It's all in the details. If you take one apart the tolerances can be as close as .0001 "..

The parts can be very small, such as the pin that keeps the ring from turning or the teflon pads (if they have one) on the end of the piston pin. some rods can only be installed one way with oil holes. As others mentioned the sleeve alignment is important also. So it isn't very complicated, but don't miss a detail. Jim