Editing BR Standard Class 8 (section)

== Design details ==
[[File:71000 DUKE OF GLOUCESTER East Lancashire Railway.jpg|thumb|left|71000 Duke of Gloucester on the [[East Lancashire Railway]], 2009. Note the [[British Caprotti valve gear]].]]

At first, Riddles wanted to develop an enlarged version of his Standard Class 7 ''Britannias'', because that design still featured a two-cylinder layout.<ref name=Herring>{{cite book |last=Herring |first=Peter |title=Classic British Steam Locomotives |section=Standard Class 8}}</ref>{{rp|188-189}} However, the size of the cylinders needed to achieve the 8P power classification would mean that the locomotive was outside the British [[loading gauge]], so a reluctant reversion to the three-cylinder layout ensued.<ref name=Langston /> That reluctance was born from experience with the Gresley ''Pacifics'', in which the conjugated valve gear was difficult to maintain due to the middle cylinder being located between the frames.<ref name=Herring /> Therefore, an alternative type of valve gear had to be found.

[[Caprotti valve gear#British Capritti|A rotary cam-driven form]] of [[Caprotti valve gear]] was settled on, as developed by [[Heenan & Froude]], using [[poppet valve]]s.<ref name=review>{{cite magazine |title=B.R. class 8 4-6-2 locomotive No. 71000 |magazine=[[Locomotive, Railway Carriage & Wagon Review]] |page=88}}</ref> It was based on Italian locomotive practice and allowed precise control of steam admission to the cylinders, as well as improving exhaust flow and [[boiler]] draughting when compared to the more conventional [[Walschaerts valve gear|Walschaerts]] and [[Stephenson valve gear|Stephenson]] valve gear.<ref name=review /> On paper, that created a free-steaming, hard-working locomotive, capable of hauling heavy loads over long distances, but in practice, fundamental design errors and undetected deviations from the drawings made during construction combined to prevent the locomotive from achieving its expected performance during British Railways ownership.<ref name=engineer>{{cite magazine |title=British Railways standard class "8" locomotives |magazine=Engineer |year=1954 |pages=8, 19}}</ref>

The main problem was known even when the locomotive was under construction, because L.T. Daniels, the representative of the British Caprotti company, recommended the use of the [[Kylchap]] [[blastpipe]], which could have coped with the fierce exhaust blasts experienced with the Caprotti system.<ref name=Herring /> A standard [[double chimney]] of the Swindon type had already been fabricated in order to cut costs and it had been installed in the [[smokebox]], supposedly before Riddles could do anything about it.<ref name=Herring />  As a result, the locomotive suffered due to the choke area of both the chimney and blastpipe being much too small for the pressure created by the exhaust, which led to poor draughting.<ref name=BR>{{cite report |title=British Railways standard 3 cylinder 4-6-2 express passenger steam locomotive No. 71000 |author=HMSO, British Transport Commission |location=London |year=1957 |series=Performance and efficiency tests |issue=15}}</ref>{{rp|29}} Further problems relating to the [[firebox (steam engine)|firebox]] of the locomotive were only discovered during its restoration, including a poorly dimensioned ashpan, and [[damper (flow)|dampers]] that were too small, starving the fire of air when operating at speed.{{r|BR|p=58}} 

Following the occasional appearance of cracks near the spring brackets of the Britannias and [[BR Standard Class 6|''Clans'']], a substantial rearrangement took place in that area, which resulted in the locomotive riding on three cast steel "sub-frames" carrying the ten front-most spring brackets, and lengthened spring brackets behind the rear driven axle.<ref name=":2">{{cite document |title=SL/DE/21642, SL/DE/21631, SL/DE/22042 |author=British Rail |publisher=[[National Railway Museum]] |type=Drawing}}</ref>