the body of the cap will be tightly held by cap retainer 45 in an arcuate shape below head 15 fig1 referring also to fig1 a guide bar 49 is secured to each sidewall 51a of a downward facing channel member 51 channel member 51 has an upper surface 51b and is secured to saddle 13 forming a part of the upper surface and sides of saddle 13 each guide bar 49 protrudes outward and has an upper and a lower surface each guide bar 49 is a rectangular member that extends a substantial distance along saddle 13 parallel with saddle axis 19 referring to fig3 and 4 two sets of upper bearings or rollers 53 will engage the upper surface of each guide bar 49 in rolling contact one set of lower bearings or rollers 55 locates below each guide bar 49 and engages the lower surface of each guide bar 49 in rolling contact the engagement of rollers 53 55 with guide bars 49 allows cap driver 27 to freely roll along saddle axis 19 however rollers 53 55 prevent any upward and downward movement of cap driver 27 relative to saddle 13 as shown in fig3 in the preferred embodiment each set of.