as gulag stated, load the pistons in the cylinders first. shove 1 and 4 up the bore to be out of the way of 2 and 3 piston pins sliding into the rod. pull 1-4 out enough to clear the pin holes and you are in good shape. install the pins from the outboard side of the pistons. If there are clips, have them in on the inboard side beforehand. If a pin does not slide all the way through by hand, turn the pin around and try it. There should not be an interference fit between a pin and piston. At temperature this could cause the piston pin boss to crack.
