nut 71 the link mechanism 70 can be composed of a link 72 and a link piece 78 the link 72 will be described below with reference to an example of link 72 comprising a first link piece 73 and a second link piece 75 which are slidable relative to each other so as to be contractable longitudinally thereof the first link piece 73 is pivoted to the resin nut 71 and tiltable forward or rearward a slide pin 74 projects from the first link piece 73 in the vicinity of a base end thereof the second link piece 75 comprises a pair of members holding the first link piece 73 therebetween and each having a slot 76 extending longitudinally thereof the slots 76 of the second link piece 75 have slidably fitted therein the slide pin 74 of the first link piece 73 further as shown in fig6 the second link piece 75 has a pin 75 a projecting from the forward end thereof a spring 77 extends between and is engaged with the slide pin 74 and the pin 75 a the spring 77 biases the second link piece 75 toward the resin nut 71 when free of any load the second link piece 75 is pulled closest to the resin nut 71 by the spring 77 the link piece 78 i e third link piece 78 is supported by the pin 75 a of the second link piece 75 the third link piece 78 is bent forward at its midportion and has one end secured to the crankshaft 62 when the.