From a
Key.aero thread:
Toward the latter end of the Merlin's military/civil life, some variants of the engine had been developed to a very high degree of longevity. Engine developed for the civil DC-4, the Tudor and the Argonaut for example - 600 & 700 series Merlins. Engines where longevity was of more importance than ultimate performance.When Transport Heads and banks are indicated it means that the cylinder blocks, cylinder heads, cams and operators are to this later specification and give an aspiration of longer life than normal over a stock Spitfire or Mustang engine.
Posted by Mark12
Transport heads were originally developed for the Avro York fitted with Merlin 24’s. These were the same engine as fitted to the Lancaster from about 1944 onwards. These were the most powerful of the engines fitted to the Lancaster 1 and were mirrored in the Lancaster III with 224’s from Packard. However the York had a greater life expectancy than the Lancaster and RR developed the 24T with improvement to the heads in particular to give a longer valve life. Up to this point development had concentrated on strengthening the pistons, improving the supercharger and better lubrication of the bottom end to improve performance.
Post war the Merlin 24T formed the basis of the 500 series that found its way into the Spanish built CASA CA2111. The 600 and 700 series engines were based on the 100 series that powered the latter Mosquito’s and Hornets, with vastly improved lubrication and were also 2 speed 2 stage engines. They were built exclusively for the civil market. It has been stated that the development effort expended on the Merlin post war was as large as that from 39 to 45, such was RR desire to gain a foothold in civil aviation with the Merlin.
Posted by TempestNut