Layout
The valves are driven from an eccentric cam mounted on one of the driving axles and linkage from the crosshead. The driving axle cam drives a rocking shaft that is mounted inside a cylindrical sleeve (known as the rocking shaft bush) above the cylinder. A drop lever and connecting link from the crosshead oscilates the sleeve to control the lap and lead of the cylinder.
Read more about this topic: Bagnall-Price Valve Gear