A piston engine is undersquare if its cylinders have a smaller bore (width, diameter) than stroke (length of piston travel). This can be a negative trait, since a longer stroke means greater friction and a weaker crankshaft. An undersquare engine usually has a lower redline than an oversquare one, but it may generate more low-end torque.
Many British automobile companies used undersquare designs through the 1950s, so many of the most famous cars of that era use this design. This includes the Austin A-Series engine and many Nissan derivatives.
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