gudgeon pins are smaller on black top rods
and im not 100% but i think its the black top rods that are longer as i had blacktop pistons on silver top rods before and the piston came about 3mm from the top of the block

i had to have my silvertop rods machined out and bronze small end bushes pressed in to take the blacktop pistons cost £140 to do the 4 rods the bushes where from a golf gti as they have the same pin size as the blacktop and the bush was big enough to fit into the opened out silvertop rod