is provided by spacers 18 and 19 the principal feature of the present invention is the fact that a first one of the pistons the expansion piston 6 which has its insulator 7 portion extending substantially into attached piston rod so that it is formed as a first rocker piston this can be accomplished if the first piston 6 is pivotally connected to the yoke 3 at the pivot axis of the wrist pin 4 which is distal that is laterally opposite the rocking lever 12 the pivot axis of the wrist pin 4 has substantially no lateral movement and therefore permits a rocker piston 6 to be used in spite of the fact it has its insulator 7 extending far into its cylinder the second piston 8 is also a rocker piston having its rigidly attached piston rod 9 pivotally attached to the pivot axis of the wrist pin 10 of the yoke 3 while the wrist pin 10 has a greater lateral movement during operation such lateral movement can be tolerated by the rocker piston 8 because it does not have the extension substantially up into its cylinder the lateral motion of the wrist pin 4 is minimized if the distance from the axis of the pin 13 to the axis of the wrist pin 4 is between 70 and 100 of the length between the pivot axes of the rocking lever pins 13 and 14 these features are particularly advantageous when the mechanism is used with a stirling cycle engine.