Hoping that this is not becoming an argument as my intent is only to discuss what I consider to be a myth. I have been involved in the automotive repair trade since 1960 and began repairing 2-stroke go kart engines around 1963. My first motorcycle in 1965 was a Suzuki 80cc, K11 (?I think?). I hold both Canadian Interprovincial Journeyman Automotive Mechanic and Journeyman Parts certifications so have some exposure to both sides of the repair industry. Even someone as unobservant as I often am is able to notice some things in that time.
A great example is that of the small frame Nippon Densu alternators used on Honda ST1100, ST1300, Suzuki Bandit, etc. Parts are readily available for these little guys through aftermarket because Honda does not make these alternators. Honda will not even supply most of the parts, presumably because there would be insufficient sales volume. Same part, far less money out of the real OEM boxes from Nippon Densu. One can also buy good quality parts for these alternators from other makers.
Check brake calipers on many vehicles for replacements and the jobber will ask what make are those installed. Why? This is because the auto maker used one of several suppliers who actually made the parts.
The Stant thermostat found as factory installed in your typical automobile is not as good quality as a better one which they will sell in the aftermarket.
Another aspect of parts selection is that of pattern failure. In many cases some issue in terms of lifespan, failure rate, or performance arises. Aftermarket makers often provide an improvement in order to addres issues but OEM tends not to provide such options. Given that this is mainly an automotive/truck option because of the tiny motorcycle market scale, but it still is true.
It's fun to be challenged in some of these areas and hope no one is upset.
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