Crankcase (Engine Cases)

The crankcase, often just called the cases, is the housing that surrounds and supports the crankshaft. On most motorcycles it is cast from aluminium in two halves split vertically or horizontally, and it also contains the gearbox, so engine and transmission share one compact unit.

In a four-stroke the crankcase usually holds or connects to the oil sump. In a two-stroke it is a sealed chamber where the fresh air–fuel mixture is pre-compressed, so the crankcase seals and gasket must be airtight; an air leak there causes a lean mixture, high idle and potential seizure.

The side covers bolted to the crankcase are vulnerable in falls. Cracked cases can sometimes be welded, but splitting the cases to replace a crankshaft or bearings is a major job best done with a manual.