I received my copy of "The Canadair North Star" by Larry Mileberry. It is an excellent read and very informative. A considerable section covers the choice and operating experience of the Rolls Royce Merlin 620 series as the powerplant.
The reasons for choosing the Merlin are clearly laid out and are persuasive. It offered better power at the higher cruising altitude desired than the Pratt R-2000 or even the R-2800. That being said, operationally the choice turned out to be not as good as expected.
First was the sheer noise. Even with the crossover exhaust system which only came along years after the aircraft went into service, it was still noisier inside than than the DC-6. The system did reduce it almost to tolerable levels.
Second, the engine was not as durable as the radials for the day in and day out slog of commercial service. I use the book which quotes authoritative sources. Ultimately, TCA was forced to reduce the power setting in cruise which negated the original reasons for the choice.
The TBO of the Merlin was not up to that of the Pratts. Spare usage and costs, particularly for pistons, rings and cylinder liners was particularly high. Fortunately for Trans Canada (TCA), Rolls had entered into an agreement with TCA known as "Won't be sorry" that committed Rolls to ensuring that the Merlin spares cost would not be no more than those for the radials. In effect, Rolls subsidized the use of the Merlin. This agreement obviously paid off in later years with the choice of the Dart for the Viscounts, Tyne for the Vanguards, Conways for the DC-8 and RB-211 for the L-1011.
It is unclear if BOAC had a similar agreement for their Argonauts and certainly the later operators would have paid heavily.
This does not imply that the Merlin was a bad engine for the North Star as they and their Argonaut cousins did yeoman service all over the world. It was just more expensive an engine to operate.
One final tidbit clearing the matter of reversible props. TCA and the RCAF North Stars did not have reverse pitch props fitted. BOAC's Argonauts did. This explains why some commentators have one experience and others a different one. Depends on who the original operator was.
I recommend the book to anyone wanting insight into a great aircraft and engine.
I still remember being a young teen and leaning over the railing on the observatin deck at KLGA and seeing the elegant North Stars taxiing in and out. They had an announcer in those days on a PA who claimed that American and TCA had daily races from YYZ down to NYC. American with their DC-6 and TCA of course with the North Stars. Sheer hype of course but great fun.