 In contrast, aftermarket parts are those made by companies other than the OEM, which might be installed as replacements after the car comes out of the factory.For example, if Ford used Autolite spark plugs, Exide batteries, Bosch fuel injectors, and Ford's own engine blocks and heads when building a car, then car restorers and collectors consider those to be the OEM parts.Other-brand parts would be considered aftermarket, such as Champion spark plugs, DieHard batteries, Kinsler fuel injectors, and BMP engine blocks and heads.