PURPOSE: To facilitate assembly work of a piston of scotch yoke type, by fitting a raised cut, provided in a crosshead, into an opening part, provided in a rear end part of the piston, and temporarily fixing both the piston and the crosshead.

CONSTITUTION: A crosshead 1 provides in its mounting part of a piston 3 a raised cut 2, while the piston 3 forms in its rear end part an opening part 4. The mounting method, fitting the raised cut 2 of the crosshead 1 into the opening part 4 of the piston 3 and temporarily fixing both the piston and the crosshead thereafter performing brazing, enables both the piston and the crosshead to be easily secured and assembled.