problems were cylinder head cracking, coolant leaks, and excessive wear to the camshafts and crankshaft main bearings. Early engines The prototype, developmental, and early production engine types were the: PV-12 The initial design using an evaporative cooling system. Two built, passed bench type testing in July 1934, generating at equivalent. First flown 21 February 1935. Merlin B Two built, ethylene glycol liquid cooling system introduced. "Ramp" cylinder heads (inlet valves were at a 45-degree angle to the cylinder). Passed Type Testing February 1935, generating at equivalent. Merlin C Development of Merlin B; crankcase and cylinder blocks became three separate castings with bolt-on cylinder heads. First flight in Hawker Horsley 21 December 1935, at . Merlin E Similar to C with minor design changes. Passed 50-hour civil test in December 1935 generating a constant and a maximum rating of . Failed military 100-hour test in March 1936. Powered the Supermarine Spitfire prototype. Merlin F (Merlin I) Similar to C and E. First flight in Horsley 16 July 1936. This became the first production engine, and was designated as the Merlin I. The Merlin continued with the "ramp" head, but this was not a success and only 172 were made. The Fairey Battle I was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936. Merlin G (Merlin II) Replaced "ramp" cylinder heads with parallel pattern heads (valve stems parallel to the cylinder bore axis) scaled up from the Kestrel engine. 400-hour flight endurance tests carried out at RAE July 1937; acceptance test 22 September 1937. It was first widely delivered as the Merlin II in 1938, and production was quickly stepped up for Fairey Battle II. Merlin III Merlin II with standardised de Havilland/Rotol SBAC propeller shaft, and dual accessory-drive. at 3,000 rpm at at +6.5 lb boost. Formed basis for the Rolls-Royce/Rover Meteor tank engine "Racing" Merlin Racing engine for 1937/38 "Speed Spitfire" world speed record attempt. Merlin III with strengthened pistons, connecting rods, and gudgeon-pins, running on increased octane fuel, developed at 3,200 rpm and +27 lb boost, a power/weight ratio of 0.621 lb per horsepower. Completed 15-hour endurance run at , 3,200 rpm at +22 lb boost. Merlin IV Merlin with pressure-water cooling for Armstrong Whitworth Whitley IV. Merlin V Merlin for Fairey Battle V. Merlin VIII Medium-supercharged Merlin developed for Fairey Fulmar I, rated at 2,850 rpm at ,