If using a aftermarket cylinder and piston, do not just assume everything fits perfect as you might with genuine Stihl parts.
Observe the piston skirt to crank counter weight clearance as well as taking a squish measurement between the piston and edge of the combustion chamber.
Clean off all burrs and compare the new parts very closely with the old parts
Stick both pistons on one wrist pin and compare deck heights as well as skirt lengths.