Big Bore (Big Bore Kit)

Big bore is used in two ways. Loosely, it describes a motorcycle with a large-capacity engine; there is no fixed limit, and what counts as big depends on the class. In tuning, it means increasing an engine's capacity by enlarging the bore, the cylinder diameter.

A big bore kit usually includes a larger cylinder or liner, a matching piston, rings and gaskets. Kits are popular for scooters, pit bikes, ATVs and enduro bikes, for example taking a 50 cc scooter to 70 cc or more. More capacity gives more torque and power, but also more heat and stress.

Often the carburettor jetting or fuel mapping, clutch and cooling need updating as well. A bigger engine may also change the vehicle's licence category and registration, so check legal requirements before fitting a kit to a road vehicle.