In an internal combustion engine, an angle defined by an axis of a boundary of an intake port adjoining a valve seat and a cylinder axis is greater than an angle defined by the axis of the valve seat and the cylinder axis. A step between a first inner wall and a second inner wall, the first inner wall being a portion of an inner wall of the valve seat that is closest to the cylinder axis and the second inner wall being a portion of an inner wall of the boundary of the intake port that is closest to the cylinder axis. The first inner wall is located on the outer side in the radial direction of the intake port than the second inner wall.

First intake valve and second intake valve for an engine cylinder bore within which a piston reciprocates are arranged side-by-side in a cylinder head for motion in unison along respective intake valve axes. First intake valve axis is nonparallel to the cylinder bore axis. A first intake valve seat for the first intake valve is disposed in a plane which is non-parallel to a plane which is perpendicular to the cylinder bore axis. First intake valve seat has a first semi-circumference toward a second semi-circumference of a second valve seat for second intake valve. At least a portion of first semi-circumference is closer, as measured along a direction parallel to the cylinder bore axis, to a plane passing through the piston perpendicular to the cylinder bore axis than is the second semi-circumference as measured along a direction parallel to the cylinder bore axis.

First intake valve and second intake valve for an engine cylinder bore within which a piston reciprocates are arranged side-by-side in a cylinder head for motion in unison along respective intake valve axes. First intake valve axis is nonparallel to the cylinder bore axis. A first intake valve seat for the first intake valve is disposed in a plane which is non-parallel to a plane which is perpendicular to the cylinder bore axis. First intake valve seat has a first semi-circumference toward a second semi-circumference of a second valve seat for second intake valve. At least a portion of first semi-circumference is closer, as measured along a direction parallel to the cylinder bore axis, to a plane passing through the piston perpendicular to the cylinder bore axis than is the second semi-circumference as measured along a direction parallel to the cylinder bore axis.

An opening/closing valve structure for an engine is provided with a valve body for opening and closing an intake passage or an exhaust passage of the engine; a pair of bush members mounted on axial ends of the valve body; and a shaft member axially passing through one of the bush members and projecting from the bush member by a predetermined length within one end of the valve body. Each of the bush members is mounted in such a manner that a part of the bush member is axially received in an end of the valve body and the remaining part thereof axially projects from the end of the valve body. A portion of the shaft member projecting from the one of the bush members is connected to the valve body in such a manner that relative rotation of the shaft member with respect to the valve body is disabled.