To miniaturize an engine by arranging a chain holding part at a position lapping over a direction in parallel with a cylinder body outer wall.

In a water cooled single cylinder engine having a water jacket 21 that is formed along the whole circumference between a cylinder bore wall 27 and a cylinder body outer wall 28, bolt holes 24 through which through-bolts 5 for connecting a cylinder body 3 and a cylinder head 4 penetrate are formed in a cylinder body outer wall 28. Portions of the wall around a bolt hole 24 protrude into the water jacket 21 to become protrusions 32A-32D. Those regions of the cylinder bore wall 27 that face the protrusions 32A-32D are cut out by flat surfaces extending in an axial direction to become hollow sections 33A-33D. Consequently, the engine can be reduced in size without reducing the exhaustion amount of the engine.

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