Your biggest thing is to find a piston with oiling holes not slots, those slots weaken the landings. Your next thing is to open the ring gaps, your normal off shelf pistons were never spec'd to rev very high or be used in a setup with lots of compression/combustion pressure so you want to open your ring gap a little to prevent ring bind which also cracks the landings off due to the incredible drag created by a bound ring.

do you do this in a lathe? and how much? because both times I hav crack'd them Its be'n the lip between the two compresion rings were it has crack'd?

Cheer's

if its between the 2 rings its usualy ring bind (if the car is tuned right and not getting ring flutter from detonation),,, you use a ring file and gap them to a suitable gap per your requirements