Dayco HD Serpentine Belt Kits Extending the time between service intervals, Dayco’s heavy-duty serpentine belt kits – which cover millions of applications – are OE-engineered to perform in the most demanding environments where safety and productivity are essential. Each Dayco kit includes all the necessary components to ensure the belt drive system performs optimally – most importantly, its ability to handle higher torque and power pulses. Each kit includes one (1) premium tensioner – either the original flat spring tensioner or the new 2-piece tensioner, two (2) heavy-duty dual bearing pulleys, and one or two (1 or 2) diesel-grade belts treated to increase flexibility, reduce rib cracking, and to run cooler.