No embolisms here. Spitfires were designed to be used for 50 hours (if they got that lucky) and thrown away. And an engine which mostly worked that long was all that was needed. Oil leaks? Immaterial. Don't you know there's a war on? (Of course, the same can be said for Messerschmitts, or any other aircraft of the day. Use them until you lose them, then get another and keep going.)
From the article: a quote from “Mass Producing the Merlin” by Paul H. Becker in Flying Magazine, March 1946.
“The secret of the this low-cost, high-production manufacturing is the assembly line. Rolls-Royce manufactures a 'fitter’s' engine with parts being brought to the unit under construction on a bench. If the part doesn’t fit, it is machined until it does meet required specifications.
“An American assembly line reverses this procedure. A conveyor belt brings the engine to the parts which always fit for by American methods all parts are made so precisely that they are always interchangeable.”
The Rah-rah Americanism is excusable. The difference in production methods is what's important to describe the difference between the two versions. Rolls used highly-trained skilled fitters to build each engine in one spot and make the parts fit if they didn't at first. Packard used minimally-trained production-line workers to put one part on the engine as it came past them and moved on to the next worker who put one part on and so on until the engine was complete. They were forced to come up with ways to make each part identical before assembly. This is the great strength of the assembly-line process which evolved in America. You know that the parts will fit because the machine they're made on is set to make them that way.