A header for a harvesting machine having a frame, a cutterbar mounted on the frame operable to cut standing crop, the cutterbar being adjustable relative to the frame in a fore-and-aft direction, and a reel rotatably supported between a pair of reel arms pivotally mounted on the frame and operable to sweep crop material towards and over the cutterbar, a hydraulic cylinder extending between each reel arm and a corresponding side wall of said header frame to effect a generally vertical movement of said reel, comprising an interlock mechanism operably interconnecting the cutterbar and the hydraulic cylinder at each side of the header for adjusting the vertical position of the reel in response to a fore-and-aft movement of the cutterbar to maintain substantially constant the spacing and avoiding interference between the reel and the cutterbar.