Caprotti valve gear

Given their need for more complex and expensive maintenance, most of these locomotives were generally withdrawn from service before those with Walschaerts valve gear, in the 1960s.

It was found that the Caprotti-fitted locomotives were more economical on coal and water than those with Walschaerts valve gear, but it was later found that some of the losses of the Walschaerts locomotives was due to leakage of steam past the valve heads, where a single wide ring was used.

[2] In the 1950s, Caprotti valve gear was improved and this British Caprotti valve gear was fitted to the last two British Railways-built 'Black Fives' 44686/7, the last 30 BR Standard Class 5s, numbers 73125-54, and the unique BR Standard Class 8 71000 Duke of Gloucester.

Although more expensive to manufacture than its rivals, the improved Caprotti valve gear is considerably more efficient than any other.

A major improvement is that much of the mechanism is enclosed, leading to reduced wear and tear from the harsh steam locomotive environment, and completely independent control of admission and exhaust.