Abstract: An actuator mechanism for detecting the presence of the piston or other movable element of a fluid cylinder for reversing switch purposes and the like is disposed in the end cap of the cylinder and is engaged by the cushion spear on the piston to provide transverse linear motion for actuation of a microswitch. The mechanism consists of a slender rod supported for movement transverse to the motion of the piston, the rod having a shoe at its lower end which receives a plunger, the latter passing through a bore in the end cap and adapted to be cammed upwardly by engagement with the cushion spear. The shoe and plunger are spring-biased toward the piston and the upward movement against the bias serves to actuate a microswitch external of the end cap to signal the presence of the piston. In a design suitable for different size cylinders, the plunger is formed of square stock of appropriate length.