Working in the automotive business, I deal daily with the dilemma of whether or not to use OEM or after-market parts in the engines and transmissions we repair or rebuild. The main concern of choosing between after-market or OEM is whether or not the quality of the original part will be represented in the after-market part. There are many after-market companies out there that do produce quality merchandise, buy the consumer needs to be aware that some parts, such as pistons and liners, may be made with an inferior grade metal. This has been true from products manufactured in Mexico. The metal there is an inferior grade to that produced in other countries, especially in Europe. Injectors have also had issues; as the after-market company orders an OEM injector from a dealer, they copy the design, and mass-market it at a cheaper price to the consumer. What we have seen is that as the OEM upgrades their parts, using different or stronger metals, better designs, etc., the after-market is sometimes behind on the current design of a part. Consumers beware.
On the bright side, there are many after-market parts available, and a myriad of companies to choose from when ordering your parts. There are times when parts have an enumerable amount of lead time from an OEM, and this is where after-market is your friend. Many parts meet or exceed the criteria of the OEM. It is a buyers market when it comes to parts, and the companies producing them know that. They are constantly upgrading, and trying to keep up on the current design of OEM parts, though as discussed above, this is not always the case. The buyer must be aware of this fact, and choose carefully the parts they want from an after-market. If the consumer is a repair shop, the best route to go is to receive sample parts from an after-market company, and compare them to the OEM. This could save the small shop thousands of dollars a year, but taking the time to research the after market, and making sure the parts meet the criteria of the OEM.
OEM manufacturers are in the business to make money. They are trying to hold the captive market on their respective auto lines, and sometimes charge outrageous prices for very common parts. They will change part numbers constantly to try and throw the after-market off balance. They are trying to monopolize on their business, which is understandable.
After-market can be your friend, your headache, or your saviour. Take the time to research what you are looking for, especially the expensive parts on an engine. You could save tons of money by finding the right after-market suppliers.