The crank chamber structure is for an engine with a cylinder head on top of a cylinder block, a head cover and a lower casing part in which the crankshaft turns, an oil bath below this and an impact plate in the oil bath. It has opposite block-side jacket sections (34) and opposite casing-side jacket sections (8) fitted to the cylinder block (4) and the lower casing part (12). These jacket sections are curved outwards so that the crank chamber (18) encompasses the motion path of the connecting rods.
