Locomotive Engine Running and Management

by Angus Sinclair

INSIDE CLEARANCE.

For high-speed locomotives, where there is great necessity for getting rid of the exhaust steam quickly, the valves are sometimes cut away at the edges of the cavity, so that, when the valve is placed in the middle of the seat, it does not entirely cover the inside of either of the steam-ports. This is called inside clearance. In many instances inside clearance has been adopted in an effort to rectify mistakes made in designing the valve-motion, principally to overcome defects caused by deficiency of valve-travel. The fastest locomotives throughout the country do not require inside clearance, because their valve-motion is so designed that it is not necessary. Inside clearance induces premature release, and diminishes the period of expansion. Consequently inside clearance wastes steam, and ought to be avoided.