Spark-ignition engine

A spark-ignition engine comprising:

• a plug hole (13) provided with a plug-attaching aperture (12) at an innermost portion thereof and formed in a cylinder head (2);
• an ignition plug (10) engaged in screw-thread relationship with the plug-attaching aperture (12);
• and
• a plug, cap (9) integral with an ignition coil and attached to the ignition plug (10) upon insertion of the plug cap (9) into the plug hole (13),
• wherein the plug hole (13) is arranged at a position outside a head cover (5) at the cylinder head (2), a head surface (2a) being opened to provide the plug hole (13) and having a plug-cap attaching flange (11) attached thereto, the plug-cap attaching flange (11) being formed with an extending cylinder-portion (15) having an extension-hole (15A) communicated with the plug hole (13), the plug cap (9), as inserted from the extension hole (15A) into the plug hole (13), having a coil portion (9C) fixed to the extending cylinder-portion (15), and
• wherein the plug-cap attaching flange (11) is attached to the head surface (2a) at a position spaced-apart from the head cover (5), the extending cylinder-portion (15) being spaced-apart from the head cover (5) and extending upwardly from the plug-cap attaching flange (11), the extension hole (15A) of the extending cylinder-portion (15) being longer than the plug hole (13), and the coil portion (9C) of the plug cap (9) being fixed to an upper end portion of the extending cylinder-portion (15) at a position spaced-apart from the head cover (5).