In 1927 Montague Napier reached an agreement with Major Frank B. Halford to act as a consultant designer for D Napier & Son. Napier was determined to enter into the the market for air-cooled engines and Halford was a natural choice. Both Montague Napier and Halford believed that this was the way ahead due to the lightweight nature of the engine and having push rod operated overhead valves. Napier’s remit to Halford was for an air-cooled engine between 6.6 litres and 11.8 litres.
The Napier Javelin was the first engine Halford designed for the Company. This was similar to the ‘Cirrus Hermes’ and the ‘De Havilland Gypsy’, having an inverted six cylinder 8.2 litre giving 170bhp at 2,100rpm. The E97 Javelin I was introduced in 1932 but later redesigned with a longer stroke and capacity as the Javelin III