I have a McCulloch 450, and 740 chain saw. The piston on the 740 had some damage and a broken ring when I dismantled it. It was a barn find. 450---Stroke is 1.5 740 stroke is 1.635 450--Bore is 2.165 740 bore is 2.165 450--Cu. In. is 5.5 740 Cu. In. is 6.04 450--CC is 90.4 740--is 99cc The top of the piston on the 740 has a small gash on the top edge and reaches down to the first ring, with several pits on top, apparently when the top ring broke .I can not find another piston for the 740. Does anyone know if I could use the piston out of the 450 in the 740? I know there is a small difference in the length of the 740 piston, which is just a little longer than the 450, but they are the same diameter. I would naturally have to change the con rod from one to the other. Would I be able to put a small spot weld with a 120 v. welder into the gap and grind it down smooth.? I dont know what material is used on the piston. If not, any idea where I can find a new or good used piston for the 740? I believe the 740 is the same as a 840 and a 550. (??) I am planning on putting one of the engines on a vintage go kart and would naturally go with the stronger engine, but is there enough difference in power between to two to make much difference, just for recreational riding in dirt?