A bash valve is a valve within a piston engine, used to control the admission of the working fluid. They are directly actuated valves, operated by contact between the piston and the valve tip.

The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and exhausts through ports at the cooler centre. By this means, the relative heating and cooling of the cylinder walls is reduced.