A header for an agricultural harvester comprising: a plurality of row units (120, 220, 320, 420) each comprising: deck plates (136, 236, 336, 436) defining a crop receiving gap (40, 440) therebetween, a drive sprocket (152, 252, 352) rotatably carried by the row unit (120, 220, 320, 420), a driven sprocket (154, 254, 354) rotatably carried by the row unit (120, 220, 320, 420), wherein the driven sprocket is spaced from and movable relative to the drive sprocket (152, 252, 352), and a gathering chain (148, 248, 348, 448) extending between the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354), characterized in that each row unit (120, 220, 320, 420) further comprises an idler member (178, 278, 378, 478) for engaging the gathering chain (148, 248, 348, 448); an adjustment mechanism (182, 282, 405) operatively connected to the idler member (178, 278, 378, 478) and operable to move the idler member (178, 278, 378, 478) toward and away from the drive sprockets (152, 252, 352) to adjust a distance between the drive sprockets (152, 252, 352) and the driven sprockets (154, 254, 354); and a linkage assembly (180, 280, 380, 409) that includes a plurality of linkages connecting the idler member (178, 278, 378, 478) and the adjustment mechanism (182, 282, 405) wherein movement of the adjustment mechanism (182, 282, 405) between first and second positions moves the idler member (178, 278, 378, 478) laterally away from the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354) between a first position wherein the driven sprockets (154, 254, 354) are at a first distance from the drive sprockets (152, 252, 352) and a second position wherein the driven sprockets (154, 254, 354) are at a second distance from the drive sprockets (152, 252, 352), the second distance being less than the first distance. a plurality of row units (120, 220, 320, 420) each comprising: deck plates (136, 236, 336, 436) defining a crop receiving gap (40, 440) therebetween, a drive sprocket (152, 252, 352) rotatably carried by the row unit (120, 220, 320, 420), a driven sprocket (154, 254, 354) rotatably carried by the row unit (120, 220, 320, 420), wherein the driven sprocket is spaced from and movable relative to the drive sprocket (152, 252, 352), and a gathering chain (148, 248, 348, 448) extending between the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354), deck plates (136, 236, 336, 436) defining a crop receiving gap (40, 440) therebetween, a drive sprocket (152, 252, 352) rotatably carried by the row unit (120, 220, 320, 420), a driven sprocket (154, 254, 354) rotatably carried by the row unit (120, 220, 320, 420), wherein the driven sprocket is spaced from and movable relative to the drive sprocket (152, 252, 352), and a gathering chain (148, 248, 348, 448) extending between the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354), characterized in that each row unit (120, 220, 320, 420) further comprises an idler member (178, 278, 378, 478) for engaging the gathering chain (148, 248, 348, 448); an adjustment mechanism (182, 282, 405) operatively connected to the idler member (178, 278, 378, 478) and operable to move the idler member (178, 278, 378, 478) toward and away from the drive sprockets (152, 252, 352) to adjust a distance between the drive sprockets (152, 252, 352) and the driven sprockets (154, 254, 354); and a linkage assembly (180, 280, 380, 409) that includes a plurality of linkages connecting the idler member (178, 278, 378, 478) and the adjustment mechanism (182, 282, 405) wherein movement of the adjustment mechanism (182, 282, 405) between first and second positions moves the idler member (178, 278, 378, 478) laterally away from the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354) between a first position wherein the driven sprockets (154, 254, 354) are at a first distance from the drive sprockets (152, 252, 352) and a second position wherein the driven sprockets (154, 254, 354) are at a second distance from the drive sprockets (152, 252, 352), the second distance being less than the first distance. an idler member (178, 278, 378, 478) for engaging the gathering chain (148, 248, 348, 448); an adjustment mechanism (182, 282, 405) operatively connected to the idler member (178, 278, 378, 478) and operable to move the idler member (178, 278, 378, 478) toward and away from the drive sprockets (152, 252, 352) to adjust a distance between the drive sprockets (152, 252, 352) and the driven sprockets (154, 254, 354); and a linkage assembly (180, 280, 380, 409) that includes a plurality of linkages connecting the idler member (178, 278, 378, 478) and the adjustment mechanism (182, 282, 405) wherein movement of the adjustment mechanism (182, 282, 405) between first and second positions moves the idler member (178, 278, 378, 478) laterally away from the drive sprocket (152, 252, 352) and the driven sprocket (154, 254, 354) between a first position wherein the driven sprockets (154, 254, 354) are at a first distance from the drive sprockets (152, 252, 352) and a second position wherein the driven sprockets (154, 254, 354) are at a second distance from the drive sprockets (152, 252, 352), the second distance being less than the first distance.