and the opening 60 defined in the finger block 50 a slide block 62 is movably mounted within the insert support 22 between the plates 38 and 40 and is guided on the insert support by a pair of guide pins 64 fig4 attached to the block 44 the guide pins 64 include sleeves 66 which are slidably received within bearing sleeves 68 defined in the block 62 fig4 and in this manner the block 62 is slidably mounted upon the insert support 22 for movement in a radially direction with respect to the feeler 20 and the openings 58 and 60 and the bearing and pin sleeves ensure accurate radially movement of the slide block 62 while preventing movement in the axially direction of the feeler 20 the movable block 62 includes plates 70 defining groove 72 receiving the fasteners 74 which adjustably locates the movable finger block 76 upon the slide block 62 by loosening positioning and tightening of the fasteners 74 the finger block 76 may be radially positioned upon the slide block 62 as desired the finger block 76 includes the axially extending radially movable finger 78 having a cross sectional configuration which will be appreciated from fig7 which includes the outer cylindrical surface 80 and the inner cylindrical surface 82 and the inner surface has a diameter substantially corresponding to the diameter of the feeler 20 at its end the finger 78 includes a convex rib 84 having a configuration best appreciated from fig5 and 8 which is of an annular segment form having an outer diameter greater than the diameter 80 and substantially equal to the fixed finger diameter of surface 54 and the convex configuration of the rib is of a form corresponding to.