Abstract: A crankshaft of an internal combustion engine includes a crank journal, a crank pin, and a crank web. The crank journal is to be supported by a crankcase and to be rotatable around a crankshaft axis. The crank pin has a crank pin axis substantially parallel to the crankshaft axis to support a connecting rod which is rotatable around the crank pin axis. The crank web is provided between the crank journal and the crank pin to connect the crank journal and the crank pin. The crank web has a hole which extends from one side face of the crank web and passes in a vicinity of a connection section between the crank pin and the crank web when viewed in an axial direction along the crankshaft axis.

Abstract: A damper unit of an internal combustion engine includes an input-side rotor, an output-side rotor, first coiled springs, second coiled springs, an input-side sliding contact surface, and a biasing member. The input-side rotor includes a first spring holder, a second spring holder, and a connecting portion. The second spring holder is arranged on a second side of the output-side rotor, has a facing side facing the output-side rotor, and has second spring accommodation portions on the facing side each of which accommodates each of the second coiled springs. The input-side sliding contact surface is provided in the second spring holder so as to be in sliding contact with an output-side sliding contact surface of the output-side rotor. The biasing member is provided between the first spring holder and the output-side rotor to press the output-side rotor toward the second spring holder.

Abstract: A damper unit of an internal combustion engine includes an input-side rotor, an output-side rotor, first coiled springs, second coiled springs, an input-side sliding contact surface, and a biasing member. The input-side rotor includes a first spring holder, a second spring holder, and a connecting portion. The second spring holder is arranged on a second side of the output-side rotor, has a facing side facing the output-side rotor, and has second spring accommodation portions on the facing side each of which accommodates each of the second coiled springs. The input-side sliding contact surface is provided in the second spring holder so as to be in sliding contact with an output-side sliding contact surface of the output-side rotor. The biasing member is provided between the first spring holder and the output-side rotor to press the output-side rotor toward the second spring holder.

Abstract: A crankshaft structure includes a crank pin section, a crank journal section, and a counter weight section arranged between the crank pin section and the crank journal section in a direction of a crankshaft. A crank arm section connecting the crank pin section and the crank journal section is formed with each of lateral edge portions extending substantially along a circumscribed tangential line connecting a base of the crank pin section and a base of the crank journal section when viewed in the direction of the crankshaft. Each of the lateral edge portions of the crank arm section includes an edge groove having a length extending between a central axis of the crank pin section and a central axis of the crank journal section when viewed in a direction to cross the crankshaft. The edge groove has a groove bottom portion formed between two reinforcing ribs.

Abstract: A crankshaft of an internal combustion engine includes a crank journal, a crank pin, and a crank web. The crank journal is to be supported by a crankcase and to be rotatable around a crankshaft axis. The crank pin has a crank pin axis substantially parallel to the crankshaft axis to support a connecting rod which is rotatable around the crank pin axis. The crank web is provided between the crank journal and the crank pin to connect the crank journal and the crank pin. The crank web has a hole which extends from one side face of the crank web and passes in a vicinity of a connection section between the crank pin and the crank web when viewed in an axial direction along the crankshaft axis.

Abstract: A crankshaft for an internal combustion engine can be an integrated crankshaft in which a thrust receiving surface is formed on each of crank arm parts around the root of a crankpin in an annularly protruding manner to restrict axial movement of a connecting rod. A recessed portion is formed in each of the facing surfaces of facing crank webs at a position near a corner portion continuing from the crankpin to the crank arm part, the position being so close to the crankpin that part of the thrust receiving surface is cut away.

Abstract: A crankshaft structure includes a crank pin section, a crank journal section, and a counter weight section arranged between the crank pin section and the crank journal section in a direction of a crankshaft. A crank arm section connecting the crank pin section and the crank journal section is formed with each of lateral edge portions extending substantially along a circumscribed tangential line connecting a base of the crank pin section and a base of the crank journal section when viewed in the direction of the crankshaft. Each of the lateral edge portions of the crank arm section includes an edge groove having a length extending between a central axis of the crank pin section and a central axis of the crank journal section when viewed in a direction to cross the crankshaft. The edge groove has a groove bottom portion formed between two reinforcing ribs.

Abstract: A bolt fastening structure for an internal combustion engine includes bolt fitting holes formed in a cylinder of the engine. An inner wall of a top portion of a crankcase includes boss portions having openings, as an extension to the corresponding bolt fitting holes, with internal threads formed therein. The crankcase further includes hollow recesses formed thereon between the boss portions and crankshaft rotational support portions. Fastening bolts, each having an external thread formed at a tip portion thereof, are extended through the bolt fitting holes, and engaged with the internal threads of the boss portion. The hollow recesses include portions of the openings of the boss portions having the internal threads opened therein.

Abstract: A bolt fastening structure for an internal combustion engine includes bolt fitting holes formed in a cylinder of the engine. An inner wall of a top portion of a crankcase includes boss portions having openings, as an extension to the corresponding bolt fitting holes, with internal threads formed therein. The crankcase further includes hollow recesses formed thereon between the boss portions and crankshaft rotational support portions. Fastening bolts, each having an external thread formed at a tip portion thereof, are extended through the bolt fitting holes, and engaged with the internal threads of the boss portion. The hollow recesses include portions of the openings of the boss portions having the internal threads opened therein.