It has been generally believed, that all of the “H” configured engines manufactured by D Napier & Son Ltd were attributable to their Consultant Designer Major Frank Halford. First there was the small 16-cylinder air-cooled Rapier being referred to by Montague Napier as “my little “H” engine”. This was followed by the much larger 24-cylinder air-cooled Napier-Halford Dagger well after Napier’s death in January 1931, and undeniably a Frank Halford “H” production lasting until 1942 with the RAF.
However the flat “H” of the massive 24-cylinder water-cooled Napier Sabre engine with sleeve valve cylinders was a quite different creature. Its origin dates from the 1929/30 period and came from a designer then residing in Cannes, France. It was in fact Montague Napier himself who designed his E96 ‘H’ Type four-stroke 24-cylinder compression ignition (diesel) engine with a 5.3/4“ bore x 5.1/2“ stroke. This was at a time when the Halford/Napier contractual design agreement specifically limited all Halford engines to a maximum cubic capacity of just 11.8 litres. Indeed when Montague started developing his “H” engine there was discussion with Halford about this resulting in him being paid £4,000 to amend the contract so NAPIER could develop this engine.
Very little information exists about the E96 engine design however the Napier Company minutes between 1929 and 1930 reveal that it was intended for marine or stationary use. Drawing office and development expenditure was authorised under Sanction No. 2230 eventually amounting to £5,500 and Sanction No. 2240 £500 towards developing a two cylinder test engine (around £500,000 in today’s money!). This development was shelved in August 1930 when the Napier Board took the decision to concentrate on developing the E97 straight 6-cylinder Napier Javelin engine for civil aviation purposes. . . for the time being.
Following the unexpected death of Montague Napier on 22nd January 1931 the new Napier Board ensured the E96 development continued by reducing the E96 capacity whilst retaining the Napier patented worm sleeve drive mechanism. The new E101 24-cylinder C.I. engine now had the identical 5” bore and 4.3/4” stroke to that which also formed the petrol fuelled E107 Napier Sabre engine of 1935 onwards.
Two and six cylinder test units (E101T and E101/6T) were built followed by a full 24-cylinder version. In keeping with the company’s tradition (since the Napier Rapier) of naming it’s aero engines after swords this new engine was named the Napier Sabre. Development work was slow and the design and the Air-Ministry realised that it was a long way behind the Junkers Jumo 204 & 205 6-cylinder opposed piston engine designs. They “persuaded” the DNS Board to abandon the E101 project in favour of building the Jumo 204 engine under licence (Napier Culverin).