The reservoir cap is a sealing component for the coolant reservoir in a motorcycle's cooling system. It is designed to maintain the correct pressure within the reservoir, prevent coolant leaks, and protect the cooling system from contaminants. Typically made of durable plastic or metal with a rubber or gasket seal, the cap ensures a secure closure and allows for proper expansion and contraction of the coolant. It often includes a pressure relief valve that releases excess pressure to prevent damage to the cooling system.