Interesting as the six page discussion on stresses in engine blocks is, the answer to the question that started this thread is simple and straightforward and explained in the second post, a case of not comparing like with like.
Three types of Merlin, all from the XX series, were used by the Lancaster Mk I. Merlin XX, 22 and 24. Aircraft fitted with Packard built Merlins were always given a different Mk number.(1) In the case of the Lancaster, when fitted with Packard Merlins it was called a Mk III. Aircraft built under licence outside of the UK were also given their own Mk number and a Lancaster Mk X is essentially the Mk I built in Canada. As you might expect the Mk X also used Packard Merlins. The Mk IIIs and Mk Xs used Merlin 28, 38, and 224, Packard built versions of the XX, 22 and 24 respectively.(2) Engine performance differs according to the source used but generally the Packard Merlin is said to have been slightly more powerful compared to the RR equivalent (see the attachment). Also they were well liked by ground crews because they came with an extensive tool kit. So the difference in performance is due to different variants of the Merlin and not to where, how or by whom they were made. There is a touch or irony here as most Lancaster Mk Is had Merlins built by Ford not RR. However, unlike Packard Merlins, it was difficult to tell if a Merlin was built by RR or Ford. The RAF did not own a single Merlin. They were all owned by RR and leased to the RAF. They had a shelf life of five years or 500 hours after which they were returned to RR and rebuilt as new. Ford did not sell any of the +30,000 Merlins it manufactured at Manchester to anyone other than RR. When RR leased them to the RAF it ensured that no 'Ford' markings appeared on the engine. If it had been different I am sure that, like the Packard Merlins, there would be comments that the ones with the blue ovals weren't quite as good as the genuine RR. As nobody could tell the origin nobody noticed a difference.
The notion that while Packard could only mass produce some sort of inferior clone of the Merlin engineering masterpieces coming out of the RR factories is risible. RR factories at Derby, Crewe, and Glasgow as well as Ford's factory at Manchester were all mass production, using similar naïve work forces as Packard. It was in fact Ford that showed RR how to mass produce the Merlin, and they were built to Ford standards, with much of the machinery to do it designed, built, and supplied by Ford.
When the Ford Motor Company Ltd opened its new factory at Dagenham in 1932, the Chairman, Sir Percival Perry, and his successor Rowland Smith, made sure the Air ministry were aware of their production facilities and expertise. Sir Wilfrid Freeman, Commandant of the RAF staff college at Andover became a friend and encouraged students from the college to visit Ford as part of their management training. Not long after the outbreak of war, Freeman, then the Assistant Chief of Air Staff, suggested that Ford should be asked to build, equip and manage a factory for their own style of production line to produce Merlin engines. The factory would be owned by the government and a licence arranged for the Merlin XX series.
Perry received Freeman's letter on 22nd October 1939. He immediately contacted Rowland Smith and the two of them met Freeman in Whitehall. A team was assembled and on 2nd November they went to Derby to meet Lord Hives the chairman of RR and his production manager H.J.Smith. The two sides already held each other in high esteem and worked well together from the start. RR offered Ford everything other than any actual manufacturing assistance. By mid-November, Smith told Freeman the cost for building and equipping the shadow factory would be in the region of £7,000,000. It was immediately accepted. The actual cost was £6.6 million. An 84 acre site at Eccles in Manchester was selected. Building began in March 1940. Smith was appointed as Controller and part of the former Ford factory at Trafford Park was used for preparatory work by draughtsmen and tool-makers. It was during this period that RR received a shock. The events were described by Sir Stanley Hooker. (3)
"A number of Ford engineers arrived at Derby and spent some months examining the drawings and manufacturing methods. One day, their chief engineer said 'You know, we can't build the Merlin to these measurements'.
"I replied loftily 'I suppose that is because the drawing tolerances are too difficult for you, and you can't achieve the accuracy'.
"On the contrary, the tolerances are far too wide for us. We make motor cars far more accurately than this. Every part on our car engines has to be interchangeable with the same part on any other engine, and hence, all parts have to be made with extreme accuracy, far closer than you use. That is the only way we can achieve mass production'."
In the following silence, one of Hooker's colleagues asked what Ford proposed. He was told complete new drawings were required, to Ford standards. It took about a year to produce the 20,000 drawings. Hooker went on to say that this led to an enormous success with Merlins coming out "like shelling peas, and very good engines they were too".
In the meantime machine tools that were a vital part of Ford's construction methods had to be designed and built and a new workforce created and trained in a new philosophy of work. Tools to make the tools that made the components had to be designed and procured from specialists. Every one of the companies capable of such work in the UK was working at full capacity. Much was not even available here at that time. A crankshaft machining tool was sourced in the USA. A U-boat put it on the bottom of the Atlantic and the same fate befell its replacement. The third finally got through. Boring equipment clearly labelled 'Ford Motor Company, Trafford Park, Manchester, England' .was shipped from Switzerland through occupied France and hostile Spain to Gibraltar and on to England. The Swiss firm warned the Germans that as a neutral country they were entitled under international law to trade with anyone, and any interference with this right would bring a halt to the supply of any similar equipment to Germany.
The first Ford designed machine tools were made as early as August 1940 and sent to the RR factory at Crewe. While the factory at Eccles was being constructed a shadow factory at Derby was set up alongside the main Derby works. 190 Dagenham employees were sent there and started proving the various machine tools and manufacturing processes. They also spent time working alongside Rolls workers in the main factory to become thoroughly conversant with all the parts of the Merlin. By September 1940 the first building for the new factory were complete. 2,300 w0rker were employed there that winter, including those that had trained at Derby. The whole factory was finished in May 1941 and the first five engines delivered in June. By 1943 Ford was making 200 a week. Ford's investment in machinery and management paid off handsomely. The 10,000 man-hours needed to produce a Merlin dropped to 2,727 in three years. The unit cost fell from £6,540 in June 1941 to £1,180 by the war's end. The percentage of engines rejected by the Air Ministry was zero. Not one engine of the 30,400 produced was rejected.
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(1) It is often said that this was because Packard Merlins were built to metric standards and required a completely different set of spares. However, those that restore and repair Merlins say that parts are interchangeable.
(2) Ford in the USA was given the opportunity of building Merlins there. Henry Ford passed up the chance, expressing the opinion that Britain was unlikely to win the war. Packard seized the opportunity and their Merlins were produced on a similar basis to the Ford versions built in Britain, but with Stromberg carburettors and one or two other differences such as silver-lead-indium bearings and a different supercharger drive.
(3) Not Much of an Engineer, Crowood Press.