The selective positioning of row units along a planter tool bar is facilitated by introducing an elongated channel of generally C-shaped cross-sectional configuration intermediate the tool bar and a plurality of row unit head brackets. The channel is welded or otherwise fixed to the tool bar with the open edge of the “C” facing away from the tool bar and the head brackets are fixed to the channel by clamping nut and bolt assemblies. The bolts extend through the clamping brackets and engage corresponding clamping nuts. The clamping nuts are positioned within the “C” and may be slid along the inner edge of the channel to any preferred position. The bolts are then and tightened fixing the head bracket at the selected at location.