I jam the locks in with a square chopstick. This way you dont scratch the piston either.

Whichever way you install the pistons, once the shortblock is built: put a thin layer of clay on the tops of the pistons, bolt the head on hand tight temporarily, spin the crank a couple times, and make sure when you take it back off, the clay isn't smashed down to the metal anywhere.

I'm sure theres a more professional way of doing both these procedures, but that's the basics. Google it.