As an automotive parts retailer, we get a lot of questions regarding the definition of "OEM" and what that means with regards to a particular part. What you know already this is a straight-forward question, dilemma is, folks use the term different ways.
Let's first start out with a few potential synonyms which you might see. These include OES, OEM, Original, Genuine to name a few. OES (Original Equipment Supplier) is normally the same thing as OEM (Original Equipment Manufacturer). We come up with a distinction between these two and Genuine that we will explain later.
First, when it concerns how parts are given and delivered on a vehicle when new. Manufacturers do not make parts themselves, but rather they sub out to specialty manufacturers like Brembo, Lemfoerder, VDO etc. When BMW requires a new control arm because of their latest car, they get the design and then contract with a supplier like Lemfoerder to make the part for them. In cases like this, Lemfoerder is the OEM/OES for this part. The part is shipped to BMW, and they also put the part on when manufacturing the automobile.
Once a car hits the streets, and may that part need replaced, there are many alternatives for folks typically available. First, they could go to their dealer, and buying a "Genuine" part, which is a part manufactured by the OEM, but supplied through the dealer supply chain. The parts will have a perfect fit since they were built to precisely the same specs as the new parts from the same manufacturer.
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As a second option, they can seek out the part from the aftermarket space as a possible OEM. This is the exact same part as the Genuine, simply not in a box labeled by the auto manufacturer. Sometimes they are in OEM branded boxes, sometimes they are white boxed. This is sometimes is due to contracts where the manufacturers are prohibited from distributing under their name through the auto manufacturers. Buying these parts when available can save sometimes 50%-70% off the dealer price.
The final option is to find the product inside the aftermarket. In this space, manufacturers make an effort to create a part that will be a direct replacement for the main part, and they are available in varying levels of quality. Here is the most difficult area to assess a particular item. There is this type of variety of aftermarket goods that range from parts which are far superior to the OEM unit, to people who many times will not even install properly. It is best to stick with major manufacturers when going this route. This is not intended to steer folks faraway from this potentially lucrative option. There are lots of quality aftermarket parts found through the product lines. Many of these are made by other primary suppliers for the auto manufacturers. For instance, BMW typically uses ZF and LUK as manufacturers of steering pumps for his or her cars. One particular manufacturer wins the bid for any specific model, but that doesn't stop the other from reproducing exactly the same item and selling it exclusively in the aftermarket.