The Merlin engine was the main power source for the aircraft used by the RAF throughout WWII. During this period the engine was continuously developed to improve its performance and to suit the operating envelopes of the aircraft that it was fitted to.
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Mark |
Cylinder Block | Pistons, Connecting Rods and Crankshaft | Crankcase | Reduction Gear | Wheelcase | Supercharger | Carburetter and Boost Control |
| II | D.8985 and D.8986. One-piece with narrow inlet ports. Ribbed liner with 0.15 in. spigot. Radiused top joint ring 0.08 in. thick. Unshrouded. May be modified to receive rectangular-sectioned top joint ring 0.100 in. thick. Partially shrouded. | Narrow piston rings, but later engines fitted with wide type piston rings. Centri-cast connecting rods on early engines. Later types fitted with loose shells with R.R. locating tangs. Replacements have Allison type locating tangs. Standard crankshaft. Repair engines as Merlin XX. | D.7445 fitted to early type engines had only one of the two reduction gear bosses drilled for propeller oil. D.10605 fitted to later type engines had both bosses drilled for propeller oil. | Ratio 0.477. D.10143 necked down propeller shaft fitted with full diameter thread. Dual drive unit optional. | Standard wheelcase. Early type engines fitted with Crypto starter gears, but later types incorporate epicyclic gearing. | Ratio 8.588 to 1 single-speed type with divided entry intake. Integral with upper half of carburetter. | A.V.T.32 type carburetter with upper half integral with supercharger. Open type progressive boost control |
| III | Early type engine as Merlin II.Later type engine as Merlin X; still later type engines as Merlin XII. | As Merlin II. Repair engines as Merlin XX. | D.108505 as fitted to later type Merlin II. Later type engines fitted with Merlin X type crankcase. | Ratio 0.477. D.10143 necked down propeller shaft fitted to early type engines. Later type engines fitted with universal propeller shaft D.11556. Dual drive unit also fitted. | As Merlin II, but all starters fitted with epicyclic gearing. | As Merlin II. | As Merlin II. |
| IV | As Merlin II. | As Merlin II. Repair engines as Merlin XX. | D.10605 as fitted to later type Merlin II. | Ratio 0.477. D10143 necked down propeller shaft fitted. Dual drive unit also fitted. | As Merlin III. | As Merlin II. | As Merlin II. |
| V | As Merlin II. | As Merlin II. Repair engines as Merlin XX. | D.10605 as fitted to later type Merlin II. | As Merlin IV. | As Merlin III. | As Merlin II. | As Merlin II. |
| VIII | As Merlin XII. | As Merlin II. Repair engines as Merlin XX. | D.11291 with combustion starter drive housing which incorporates oil relief valve fitted to early type engines. Later type engines fitted with Merlin XII crankcase D.13069. | Ratio 0.477. Combustion starter type reduction gear. Universal propeller shaft and dual drive fitted. | As Merlin III, but with epicyclic starter gear removed. Electric starter and gears can be fitted if required. | Ratio 6.313 to 1. Otherwise as Merlin II. | Carburetter as Merlin II, but is fitted with enclosed type progressive boost control. |
| X | Early type engines fitted with D.11521 and D.11522 blocks. One-piece, with wide inlet ports. Alternative types ribbed liner with 0.15 in. spigot. Rectangular section top joint ring 0.100 in. thick. Partially shrouded. Later type engines fitted with Merlin XII type block. | As Merlin II. Repair engines as Merlin XX. | D.9745 fitted incorporating strengthened side panels, deeper main bearing caps and with two dowel bolts instead of one on the centre cap. | Ratio 0.420. Propeller shaft as in Merlin III. | Deeper type to house the two-speed gears, otherwise similar to Merlin III. | Ratio 6.39 and 8.75 to 1. Two-speed type with divided entry intake. Integral with upper half of carburetter. Deeper front half casing. | As Merlin VIII. |
