Locomotive Engine Running and Management

by Angus Sinclair

THE EXTENT OF LAP USUALLY ADOPTED.

In locomotive practice, the extent of lap varies according to the character of service the engine is intended to perform. With American standard gauge engines, the lap varies from ½ inch to 1¼ inch. For high-speed engines, the extent of lap ranges from ⅞ to 1¼. Freight engines commonly get ⅝ to ¾ outside lap, and from 1/16 to ¼ inside lap. With a given travel, the greater the lap the longer will the period for expansion be.