I believe that overstates the Rolls Royce built Merlin manufacturing issues. And they were coming off 3 production lines located in Derby, Crewe and Glasgow. While Derby & Crewe relied on many external subcontractors to produce components, Glasgow manufactured virtually everything on site. That would have made quality control much easier to achieve.
Then consider that when Ford of Britain was brought into the Merlin programme in May 1940 (first Ford engine appeared in June 1941), they redrew the Merlin blueprints to much finer tolerances to allow their mass production techniques to be applied. They concentrated on bomber versions and produced over 30,000 engines before production ceased in March 1946.
en.m.wikipedia.org
Henry Ford turned down the opportunity to manufacture Merlins in the USA. But Ford of Britain was a separate listed company, despite Ford USA being the major shareholder, that had a very large degree of independence from Ford USA.
Production at the various factories was:-
Derby - 32,377 from
Crewe - 26,065 from 1939
Glasgow - 23,675 from Nov 1940
These factories also went on to build Griffons alongside the Merlins later in the war.
About 12,000 of the Spitfires came from the Castle Bromwich Aircraft Factory, located in the heart of the industrial Midlands and the pre-war British motor industry. There were plenty of skilled metal workers in that region able to feed that factory. And production methods were refined to incorporate unskilled labour.
Discover Castle Bromwich Aircraft Factory: its pivotal Spitfire and Lancaster production, wartime challenges, and legendary pilots who shaped WWII.
www.warwickshireias.org