Abstract: Adjustment capabilities are useful in gearing mechanisms so that backlash settings between gears may be changed to accommodate for normal operation, wear, or maintenance disturbances. The subject backlash adjustment mechanism allows for adjustment of center distances between an adjustable idler gear (23) and a camshaft gear (24), while providing a geometric alignment (47,82) relationship to maintain a substantially fixed center distance between the adjustable idler gear (23) and another mating gear (22). The subject backlash adjustment mechanism (26) has the adjustable idler gear (23) rotatably mounted on an idler gear mounting (31,58) having a slotted hole (33,62), with the idler gear mounting (31,58) being releasably connected to a cylinder block (11).