With the YZ you have to weld the bottom of the exhaust to prevent the piston from uncovering the bottom of the exhaust port at TDC. You also have to bore the crankcases to clear the rod and cut the head deeper. I wouldn't waste the money stroking this model, boring makes a much bigger difference because the cylinder and head offer more control over tuning factors. Possible sizes for this model are 265, 275, & 285. I've tried them all, 265 is most reliable.