A cutting tool the most suitable for use in boring a valve hole in the cylinder head of an engine. A groove (11) is formed along a generator of a generally conical tool main body (1). A slider (16) with a cutting blade tip (5A) is slidably mounted on the main body (1). A spacer (12) having a serration groove (12a) is detachably interposed between a wall surface (11a) of the groove (11) and the slider (16). A serration groove (16a) is formed in a side of the slider (16). The two serration grooves (12a, 16a) are engaged with each other. A biassing means (15) comprising a pressing pin (17) , a coned disc spring (18) and a set screw (19) is provided on a wall surface (11b) of the groove (11) so that the slider (16) is elastically biassed and pressed against the side of the wall surface (11a). Due to this, wear and deformation can easily be coped with only by replacing the spacer (12), and the looseness of the slider (16) is suppressed whereby the life of the tool can be prolonged and the maintenance labour can be reduced.
