BR Caprotti Black Fives

In 1947, as part of an experimental programme by George Ivatt to try to improve the already good design, (4)4767 was built with Stephenson link motion.

[1] The last two of the 842 LMS Stanier Black Fives, numbers 44686 and 44687 were constructed by British Railways at Horwich Works in 1951.

They were fitted with Caprotti valve gear, raised running plates without splashers, a double chimney and SKF roller bearings on all axles.

[4] Caprotti valve gear had previously been fitted to a batch of twenty Black Fives, nos.

The smokebox was extended by 4 in (10 cm) and the chimney moved forward to allow room for the Caprotti valve box and associated steam and exhaust pipes.

The locomotives fitted with Timken bearings had large split cannon-type axleboxes between the frames.

[6] All locomotives had a modified cab and a lowered running plate, necessitating the use of splashers over the wheels.

In service, the Caprotti Black Fives were slow in acceleration, but were powerful at high speeds, and coasted freely.

These were addressed in the design of the last two Black Fives to be built, 44686-7, which emerged in 1951 with Caprotti valve gear driven by two external shafts on either side of the locomotives.

[4] The exposed driveshafts were easier to access and negated the need for splashers, while the high running board allowed clearance for the valve boxes and subsequent sharp angle of the shafts, Later BR built thirty of the closely related BR Standard class 5 with Caprotti valve gear,[7] and it was also employed on the standard class 8 71000 Duke of Gloucester.