of bearing surfaces 62 and 64 a central bore 70 extends axially of clutch shaft 34 one end of the central bore 70 is counter bored presenting a larger diameter bore 72 along the length of central bore 70 keyway 74 extends axially from one end of the clutch shaft 34 to the counter bore 72 at the end of clutch shaft 34 opposite counter bore 72 a pair of tapped bores 76 spaced at right angles to each other extend radially through bearing surface 74 to central bore 70 screw hub 60 has a central hub 78 having an internally tapped bore 80 extending axially thereof the screw thread of bore 80 is complementary with external thread 66 of clutch shaft 34 preferably the threads can be one of any known threading suitable for power transmission extending from opposite ends of hub 78 are flanges 82 presenting a substantially planar frictional surface 84 the inside corner between flange 82 and hub 78 is illustrated as having a weld 86 however hub 78 and flanges 82 can be manufactured as an integral unit sprocket 36 has an axially extending central bore 88 having at each end of the bore a counter bore 90 bearing 92 is placed within counter.