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A factory part breaks or wears out. Independant shops I have used insist on replacing with the same OEM part when reputable aftermarket parts are available at less cost. Given hard evidence of particular factory part unreliability is in hand, it seems logical to replace with a part from a second source and the "OEM only" practice appears dubious.

Why would an independent shop prefer OEM parts?

I ask because GM, for example, calculates their parts are only reliable to just 36k miles. It seems customer/shop enthusiasm for OEM parts exceeds the that of the analysts in the factories' own warranty department. Is there something motivating the independant shop business other than preceived quality (easier to order?, faster delivery, distributor floats payment or extends credit?, higher priced parts allows more markup?). Or are independant shops really have data and expertise to do a proper lifespan analysis, (with numbers) that show OEM parts last longer?

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I haven't worked in or managed a shop, but I have been told in the past that work would not be warranted if I wanted to use my own parts, or if I wanted the shop to try and get cheaper parts. Extrapolating from that...

For the shop to trust the work they do, they have to put their trust in all of their suppliers. There are many many after-market companies making parts. Some are super cheap copies with no engineering work behind them that will fail quickly, and some are performance parts that are much much better than OEM, but unless the shop has used a certain supplier several times and trusts them, they will be hesitant to put it in a customer car. They just don't know what's good and what's junk for all the suppliers of all the cars they work on.

OEM is inherently trusted because by definition, its what was already installed on the car. It's what a dealer would use for warranty work, and they know that the manufacturer has vetted the product.

There's a saying in the computer world: No one ever got fired for buying IBM (or HP, or Oracle, etc.). Its the same principle. OEM parts can be trusted. They might not be the cheapest, but they are known to work.

Of course there are exceptions, and like you say, some OEM parts are known to be terrible and some cheap aftermarket parts could be much better, but as a general rule, its safer for the shop to stick with OEM.

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My evidence is likely anecdotal at best, however..

For the longest time I went with aftermarket OE-equivalent parts because I'm cheap. After buying a high strung performance car (and having a wholesale hookup at the dealership), I moved over to proper OEM parts. The difference is astounding.

Gaskets have generally been paper from Felpro, Beck/Arnley, etc.. Many OEM gaskets are metal, some with rubber on them, some multi-layer. For one of my cars, the OEM water pump outlasts aftermarket OE-equivalent pumps by 30-60 thousand miles or more.

If, as you say, an OEM part is known to suck or an OE-equivalent part is known to be superior, a basic judgement call should be made factoring price/expected life. Most reputable shops will make this call for you. Many will ask in the event that personal preference is the only difference.

A counter-argument would be "if it ain't broke, don't fix it." If the original part lasted 130k miles, why would you gamble with a different manufacturer?

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