The term square or over square referrs to the the bore to stroke ratio. An engine is said to square if the bore equals the stroke, over square means the bore is bigger than the stroke. Rover V8s are over square. The shorter (over square)stroke means lower piston speed (ie for every rev the piston has to travel less distance in the same time so it is travelling slower) which results in less ring and bore wear, Generally most petrol engines built since the 60s are square or over square. Longer stroke results in more torque, shorter stoke engines generally are safer to rev as the angles of the conrods when rotating are not as great.
