A header (12) is provided for an agricultural harvester (10). The header (12) comprises a plurality of row units (20, 420). Each row unit comprises deck plates (136, 436) defining a crop receiving gap (40, 440) therebetween, a drive sprocket (52) rotatably carried by the row unit (20, 420), a driven sprocket (54) rotatably carried by the row unit (20, 420), wherein the driven sprocket (54) is spaced from and movable relative to the drive sprocket (52), a gathering chain (48, 448) extending between the drive sprocket (52) and the driven sprocket (54), and an adjustment unit (170, 470) for engaging the gathering chain (48, 448). An adjustment mechanism (405) is operatively connected to the adjustment units (170, 470) and operable to move the adjustment units (170, 470) toward and away from the drive sprockets (52) to adjust a distance between the drive sprockets (52) and the driven sprockets (54). A linkage assembly (409) includes a plurality of linkages connecting the adjustment unit (170, 470) and the adjustment mechanism (405). Movement of the adjustment mechanism (405) between first and second positions moves the adjustment units (170, 470) between a first position wherein the driven sprockets (54) are at a first distance from the drive sprockets (52) and a second position wherein the driven sprockets (54) are at a second distance from the drive sprockets (52).
