Early Rolls-Royce Merlins under construction at Derby.

The British airframe industry would have been nothing without the magnificent Rolls-Royce Merlin aero-engine. The Merlin was a liquid-cooled, V-12, piston aero engine of 27-litre capacity. Rolls-Royce Limited initially designed and built the engine as the PV-12 - PV standing for ‘Private Venture’.

The PV-12 first ran in 1933 and, after a number of modifications - and a name change to ‘Merlin’ following RR’s convention of naming their engines after birds of prey - the first production variants were built in 1936. A series of rapidly applied developments brought about by wartime needs markedly improved the engine’s performance and durability.

Considered a British icon, the Merlin was one of the most successful aircraft engines of the World War II era, and many variants were built by Rolls-Royce in Derby,  Crewe and Glasgow, as well as by Ford of Britain at their Trafford Park factory, near Manchester. The Packard V-1650 was a version of the Merlin built in the United States. Production ceased in 1950 after a total of almost 150,000 engines had been delivered.


Merlins were built up on stands that could be rotated for ease of access. Some stands allowed for the engine to be completely inverted.


Literally thousands of components went to make up a single R-R Merlin, from the smallest of washers to the large castings. Here the crankcase is fettled - that is cleaned after removal from the casting molds. Pre- and post-war Merlins were finished to a different standard than wartime engines, many of which were not polished at all, just painted black.

The Merlin II and III were the first main production versions of the engine. The Merlin III was manufactured with a ‘universal’ propeller shaft, allowing either De Havilland or Rotol propellers to be used.

The next major version was the XX which ran on 100 octane fuel. This allowed higher manifold pressures, achieved by increasing the boost from the centrifugal type supercharger. The Merlin XX made use of two-speed superchargers resulting in increased power at higher altitudes than previous versions.

The process of improvement continued, with later versions running on further-increased octane ratings,  delivering ever higher power. By the end of the war the ‘little’ engine was delivering over 1,600 horsepower in common versions, and as much as 2,060 horsepower in the Merlin 130/131 versions.

Many of the early Merlins were hand built by two-man teams - later they were built more more on production line principals.



From a contemporary report: ‘After being reassembled the engine is fitted to the test bed, again primed with hot oil, and run light for half an hour. Three electric starts and one hand start are then made- to check magnetos and a half-hour incremental running carried out up to maximum continuous cruising conditions on the same lines as the previous incremental test. During this time power and consumption checks are made to prove satisfactory functioning of the engine.

The engine is then run for 25 minutes at maximum continuous cruising conditions, followed by 5 minutes at maximum climbing conditions, single ignition checks are then taken.

The same reading of revolutions per minute and brake horsepower are taken every 15 minutes as during the endurance tests, and the engine is then passed through the same six check tests that were made at the end of the endurance test, and in addition to checking the power curve at rated altitude (100 octane fuel). The exhaust manifolds removed, carburation and each magneto checked for slow running, maximum speed 480 r.p.m. with brake set for maximum continuous cruising condition, then three accelerations are carried out.

The engine is then passed off and finally inspected, filters examined and oil drained from the engine. One engine in every ten must complete a rated boost curve with points at 3,000, 2,850, 2,600 2,400 and 2,200 r.p.m.

On final assembly all external locking devices are brought into use, namely, split pins, wire and locking plates. In the first place only internal and a few important external nuts and unions need be locked. Lastly, all external pipes are enameled and main units and accessories are touched up if necessary. After final testing, exhaust ports, sparking plug holes, air intakes and pipe connections are each blanked off and fresh oil poured into each cylinder head for protection. The engine is then ready for despatch’.


Later in the war Merlins were later built on completely rotatable stands on production lines that ran around the building.

‘Engines are delivered to the test-house by road, one vehicle transporting four engines, each mounted on a castor-wheeled cradle. In the receiving bay, they are unloaded by a hoist, controlled by push-buttons, on to an overhead runway, then wheeled to position for preliminary attention. Two operatives in about one hour remove all sealing blanks and fit sparking-plugs, exhaust stubs and gauge adaptors in readiness for testing.’



Merlins were tested in individual cells, each fitted out with an hoist and adjustable platform lift for erecting and dismounting the engine. The engine itself was mounted on a circular steel fixture some four feet in diameter restrained by steel cable looped around its periphery.

A number of Serck coolers - the three circular objects seen below the engine on test - provided temperature regulation for lubricating oil and engine coolant. Immediately to the rear of these coolers was a ‘depression box’ that was connected to the carburettor which through a shutter device, allowed the simulation of different altitudes to be made.


Staging was provided on both side of the engine so that adjustments could be made while the engine was being run - definately a draughty job for the technican!

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