Undersquare engine (long stroke)

An engine is undersquare, or long-stroke, when the piston stroke is longer than the cylinder bore, giving a bore-to-stroke ratio below 1:1. The opposite is an oversquare (short-stroke) engine; when bore and stroke are equal, the engine is called square.

A long stroke means the piston travels a long way each revolution, so mean piston speed rises quickly with revs. This limits the maximum safe rpm, while the long crank throw gives strong torque at low engine speeds. Many cruisers, classic singles and big touring twins are undersquare.

High-revving sports and racing engines are nearly always oversquare, with large pistons and room for big valves. Neither layout is better in itself; it depends on how the bike is meant to be ridden.