Radiator Cap Quection

I am checking the radiator pressure cap that is on my 230CE. It has the marking 120 which I think is the pressure rating. It has the part number 123 501 02 15.
Does anyone know what unit is? I am trying to convert it to PSI.
The 120 cap goes to about 22 PSI on the pressure tester.
Thanks
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I agree that the units that we use in the US suck. I am a part time machinist at my job. It would be so much easier and intuitive if we were on the metric system. For example if I wanted to tap for an 8-32 bolt I have to look at a chart to find what the tap drill size should be. Then go through one of three drawers (Number size, Letter size, and Fraction size) to get the correct drill. If I were working metric, using a 6 mm X 1.5 bolt I would subtract 1.5 mm from the 6 mm and know that the correct drill size is 4.5 mm. All the metric drills are lined up in one drawer. To make a clearance hole for the 8-32 I need to measure the diameter of the bolt then refer to the chart to find the right size drill. With metric I'll just grab a 6.25 mm drill.
When working with of-rings you need to refer to another chart using the US system. With metric you use the outside diameter and the cross sectional diameter. Last year I needed a metric of-ring and went to about ten auto parts stores and nobody had any. Just ones in the US system. This is 25 years after American cars converted to metric.
I could go on with more examples but do not want to belabor the point.
One of the engineers that I work with works in metric. He says it is so much better.
I think that the US could do better in science and engineering if we could learn and use the metric system because it is intuitive. None of this 32 ounce = one quart, 4 quart = one gallon crap. We expend too much energy trying to learn all this stuff by rote. Remember learning fractions in school? How many kids said "I hate fractions?" How many of them got turned off of math at this point?

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Commuter wrote:

Disclaimer: I'm in Holland and we're fully metric.
I did hear someone make a point once that 'forcing' children to learn hard things actually stimulates their intelligence. The point was made in regards to the Chinese writing system, which is of course extremely hard to learn compared to out alfabet. As you may know, Chinese have a higher IQ on average than westerners. Perhaps the illogical American measuring system serves a similar purpose. ;)
But then it's probably not hard enough...
X.
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