Undersquare

A description of an engine whose cylinders have a smaller bore (width, diameter) than stroke (length of piston travel). Having a longer stroke usually means greater friction, a weaker crankshaft, and a smaller bore means smaller valves which restricts gaseous exchange.

Although an undersquare engine is generally less reliable, wears more, and cannot be run at as high a speed than an oversquare engine, they tend to offer better fuel economy and somewhat better emissions. An undersquare engine does not overheat as easily as similar oversquare engine.