The difference between a RR Merlin and a Packhard Merlin, RR the oil is on the outside, Packhard, the oil is on the inside, sort of like English and Japanese motor bikes. The American engine had better oil seals, the RR engines had felt seals, I used to have a book with all the modifications that were instituted for mass production.
There was a story in ME proberbly about 20yrs ago about someone who produced a domestic sewing needle drilled lengthwise, there was a bit more to the story, but I can't remember what it was.
As a nurse I'v stuck plenty of needles in people, but I never really thought much about the method of construction. Ian S C