Re the 5625/Poppet Valves, the January 1995 Railfan & Railroad Magazine had an engrossing article about trouble shooting the 5625, after it had been converted to poppet valve in early 1942. Written by Vernon L. Smith, a mechanical engineer working for Lima Locomotive + Franklin RY supply and, later, CMO for the Belt Ry of Chicago, if memory serves me.
Problems included warpage of valve chests, valve seats, inadequate oil feed to the valve stem guides......on and on ! But, the 5625 performed, rushing high priority Chicago-Denver freights, weighing 3500-4500 tons comfortably at 60 mph - in cold, winter weather (journals are a bit resistant in cold weather). Alas, 5625 was the 2nd O5 to be retired in 3/54.
