Abstract: An aluminum piston is mounted onto a steel piston rod by means of an intervening, steel hub. The piston, hollow with an opening at one end, pilotingly receives the hub in the opening. The hub has a throughging bore in which the piston rod is pilotingly received. The rod has a prominent shoulder against which a rim on the hub seats, and a hub nut prestresses the hub against the shoulder. The hub has a flange with a series of bolt holes formed therein, the latter being aligned with a like series of tapped bores in a ledge of the piston. Studs with locknuts, engaged with the holes and bores, make the flange fast to the ledge.