Caprotti gear was the most extensively applied and probably the best poppet valve gear ever evolved. It was developed in Italy and first allied to a locomotive by Ing. Arturo Caprotti in 1921, hence its name.
From that date numerous applications were made over practically the whole world.
Most of the leading European countries used this gear, including France, Spain, Italy, Germany, Romania, Austria and scattered across the world one could find many Caprotti locomotives in such places as Argentina, the USA, South Africa, Egypt, India, Pakistan, Malaya and Thailand.
In Gt Britain, the L.M.S.R. gave 10 engines a trial on a conversion of the 4-cylinder Claughton class and the LNER on 2 engines of the Lord Farringdon class (B3).
The L.M.S.R. came near to achieving with the Claughton's 100k miles between shopping’s,
after World War 2, when the L.M.S.R. were endeavouring to obtain over 100k miles between shopping’s. This was remembered, and the gear was chosen for 20 engines Stanier Black 5’s. No's 44738 - 44757.
By War time the British team had been developing the gear to eradicate earlier weaknesses of the Italian design, These Black 5’s settled down to some good steady work proving capable of sustaining the required mileages between shopping’s.
The gear was developed still further and the last two of 842 Stanier Black 5’s No's 44686 and 44687 were built (an exhibit of the revised valve gear was shown at The Festival of Britain in 1951). This new version of British Caprotti design was outstanding.
As the result of the success of these two engines the gear was used on 30 B.R. Standard Class 5's, No's 73125 – 73154 ( preserved No. 73129 ), but in 1954 it found its most famous application on the largest and most powerful of the new B.R. Standard range, Class 8P, named Duke of Gloucester. No. 71000 (now preserved).