Aragon
member
That's not true.
First, Rolls-Royce's drawing were complete and accurate. Second, it was not Packard, but Ford (UK Ford).
Some time in mid 1939, the UK Air Ministry asked Ford (UK) if they could make the Merlin after showing them Rolls-Royce's drawings in a meeting with the AM and RR. They said "no, those drawing are not to your standards."
The problem was not that the drawing were incomplete or lacking information. They had numerous places were things were noted as "FILE TO FIT" or "HAND LAP". Ford felt that improved tolerancing callout would improve interchangeability and speed production. Ford (UK) redrew the drawing along with RR engineers the establish good production tolerances and simplified a few areas for mass production.
Those were the drawing that were given to Packard. Packard may have redrawn them to meet their needs, but the heavy lifting was already done, and the Merlin was produced at Packard without much difficulty. They were given the plans in Sept 1940, and the first Merlin XX rolled off the line in August 1941. That's pretty good.
In addition Ford (US) either could not or would not build this engine in the US. There are a number of potential reasons for this. Ford was already involved in a huge amount of military contracting for the war effort. Henry Ford I was also becoming a bit more "unstable" (and testy) due his advanced age (his battles with Gen. Wm. Knudsen were well known) and FoMoCo's expertise was simply better put to use elsewhere.