Engine cases

How it works

The crankcase is the aluminium housing that holds the crankshaft, the gearbox (on most bikes) and the oil. It is usually made of two halves bolted together, with side covers enclosing the clutch, alternator, oil pump or ignition. All these joints are sealed with paper gaskets, O-rings or sealant, and the shafts leaving the case (gear-shift shaft, gearbox output sprocket) pass through oil seals. Over time, heat and repeated removal can make these seals weep: an oil leak is far more often a gasket or loose bolt than a cracked case. The case can also be holed or cracked in a crash or by hitting an obstacle, especially off-road, which is why skid plates and case covers exist. The drain plug threads into aluminium: overtightening can strip the thread. To find a leak, clean everything, ride a little and look for the highest point where oil appears.

Maintenance tips

• ✓To find a leak, degrease the whole engine, ride a few kilometres and look for the highest point where oil reappears.
• ✓Torque the drain plug and cover bolts to the manual's value: aluminium does not tolerate overtightening.
• ✓Fit a new gasket every time a cover is removed and clean the mating faces well.
• ✓Off-road or on an exposed bike, a skid plate or case covers prevent cracks in a fall.
• ✓A cracked case or stripped thread should be repaired by a specialist (aluminium welding, thread insert).