Bulleid chain-driven valve gear

It was peculiar to the Southern Railway in Britain, and borrowed from motor-vehicle practice in an attempt to create a compact and efficient design with a minimum of service requirements.

Bulleid's decision to have three cylinders, all driving the middle coupled axle of his Merchant Navy and West Country / Battle of Britain classes, gave rise to several problems.

A sealed oscillating shaft drove the offset upper rocker arm and link, and the whole assembly worked inside the exhaust space of the steam chest.

The advantage of having admission steam working on the outside faces of the valves meant that the volume under maximum pressure was completely sealed at the ends with no glands susceptible to leakage.

Bulleid asserted that a slack of 3 inches (76.2 mm) would be absorbed by the chain coming under load, with the remaining valve irregularity corrected by adjusting the cut-off.

These rockers alone would have made accurate valve timing difficult to achieve, giving rise to stress in the drive mechanism, a symptom of which would be an increase in the likelihood of chain-stretch.

This would result in the locomotive taking off like an unleashed race horse, high coal consumption, throwing the fire out of the chimney and running the risk of a violent high-speed slipping.