Say you have an Sl70 72cc engine and you buy that 88cc larger bore kit. That doesnt change the stroke at all but just increases the bore and combustion volume-correct? For stroking dont you have to buy a slightly longer piston rod and have a deeper head? Years ago I seem to remember that when you mill the head that brings it closer down to the piston head and yields more compression . And porting was done to the valves somewhere (I forgot).