Thule TB60 - Top Tracks are a bolt through custom application to create a multi-purpose rack system on fiberglass truck bed caps. The tracks are designed to allow the bars to be easily repositioned for various cross bar spreads up to 55 inches. The tracks are made of black anodized aluminum that resists corrosion. Bolts and capped nylon insert lock nuts are included. To create a weather tight seal, silicone, plastic gasket and nylon washers are included as well. See product manual for installation instructions.