An impact mechanism for use with a drive motor, said impact mechanism comprising: a drive engaging member for engaging a rotatable output of a drive motor for rotation therewith about a longitudinal axis; a tool bit retaining member operatively inter-connected with said drive engaging member for rotation with respect to said drive engaging member about said longitudinal axis, said tool bit retaining member having a main body portion, an anvil portion securely attached thereto for co-rotation with said main body portion, and a tool bit retaining means securely attached thereto for co-rotation with said main body portion; a hammer member mounted on one of said drive engaging member and said tool bit retaining member for movement between an anvil contact position whereat force is transmitted from said hammer member to said anvil portion so as to create a moment about said longitudinal axis, and a release position whereat said hammer member is temporarily removed from said anvil portion; guide means for moving said hammer member between said anvil contact position and said release position when said drive engaging member is rotated with respect to said tool bit retaining member; spring means operatively interconnected between said drive engaging member and said hammer member for biasing said hammer member to said anvil contact position; and, a selectively adjustable spring compression mechanism, for permitting selective compression of said spring means; wherein, in use, rotation of said drive engaging member about said longitudinal axis causes said hammer member to move from its anvil contact position towards its release position, thereby storing potential energy in said spring means; and, wherein, when said hammer member reaches said release position, said hammer member is forcefully propelled by said spring means and the rotation of said drive engaging member to impact on said anvil portion, thus urging said tool bit retaining member to forcefully rotate about said longitudinal axis; and characterised in that said selectively adjustable spring compression mechanism comprises an annular main body member having an internal thread and a manually grippable portion, the annular main body member threadibly engaged on a co-operating external thread on said drive engaging member, and said spring means is operatively interconnected between said annular main body member and said hammer member, to thereby permit selective compression of said spring means through rotation of said manually grippable portion. a drive engaging member for engaging a rotatable output of a drive motor for rotation therewith about a longitudinal axis; a tool bit retaining member operatively inter-connected with said drive engaging member for rotation with respect to said drive engaging member about said longitudinal axis, said tool bit retaining member having a main body portion, an anvil portion securely attached thereto for co-rotation with said main body portion, and a tool bit retaining means securely attached thereto for co-rotation with said main body portion; a hammer member mounted on one of said drive engaging member and said tool bit retaining member for movement between an anvil contact position whereat force is transmitted from said hammer member to said anvil portion so as to create a moment about said longitudinal axis, and a release position whereat said hammer member is temporarily removed from said anvil portion; guide means for moving said hammer member between said anvil contact position and said release position when said drive engaging member is rotated with respect to said tool bit retaining member; spring means operatively interconnected between said drive engaging member and said hammer member for biasing said hammer member to said anvil contact position; and, a selectively adjustable spring compression mechanism, for permitting selective compression of said spring means; wherein, in use, rotation of said drive engaging member about said longitudinal axis causes said hammer member to move from its anvil contact position towards its release position, thereby storing potential energy in said spring means; and, wherein, when said hammer member reaches said release position, said hammer member is forcefully propelled by said spring means and the rotation of said drive engaging member to impact on said anvil portion, thus urging said tool bit retaining member to forcefully rotate about said longitudinal axis; and