During difficult driving conditions, your Mx5's cooling system becomes highly pressurized. Stock caps are not equipped to handle higher pressure and that causes fluid to leak into the overflow reservoir. The Mishimoto High-Pressure 1.3 Bar (19 psi) Radiator Cap does an excellent job of keeping the coolant where it needs to be, flowing within the system and provides a safer and more efficient cooling system. Ergonomically designed for easy removal and installation, this radiator cap comes with Mishimoto Lifetime Warranty.