For Sale is a good used McCulloch piston, with thick cast Iron Rings.  The rings still have a lot of spring so they would probably work fine as is, piston is in good shape with no scoring: 

This fits:

McCulloch 1-41, 1-42, 1-43, 200, MAC 15, 1-51, 1-52, 1-61, 1-62, 1-63, 1-50, 1-60, D-36, 1-40, D-30, S-44A, S-55A, 1-70, 1-80, 1-71, 1-81

McCulloch 250, Super 250, 300, 380, 380A, 440, 640, or 660 Gear Drive Chainsaws

McCulloch Karts MC 20 and MC49, this piston has thick rings, but will work on these karts if desired.