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 hole, and a piece of new copper plate is marked off and cut out slightly larger than the hole, so that it forms a lap all round. The patch is then carefully bedded down and the stay holes and rivet holes marked off in the new piece after which it is riveted or fastened by stud bolts according to the position which it occupies, for in some parts of the firebox riveting cannot be done. It should be noted that such patches are never put over the cracks in the old plate, but the latter is always cut out, for the reason that were the old plate left there would, after the patch has been put on, be a double thickness of plate, which would reduce the conductivity, so that overheating and burning would result.

If a firebox plate be in bad condition, the lower half may be cut away, and a new half-side, or half tube plate may be riveted to the old upper half.