Abstract: A hydraulic tappet comprises an open-ended cylinder and a piston slidably accommodated in the cylinder. One end of the piston projects out of the open end of the cylinder and engages a valve rocker. A gear mechanism is provide to effect rotation of the cylinder about its axis whilst the piston remains stationary. An oil chamber is provided within the cylinder beyond the other end of the piston. A pressurized oil space is arranged to supply oil to the chamber via a non-return valve. An oil discharge space communicates with the chamber via a pathway including an opening in the cylinder wall. The pathway is interrupted by the piston as the piston moves into the cylinder. The oil discharge space is at a pressure substantially below that of the pressurized oil space. A plurality of spaced holes is arranged in a spiral line formed in the cylinder wall but only one of the holes communicates with the oil discharge space at any one time.