It may be worth explaining that the decision to design a one piece block, to replace the troublesome ramp head on the Merlin I, was made because Rolls-Royce had a good understanding of the one piece block design on the Kestrel. This was essentially scaled up and improved for the Merlin, even though it had some known weaknesses they were not as bad as the ramp head and, the redesign work could be achieved in a relatively short timescale. The brave decision to go for the simpler but faster fix of the one piece block in 1937 meant that enough Merlin II and Merlin III engines were there in time for the RAF to be able to win the Battle of Britain in 1940.
Ultimately, the two piece cylinder and head assembly allowed good reliability at very high powers. The Packard designed two piece block was used on their early versions although, they also reverted to the later Rolls-Royce designed two piece block after its introduction later in the war.
OAP