Napier Javelin

The Napier Javelin was a British six-cylinder inline air-cooled engine designed by Frank Halford and built by D. Napier & Son.[1] First flown in March 1934 in the prototype of the Percival Mew Gull racing aircraft, the engine was also used in the Spartan Arrow biplane and the Percival Gull.[2]

Type Piston aero-engine
Manufacturer D. Napier & Son
First run 1932
Major applications Martin-Baker M.B.1 (one aircraft only)
Percival Mew Gull


The six cylinders were arranged inverted and inline. Unlike the similar de Havilland Gipsy Six the poppet valves were operated by a single gear driven overhead camshaft.


Javelin I

Initial version producing 150 hp.

Javelin III

A longer stroke and increased capacity version of 160 hp.

Javelin IIIA

A Mark III with a starter motor, generator and other improvements for the Martin Baker MB.1.


Specifications (Javelin I)

Data from [1]

General characteristics

  • Type: 6-cylinder inverted inline air-cooled engine.
  • Bore: 4.5 in (114.3 mm)
  • Stroke: 5.25 in (133.35 mm)
  • Displacement: 501 cu in (8.21 L)
  • Length: 58.5 in (1486 mm)
  • Width: 26 in (660.4 mm)
  • Height: 31.5 in (800 mm)
  • Dry weight: 410 lb (186 kg)


  • Valvetrain: Overhead cam poppet-valves
  • Fuel system: Twin updraught carburettors
  • Fuel type: Petrol
  • Cooling system: Air-cooled
  • Reduction gear: Direct drive, left hand tractor


See also

Comparable engines

Related lists



  1. Lumsden 2003, p.173.
  2. Lumsden 2003, p.174.


  • Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.
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