From Enginehistory.org
https://www.enginehistory.org/Collec...asteners.shtml
Packard Built Merlins
An often asked question is; "did Packard replicate the British thread system when they built Rolls-Royce Merlins under license during World War II?" The answer is yes; all threads that were used on the Merlin were accurately replicated by Packard. This would include BSW (British Standard Whitworth), BSF (British Standard Fine), BSP (British Standard Pipe) and BA (British Association). Having said that, however, Packard Merlins used U.S. built Bendix injection carburetors; PD-16 for single stage engines and PD-18 for two stage engines, both of which used U.S. Unified threads. British built Merlins employed S.U. carburetors using Whitworth threads. The job facing Packard when they undertook manufacture of the Merlin was daunting to say the least. It’s bad enough having to build a complex product like the Merlin but exacerbating the situation was the fact no tool maker in the U.S. made Whitworth taps or dies. Therefore, Packard were forced into making their own. Although this created a significant hurdle to overcome, the effort was well worth it, Packard and Rolls-Royce components were interchangeable.