A replacement diaphragm for a fuel pressure regulator. The diaphragm is the component that does the actual regulating, flexing against spring pressure to open and close the bypass, and it is also the part that wears. Ethanol blends and long service intervals eventually harden or perforate it, and the symptoms are easy to recognise: pressure that will not hold steady, a base setting that drifts, or fuel appearing where it should not. Fitting a fresh diaphragm restores proper control for a fraction of the cost of a new regulator, and it is a straightforward workshop job. Worth keeping as a spare if you run alcohol fuels, which are harder on diaphragms than petrol.