The Napier Javelin was a British six-cylinder inline air-cooled engine designed by Frank Halford and built by Napier & Son. 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.
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.
- Initial version producing 150 hp.
- A longer stroke and increased capacity version of 160 hp.
- A Mark III with a starter motor, generator and other improvements for the Martin Baker MB.1.
Specifications (Javelin I)
Data from 
- 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
- Comparable engines
- Related lists
- ^ a b Lumsden 2003, p.173.
- ^ Lumsden 2003, p.174.
- Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.