Peter Stickney wrote:

snip

At about the same time, The Air Ministry
approached British Ford to second-source Merlins. This, in fact, was
done, after much grunting and swearing. (Rolls really didn't know much
about mass production, and the Merlin required a lot of work to build
on a high volume basis.)

Apparently British Ford had to completely redo the tolerances to permit mass
production. RR had very loose tolerances and depended on part
selection/hand fitting.

regards

jc