The engine was of conventional construction with a one-piece crankcase and cylinder block in cast iron with the crankcase extending down to the lowest level of the main bearing caps; with a cylinder head, also usually in cast iron, and a sump made from pressed steel.
The B-Series shares many design features (such as the heart-shaped combustion chambers and siamese inlet ports designed by Harry Weslake), as well as its basic layout and general appearance, with the smaller BMC A-Series engine.