HPI Optional Front Belt Tensioner for the Nitro RS4, RS4 2, Racer, and Nitro Mini Car Kits. This Tensioner installs on the upper deck where the belt passes through and adjusts to increase efficiency. Constructed of anodized purple aluminum.Maintains proper pressure on belt for more efficient power delivery to front wheels. This product includes one front belt tensioner for the HPI Nitro RS4, Racer and Nitro Mini with: One A960a Belt Tensioner Holder. One A960b Pilot Shaft 4mm x11mm. Two A960c 4mm x 10mm x 8mm Washers. One A960d 3mm x 6mm Hex Head Screw. Two Z242 E-clips.Two B017 4mm x 8mm x 3mm Ball Bearings and instruction sheet.