A boxed set of nitrile O-rings, the seal used in O-ring boss adapters, filter housings, fuel rail fittings and countless other joints across an AN plumbing system. Nitrile is the standard general-purpose elastomer for petrol and oil service and is what most ORB adapters ship with as original equipment. Because O-rings take a set once compressed and go hard with age and heat, they should be renewed rather than reused whenever a fitting is disturbed, and having an assortment box on the shelf makes that practical instead of an excuse to refit an old one. Note that nitrile is not the ideal choice for methanol and some race fuels, where a different compound is preferable.