provided with an upper rounded end 74a and a lower rounded end 74b the drive slots 73 and 74 extend 90 u00b0 about the lower portion 69b of drive cam 69 the extent of drive slot 73 and the extent of drive slot 74 are indicated in fig6 by arrows c and d respectively reference is again made to fig2 in fig2 it will be noted that the lower end 36b of valve stem 36 is threadedly engaged in the threaded axial bore portion 70a of drive cam 69 the lower portion 69b of drive cam 69 is received within the bore portion 58 of coupling 56 the coupling 56 near its upper end is provided with a pair of diametrically opposed perforations 75 and 76 see fig7 through 9 a drive pin 77 joins the upper end of coupling 56 to the lower portion 69b of drive cam 69 by passing through coupling perforations 75 and 76 and drive cam drive slots 73 and 74 to complete the structure the yoke 27 is provided with a second pair of radially extending bosses 78 and 79 having internally threaded bores 80 and 81 respectively the threaded bores 80 and 81 are adapted to receive guide pins 82 and 83 respectively as in the case of cam pins 67 and 68 guide pins 82 and 83 are in the form of hex head bolts having forwardmost ends of slightly lesser diameter and free of threads the.