Because US assumed Chinese copies of anything-they-could-lay-hands-on, US embargoed much, under COCOM rules, until Nixon chose to open contact, 7/71. UK did not take the same line, having recognised PRC de jure (HKG-sensitivity). So UK was able to deliver Viscount (1963) and Trident (1972), but must remove embargoed US content. China chose not to spend on copying those, but bought a massive upfront spares pack (the sole 8-engined Viscounts), as they in turn assumed we would sell-and-forget, like USSR had done.
No.1 707-320B went straight to (later: SAIC), 8/73, not delivered to the airline for several years; its JT3D-7s were removed into the Shanghai Engine Factory; interesting components were sent to specialist sites, all to be reversed by folk who had done just that after Mao and Khrushchev fell out, 5/60 and all USSR men and most data vanished overnight. They did reproduce much inc. H-5 Badger, its engines and components, self-sufficient even unto silica for transparancies.
Y-10 was...inspired by 707-320B; PRC tried hard but failed to reproduce JT3D-7 or such components as Sundstrand CSD, because Western kit differed from USSR at levels of material specifications and tolerances with which PRC was unfamiliar. Efforts were abandoned when the Party judged Westerners would happily sell much for money. Aerospatiale showed the way on Super Frelon (Z-8). At very great expense the Party (then dominated by Shanghainese) 3/85 made an MDC MD82 licence for the plant which had produced 2 Y-10s, assuming the airline parastatal CAAC would be enthusiastic to take $-sparing product. Instead they sought anything-but-local-produce: like BOAC/BEAC, Chinese airlines wanted to buy proper aeroplanes from firms that understood Product Support. Seattle and Long Beach answered the telex 24/7.
