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126 machines were formerly worked by hand, but now an electric motor actuates them through suitable gearing. The bores are carefully callipered, and just sufficient metal is removed to make them truly circular. New piston heads of greater diameter are required when the cylinders have been re-bored.

For cracks and breakages the cylinders must be removed from the frames, and if the cracks are serious the cylinders are replaced by new ones. Some cracks on the exterior are patched with gun metal patches cast from a wooden pattern specially made to fit the contour of the defective part. If the tops of the cylinders are corroded by the smokebox ashes, which always contain sulphur, a patch of copper plate is made to fit and bolted on. If the port faces are badly worn they are faced-up and plates or false faces are screwed on, which are machined to the original dimensions. Cracked “bridges” between the ports may be repaired by cutting away the metal on each side of the crack and dovetailing a piece into the space formed.

Acetylene welding has been used, chiefly in America, for repairing cylinders, especially in cases where flanges are broken.

Wheels and Axles. With the exception of the boiler these require more attention than any other part of the engine. The wheel “centres” themselves very rarely crack now that they are of cast steel. The former wrought iron driving