I bought mine recently from cox international (ex xenalook)

Whatever tool you have, the most important way to not mushroom the crown is to place your reset tool on a hard surface. Placed on a smooth surfaced anvil on a concrete floor is good. An anvil on a wooden table won't do as well. You have to have the tool supported on something that won't flex at all. Test your cylinder fit and check the SPI gap to make sure it has not closed up any. This happens sometimes.

Practice on an old worn out piston first because you have to get the feel of how hard to tap and how many taps it takes.

Cleaning castor oil varnish from cylinder wall is also needed when motor tightens up. I twirled a dowel wrapped with fine steel wool and toothpaste by hand and cleaned residue carefully with fuel and paper towel.