because it could cause highly irregular valve events, a problem compounded by the fast-moving Bulleid steam reverser. The entire system was located in a sealed oil bath, another unique design, providing constant lubrication to the moving parts.

The locomotives were equipped with the unusual 6 ft 2 in (1.88 m) Bulleid Firth Brown (BFB) driving wheels which both lighter and stronger than the spoked equivalent. These proved to be successful and were