The PRC uses only two units of measurement, "Universal Metric" and the "Shizhi". "Universal Metric" is use for domestically produce foreign export and the "Shizhi" for domestically produced domestically used products, including "Military". The Shizhi is based on units of 16 and not 10 like universal metrics, with an added oddity that their Screw Pitch is Left-Handed, not Right-Handed. So anyone Foreign Nation State is forced to buy replacement parts from the PRC. As an example, in 2013 Cuba had to outsource to North Korea for replacement parts for their aging fleet of MiG-21's, because the Russian Federation stopped producing them and the parts made in the PRC wouldn't fit because of their Left-Hand pitch screws. I suspect the Soviets had the same problem, when trying to reverse engineer the three B-29's that fell into their hands, because the American Aircrews were forced to land in Neutral (Not Fighting the Japanese) Soviet Union. Though the Soviet Engineers were forced literally at gunpoint to make exact copies of the B-29's in their possession, and didn't have the US-design manufacturing equipment to produce them, Or producing them (i.e. "a close approximation as possible") while using Universal Metrics. Which is probably why only 470 of them (i.e. Tu-4's) were ever produced...