242A1's posting on the Gresley V4 thread, http://www.national-preservation.com/threads/gresley-v4.802580/page-4 post #80, highlights the large number of choices to be made in designing a steam locomotive. One of them is whether to put the valve gear inside or outside the frames. Outside valve gear has some obvious advantages: notably that return cranks are easier than eccentrics to assemble and disassemble, and better accessibility for oiling up. So why did the GWR persist with inside valve gear? Did they use it on the 2-cylinder engines only because they thought outside Stephenson's wouldn't fit (disproved by LMS 4767), and on the 4-cylinder engines only as an obvious development from the original scissors gear? Or does it have some other advantage?