Spark unit for combustion-powered driving tool

A spark unit (10) for an internal combustion fastener-driving tool comprises a spark plug body (32) mounted within a boss portion (30) of the tool cylinder head (12), and a spark unit cathode electrode (34) is mounted within the spark plug body (32) in such a manner that the tip portion (36) of the spark unit cathode electrode (34) projects below or away from the bottom or undersurface portion (38) of the cylinder head mounting boss (30). In this manner, the tip portion (36) of the spark unit cathode electrode (34) is not disposed within a recess or pocket region so as not to tend to accumulate dirt or oil deposits which can tend to foul or otherwise adversely affect the sparking and ignition characteristics of the spark plug assembly and the operation of the fastener-driving tool.