A snowmobile includes an improved internal combustion engine. The engine comprises a crankcase, a crankshaft rotatably journaled within the crankcase, a cylinder block assembly extending from the crankcase, and a cylinder head assembly connected to an end of the cylinder block opposite the crankcase. A cam drive mechanism including at least one camshaft is rotatably journaled within the cylinder head assembly. A transmission is coupled to the crankshaft to transmit power from the engine to the drive assembly. The cam drive mechanism is connected to the crankshaft at a first end portion of the crankshaft. The transmission is connected to the crankshaft at a second end portion of the crankshaft opposite the first end portion.