED! The head clearance is around 0.020"with NO SHIMMS. this will be good for 20-30% Nitro.
The only way to check is to measure it.
Take out the plug and insert a piece of solder.
Turn over the motor fully so that the piston flattens the end of the solder
Measure the flat spot with a vernier or a micrometer
It should be 0.018" for 16% Fuel or 0.022 for 30% fuel.
If the head clearance is too big the motor will overheat as the fuel burning happens after the piston has gone over the top (advanced timing).