Many engines, especially the larger ones, have openings in some part of the engine frame. (See figs. 3-4 and 3-6.) These openings permit access to the cylinder liners, main and connecting rod bearings, injector control shafts, and various other internal engine parts. Access doors are usually secured with handwheel or nut-operated clamps and are fitted with gaskets to keep dirt and foreign material out of the interior of the engine. On some engines, the covers (sometimes called

Figure 3-6.A 251-C Alco cylinder base.

doors or plates) for access openings are constructed to serve as safety devices. A safety cover is a cover that is equipped with a spring-loaded pressure plate. (See fig. 3-7.) The spring maintains a pressure that keeps the plate sealed

Figure 3-7.A safety handhole cover.

under normal operating conditions. In the event of a crankcase explosion or extreme pressure within the crankcase, the excess pressure over-comes the spring tension, and the plate in the safety cover acts as a vent to allow combustible gases to escape to the atmosphere. This release of excess pressure serves to minimize damage to the engine.