Abstract: An idler sheave is carried by a bracket pivoted in a mount and fabricated as a part of the gear box housing of the tiller and is movable toward and away from a drive belt extending between two spaced sheaves, one a driven sheave and the other a relatively smaller drive sheave. The mount is located in the joint between die-cast halves of the gear box housing. The idler sheave engages the belt in the area of the larger driven sheave. A tension spring connects the idler sheave bracket to a second bracket which is in turn associated with a control mechanism capable of pivoting the second bracket and, through the tension spring, causing pivotal movement of the first bracket to move the idler sheave against and tension the belt on the drive and driven sheave. When the manipulative force on the control mechanism is released, gravity causes the idler sheave to fall away from the belt, that movement being transmitted through the tension spring to the second bracket and to the control mechanism.