The main difficulty with this valve gear was that at high speeds, inertial forces caused the long conjugating lever to bend or "whip". This had the effect of causing the middle cylinder to operate at a longer cutoff than the outer cylinders, therefore producing a disproportionate share of the total power output, leading to increased wear of the middle big end. Sustained high speed running could sometimes cause the big end to wear rapidly enough that the increased travel afforded to the middle piston by the increased play in the bearing was enough to knock the ends off the middle cylinder. This happened during the run of "Silver Fox". Although the problem could be contained in a peacetime environment with regular maintenance and inspections, it proved to be poorly suited to the rigors of heavy running and low maintenance levels of World War II. This gave rise to big-end problems on the centre cylinder connecting rod on the famous A4 class of streamlined Pacifics and many of these locomotives were fitted with a reduced diameter piston and had the inside cylinder sleeved down as a temporary measure. LNER Class A4 4468 Mallard suffered centre cylinder big-end damage (indicated to the driver by the fracture of a "stink bomb" attached to the bearing, which fractures during overheating of the white metal) during its world record run and was forced to limp back to its depot for repairs afterwards. Gresley's successor at the LNER, Edward Thompson, was critical of this particular valve gear. As well as introducing new two-cylinder designs, he set about rebuilding Gresley locomotives with separate sets of Walschaerts valve gear for each cylinder.