A mechanical anti-backdrive chain tensioner includes a four-sided planar piston and plastic chain shoe assembly. A pair of helical compression springs of opposite helices urges the piston and shoe assembly into the chain, and a ratchet plate pawl prevents backdrive of the piston but is designed with a predetermined amount of backlash. The planar-sided piston permits a large engagement area with the mating ratchet plate pawl. The large pawl contact area of the planar piston reduces tooth stress and withstands much higher anti-backdrive loads than prior art tensioners. The outer helical spring preferably has an outer diameter equal to the diameter of a bore hole in the piston and an inner diameter equal to the outer diameter of the inner spring to produce frictional damping upon extension and compression of the springs.