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Rh corners. To repair them the frames may be either patched or welded. A patch is a piece of plate about ⅞ in. thick cut out to embrace the upper portion of the hornblock and cover the fracture. Welding is now becoming more usual either by the oxy-acetylene or the electric arc process. In both processes, the crack is chipped out, and a V groove formed, into which molten metal is deposited by the flame or arc to produce a solid “weld.” Great care has to be taken, especially with the oxy-acetylene process, that the contraction of the frame when cooling after the operation does not cause a new breakage.

Cylinders. The defects which arise in cylinders are principally oval wear of the bore, wear of the port faces, over which the valves work, and cracked or broken cylinders. In the case of cylinders with circular ports for piston valves, such as those shown in Fig. 22, these ports are provided with internal circular liners, through which the necessary port holes are cut. As these wear through the action of the rings of the piston valves, they can be replaced by new liners, and the cylinder casting itself remains uninjured and intact.

For refacing the port faces of slide valve engines, and for reboring the cylinders themselves, the latter are not taken out of the engine frames. To do this would be too expensive an operation. Portable facing and boring machines are used which are fixed in front of the cylinders, the buffer beam having been taken down. These