An engine having a crank case (10) splitted into upper (23) and lower (24) crank cases, and including a crank shaft (18) and a transmission drive shaft (35) disposed on the split plane (A) between the upper crank case and the lower crank case and a transmission main shaft (32) carried by either one of the upper or lower crank case. The length and the size of the engine can be reduced by the arrangement according to the invention, and a starter motor, a transmission drum and balancers can be arranged efficiently to save space required for mounting the engine.
