Thomas the Tank Engine - main drive cylinder block

Thomas the Tank Engine, the children’s favourite made famous by Rev. A. Awdry, has recently undergone a serious repair by Metalock Engineering for Quainton Railway Society in Buckinghamshire.

Originally built by North British Loco in 1939 for ironworks service, 24564 Coventry No 1, now with its alter ego as Thomas the Tank Engine and operated by QRS, was suffering from an extremely worn and corroded main drive cylinder block. On this design the cylinder block is located between the frame, underneath the smokebox. Such was the condition that there was a hole between the smokebox and one of the cylinders, considerably reducing the loco’s efficiency. The solution was to line bore the cylinders, fit sleeves and undersize pistons. Having heard that Metalock Engineering had done similar work, QRS contacted them and following a survey Metalock undertook to carry out the necessary reboring work, manufacture and install two new cylinder liners. As soon as the cylinder block had been dismantled and the pistons removed, QRS engineers set about remachining both pistons and fitting new piston rings, thus defining the cylinder bore diameter required by Metalock to complete manufacture of the cylinder liners.

To remove the ovality from both cylinders Metalock removed the minimum amount of material to provide finished diameters of 18.25 inches(463 mm). At the same time they manufactured two 17.00 ± 0.005 inch (432mm) internal diameter, 26 inch (660mm) long cylinder liners and shrink fitted them into position. The liners effectively sealed off the cylinders from the smokebox and restored the original running clearances on the bores.

QRS’s Thomas the Tank Engine is now enjoying a new lease of life at the Buckinghamshire Railway Centre pulling carriage stock on special open days, to the delight of children and enthusiastic parents alike. Thomas the Tank Engine now has a new lease of life at QRS with a fully restored main drive cylinder block and new liners.