Bore

The bore is the inside diameter of an engine cylinder, measured in millimetres. Together with the stroke, the distance the piston travels, it determines the engine's capacity: the area of the bore times the stroke times the number of cylinders.

An engine with a bore larger than its stroke is called oversquare; it usually revs higher and makes peak power at high rpm, which suits sports bikes. A long-stroke, undersquare engine tends to have strong low-rev torque.

Over time the bore wears and becomes oval or tapered, causing blow-by and oil use. On iron-lined cylinders it can be rebored to the next oversize and fitted with a larger piston. Coated aluminium cylinders are usually replated or replaced instead. A larger bore is also the basis of a big bore kit.