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Rolls-Royce Merlin 25 Engine

The Rolls-Royce Merlin engines fitted to the Lancaster should be the Merlin XX series. However, the BBMF Lancaster operates using Merlin 25, 225 and 500 engines. These are very similar in specification, and we will take a closer look at the Merlin 25.

The Merlin 25 is a 12-cylinder, 60°, V12 liquid-cooled engine. It has a compression ratio of 6:1 and its dry weight is 1,430lb (687 kg).

The two-piece cylinder block is cast from aluminium alloy. There are six cylinder liners in each block, manufactured from high carbon steel. Each piston has three compression and two oil scraper rings. The connecting rods are nickel steel forgings, machined to H-sections. The one-piece six-throw crankshaft is machine forged from chrome-molybdenum steel. The crankcase is cast from aluminium alloy. Two inlet and two exhaust valves are fitted to each cylinder head and each exhaust valve has a sodium-cooled stem.


The two cylinder assemblies, mainly the right-and left-hand blocks, are known as the A and В blocks respectively. Each comprises six cylinders, the upper camshaft drive unit and the camshaft and rocker mechanism, which operates the valves incorporated in the cylinders. Each block consists of a separate alloy skirt, head and six detachable wet steel liners which, when bolted together, form the cylinder block proper. In addition to providing part of the coolant jacket, the head also forms the roofs of the six combustion chambers.

Cylinder liners

Each cylinder liner is shouldered and spigotted at the upper end to enter its respective recess in the bottom of the cylinder head. The cylinder liner is also provided with a sealing ring at its lower end to form a joint with the crankcase, to which it is drawn by the cylinder holding-down studs.

Cylinder block covers

The covers are secured to their respective cylinder blocks by studs and nuts, with a gasket between the two contacting faces. The main difference between the A and В covers is that the latter incorporates an engine speed indicator drive whilst the former is plain.


There are two inlet and exhaust valves per cylinder. Both valves are of the trumpet type and have satellite-ended stems. The valve guides are cast iron for the inlet and phosphor-bronze for the exhaust. The valves are not interchangeable. Each valve guide is pressed into its respective bore in the cylinder block until a conical collar near its top end is seated on, and is flush with, the roof of the cylinder block.


A single central camshaft for each cylinder block is mounted in pedestal brackets and operates both inlet and exhaust valves through rockers fitted with adjustable tappets. The camshafts, which are similar for both blocks, are driven from the wheel case by inclined shafts ending in bevel pinions, which mesh with bevel wheels at the end of each camshaft.

Camshaft auxiliary drive

The air compressor and hydraulic pumps for the turrets are mounted on the rear ends of the A and В cylinder heads. Both are driven from the spur gear wheels attached to the camshaft-driven bevel wheels.


The pistons are attached to the connecting rods by fully floating gudgeon pins. The connecting rods are of the forked and plain type, the forked rod being fitted to the B-side of the engine. The pistons are machined from light alloy forgings and are fitted with three compression rings above and one grooved scraper ring below the gudgeon pin. The gudgeon pins are made from hollow steel and are retained in the piston by spring wire circlips. A pair of oil holes are drilled obliquely and upwards towards the centre to meet in the metal above each gudgeon pin bore to assist in cooling the piston.


The crankcase consists of two halves bolted together. The upper portion incorporates the rear half of the reduction gear housing at its front end, and has a facing and spigot at the rear end to which the wheel case is attached. The lower portion incorporates the scavenge pumps, filters, and the mounting for the hydraulic system pump. This is only fitted to the number 2 and 3 engines.

Reduction gear

The reduction gear is of a single-spur layshaft type and is housed partly in a casing bolted to the front end of the crankcase and partly in the crankcase itself. The casing is located on the crankcase by means of a spigot concentric with the propeller shaft. The driving pinion is concentric with, and is driven by, a short coupling shaft from the crankshaft. All these shafts run in roller bearings, the propeller shaft having, in addition, a thrustable ball bearing, enabling the engine to be used with tractor (pulling) or pusher propeller. A dual-drive unit is fitted to the reduction gear cover for the purpose of driving the propeller constant speed unit (CSU) and vacuum pump unit, which are both bolted to it. The vacuum pump is used for the operation of certain navigational instruments.

Wheel case assembly

The wheel case is bolted to the rear end of the crankcase and carries the magnetos, coolant pump, generator drive, electric turning gear and fuel pump unit. It houses the spring-drive unit and certain shafts through which the magnetos, camshafts, electric generator, fuel, oil and coolant pumps are driven.

Coolant pump

The coolant pump is of the bottom-fed centrifugal type and serves each cylinder block from a separate outlet.

Fuel pump

The fuel pump unit, mounted on the facing on the port side of the wheel case, consists of two separate pumps operating in parallel. Each pump is capable of operating independently of the other and each is of sufficient capacity to supply more than the maximum amount of fuel required.

Electrical generator drive

The electrical generator is mounted upon a cast light alloy right-angled bracket that is located on the left-hand side of the crankcase.


The two-speed, single-stage, liquid-cooled supercharger is of the high-speed centrifugal type, embodying a semi-shrouded impellor which is driven from the rear end of the crankshaft through a two-speed gear.


The SU twin-choke up-draught type Merlin carburettor fitted to this engine is, with the exception of the separate boost control unit, entirely self-contained and is arranged to be fully automatic in its functioning, with the result that the responsibilities of the pilot are reduced to their simplest form and the danger of engine damage resulting from incorrect setting is reduced.

The SU type Merlin AVT40/193, or 214 carburettor, embodies two complete carburettors in one unit. The flow of fuel through the left-hand carburettor main jet is directly controlled by the induction pressure-operated mixture control, whilst the right-hand jet is directly controlled by the atmospherically operated mixture control. Both these mixture controls are automatically controlled through linkwork by separate aneroid capsules incorporated on each side of the carburettor. The upper half of the carburettor casting is mounted on studs on the lower face of the supercharger intake elbow. A large specially shaped gasket is used at the joint faces to provide a gas-tight and fuel-tight joint.

Both the vertical intake passage and throttle barrels are coolant jacketed, these jackets being included in the coolant return from the В cylinder block to the inlet side of the coolant pump. A system of oil-heated throttle valves is also included to prevent freezing and to allow for a portion of the scavenge oil being directed through the hollow interior of the throttle valves before it is returned to the oil tank.

Ignition system

The ignition system comprises two magnetos, which are attached to the wheel case, one on the left, and one on the right. Attached to these are the high-tension wiring harnesses for the spark plugs, which are metal screened and serve a dual purpose by acting as a collector for the induced field around the high tension wires and returning the resulting electrical current to earth, and also preventing radio interference. Each cylinder has two spark plugs - one magneto provides the spark for the inlet-side plugs and the other for the exhaust-side plugs. This ensures that if one magneto fails, the engine will still run using the other.

Engine lubrication system

There are four oil circuits in the lubrication system of the engine: the main pressure feed circuit; the low pressure feed circuit; the front sump scavenge circuit; and the rear sump scavenge circuit. The main and lower circuits are served by a single pressure pump and suitable relief valves, while each scavenge circuit is served by a separate scavenge pump.

Engine lubrication system

There are four oil circuits in the lubrication system of the engine: the main pressure feed circuit; the low pressure feed circuit; the front sump scavenge circuit; and the rear sump scavenge circuit. The main and lower circuits are served by a single pressure pump and suitable relief valves, while each scavenge circuit is served by a separate scavenge pump.


  • "AVRo Lancaster. 1941 onwards (all marks)" /Owners' Workshop Manual. Jarrod Cotter and Paul Blackah/