Abstract: A chain saw is provided in which the tension of a cutting chain extending along a periphery of the guide bar may be adjusted without use of a tool. A locking knob is provided which engages with a cover of the chain saw. Protrusions of the locking knob fit within corresponding notches of the cover. The shape of the protrusions and the notches allow rotation of the locking knob in a first direction but not a second direction. Upon disengaging the locking knob from the cover, the guide bar may move relative to the engine housing and clutch cover to tighten the cutting chain extending along a periphery of the guide bar.