To install the piston into the cylinder liner of the SuperTigre GS-40, you must first install the piston/rod assembly into the case. make sure the rod is positioned so that the small chamfer on the inside of the lower-end bushing faces the crankshaft counterweight. The piston should be oriented so that the ring gap will pass over a web in the liner when it's installed.

You'll likely have to heat the crankcase to slide the liner it. The liner must slide over the piston. There is a bevel on the inside of the liner that will compress the ring when the liner's slid over the piston/ring assembly. When everything's lined up, it slides right on.