Hmm....Which pump do you think it is? Engine oil or hydraulic fluid?
The difference in O rings is not a matter of quality or technology, it's a matter of the different manufacturng standards in place for making something as simple as an O ring.
There are two basic standards. One standard is for METRIC O rings and another for the USA O rings in the inch system.
Both of them offer a huge selection of O rings. But they differ in their emphasis. To get an idea of just how large this O ring world is, take a look at:
McMaster-Carr
McMaster-Carr
The simple way to look at the problem between the two standards is that the METRIC standard allows for smaller increments in size, so a lot of times you simply cannot find a USA standard O ring that is a direct replacement. And sometimes close enough is good enough to not be a problem until the O ring encounters hot oil and high pressure.
Because it's not just a difference is not just in how they are measured. Remember that these O rings run in hot oil. Old mechanics will remember opening up machinery and finding O rings that had disintegrated under heat and pressure. It used to be a big problem.
The METRIC system took up the challenge by specifying a wider range of materials, elasticity (durometer), torsional strength, compressibility, and chemical compatability, as well as more temperature ranges. The METRIC world has whole separate standards specifically for aerospace and many other special applications. For special applications the USA O rings also offer different O ring materials, but not as many types, whereas the metric varies both material and tolerances within the material types they offer. Another way to say the same thing is that the metric O ring industry is very mature in the selection of sizes, tolerances, and materials. They make a hugely wider range of O rings available to the designer.
The USA O ring industry has focused more on special geometric design. That is where the US industry is argueably more mature. You saw some of that displayed as you looked through the McMaster web site above.
So when a mechanical designer in a metric based country like Japan sits down to design something like your YM240 pump, he can open up an O ring catalogue and pick a metric O ring that does just exactly what he wants. And since the Japanese O ring industry is mature and most of the world is metric, he knows he isn't going to have a problem with sourcing the exact O ring he wants to use. He is happy, his boss is happy, and neither one of them probably bothers to check if there is an exact cross reference to the USA O ring standard. Or if there is, if that O ring material is going to be able to stand up to synthetic oils and exotic additives.
Sometimes yes, sometimes not. But you are better off using an OEM O ring is you can find it. Gaskets are more forgiving.
rScotty
