These 11mm fuel injector O-rings are made from Fluoroelastomer and are necessary to properly seat the injector.  The Fluoroelastomer o-rings are the perfect choice for demanding sealing applications with resistance to many aromatic hydrocarbons, i.e. - E85

These O-rings are available in 11mm and 14mm diameters and sold individually and in sets of 4 or sets of 8.