| XII | D.13256-1 and D.13257-1. One-piece, with narrow inlet ports. Ribless liner with 0.20 in. spigot. Radiused top joint ring 0.080 in. thick. Fully shrouded. | As Merlin II, but with improved driving bush in rear end of crankshaft. Repair engines as Merlin XX. | Early type engines as Merlin VIII. Later type engines fitted with D.13069 incorporating stiffened side panels, deeper cylinder stud bosses and longer cylinder studs. Combustion starter drive housing incorporates oil relief valve units. | As Merlin VIII. | As Merlin VIII. | Ratio 9.09 to 1. Otherwise as Merlin II. | As Merlin VIII. |
| XX | As Merlin XII. | As Merlin II. Later engines as Merlin XII with No. 4 journal sealed and two oil holes added. | As Merlin X. | As Merlin X. | As Merlin X. | Ratio 8.15 and 9.49 to 1. Two-speed type with central entry intake. Deep front half casing. Entirely separate from carburetter. | A.V.T.40 type carburetter. Entirely separate unit. Enclosed type progressive boost control. |
| 21 | As Merlin XII. | As Merlin XX. | As Merlin X. | As Merlin X | As Merlin X | As Merlin XX. | As Merlin XX. |
| 22 | Fitted with two-piece block, the head bolting on to the skirt. Incorporates new liners and top joint rings. Narrow inlet ports with brass inserts for setscrews securing manifolds. | As Merlin XX. | As Merlin X. | As Merlin X. | As Merlin X. | As Merlin XX. | As Merlin XX. |
| 30 | As Merlin XII. | As Merlin XII. Repair engines as Merlin XX. | As Merlin XII. | As Merlin VIII. | As Merlin VIII. | Ratio 8.588 to 1. Otherwise as Merlin II. | As Merlin VIII. |
| 32 | Fitted with two-piece block as Merlin 32. | As Merlin XX, but has strengthened connecting rods. | As Merlin XII. | As Merlin VIII. | As Merlin VIII. | Ratio 8.588 to 1. Single-speed type with central entry intake separate from carburetter. | A.V.T.40/213 type carburetter with diaphragm-controlled fuel supply system. |
| 45 | As Merlin XII, but later engines have front and intermediate camshaft bushes deleted. | As Merlin XX. | As Merlin X. | As later type Merlin III. | As Merlin III. | Ratio 9.09 to 1. Single-speed type with central entry intake. Entirely separate from carburetter. | As Merlin XX. |
| 46 | As Merlin 45. | As Merlin XX. | As Merlin X. | As Merlin 45. | As Merlin III. | As Merlin 45, but with larger diameter rotor, improved guide vanes and diffuser vane ring. | As Merlin XX but with larger diameter chokes. |
| 47 | As Merlin 45. | As Merlin XX. | As Merlin XII, but combustion starter drive used to drive cabin supercharger. | As Merlin VIII. Gears to suit cabin supercharger. | As Merlin III. | As Merlin 46. | As Merlin 46. |
| 50 | As Merlin 45. | As Merlin XX. | As Merlin X. | As Merlin 45. | As Merlin III. | As Merlin 46. | As Merlin 32. |
| 56 | As Merlin 22. | As Merlin XX. | As Merlin X. | As Merlin 45. | As Merlin III. | As Merlin 46. | As Merlin 32. |
| 60 | As Merlin XII. | As Merlin XX. | As Merlin XII, but combustion starter drive used to drive cabin supercharger. | 0.42 ratio, but with gears to suit cabin supercharger drive. | As Merlin X. | Two-speed two-stage with liquid cooled intercooler. | A.V.T.44 type with 4.5 in. dia. chokes (float type). New type boost control. |
| 61 | As Merlin 22. | As Merlin 32. | As Merlin 60. | As Merlin 60, but with modified gearing for cabin supercharger drive. | As Merlin X. | As Merlin 60, but with modified gear ratios. | Early type engines as Merlin 60 design. Later type engines fitted with A.V.T.44/209 carburetters having diaphragm controlled fuel feed system. |
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62 |
As Merlin 22. | As Merlin XX. | As Merlin 60. | As Merlin 60. | As Merlin X. | As Merlin 60. | As Merlin 60. |