For an authoritative source see Jack Bruce’s British Aircraft 1914-1918. See pages 449-450:
“Unfortunately the 200hp Hispano-Suiza was dogged by ill luck; consequently the SE5a was not as effective as it might have been. Really large scale production was still some way off, owing to difficulties in obtaining engines. At the insistence of the Admiralty the Air Board had ordered 8000 H-S engines in November 1916, principally from the Mayen concern in France. This firm built a large factory for the purpose, using some £2 million advanced from the British Government, but the first engines did not come off the lines until late 1917. Meanwhile, production of the 200hp engine was undertaken in England by the Wolseley company; and other H-S made in France by Brasier were fitted to SE5a’s.
The Wolseley-built engines were not at first successful …………. this serious situation was further aggravated by the unreliability of the first 200hp engines delivered from the French firm Brasier. Failures were frequent and the engines had to be sent to the Clement Talbot works for overhaul.”
There is much more about this but in the end “the Admiralty’s foresight in 1916 saved the situation, which had become very ugly indeed; for in the early months of 1918 the first deliveries of Mayen-Built Hispano-Suiza’s were made against the order.”
Of course this does not say that these engines were the most reliable.
Peter,
Whilst you do not state the reason for your query I suspect that it may be due to an article in the April issue of The Automobile magazine. David Burgess-Wise (another eminent historian) wrote of the career of Pol Ravigneaux; who, amongst many other accomplishments, was head of design at the Emil Mayen company from 1916-1918;
“This was at a period when…….The Admiralty had insisted that the Air Board order 8000 engines from French manufacturers……especially Emil Mayen, whose engines had proved particularly reliable.”
I don’t know where to find details of contracts or delivery records, let alone reliability problems. Perhaps D B-W would tell you more if you contacted him via the magazine,