Bottom End

The bottom end is the lower half of an engine. It includes the crankcase, the crankshaft with its main bearings, the connecting rod or rods, and on most motorcycles the gearbox and often the clutch, which share the same cases. The top end is the cylinder, piston, cylinder head and valves.

The distinction matters most for off-road and race engines, where the top end (piston, rings, sometimes valves) is serviced often, while a bottom end rebuild with new crank or rod bearings, main bearings and seals is done less frequently.

Warning signs of bottom-end trouble include a deep knocking or rumbling that changes with revs, play in the crankshaft, metal particles in the oil or on the drain plug magnet, and gearbox problems. Regular oil changes with the correct grade are the best protection.