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120 Cracks and fractures as at B, Fig. 46, are frequently found across the tubeplate between the holes in which the tubes are secured; such cracks also appear between the stay holes in all the plates. As these cracks extend rapidly great care is taken in looking for them. As the tubeplate expands in a vertical direction when the boiler warms up, the tube holes gradually assume an oval shape and leakage then occurs round the tubes. Overheating of the plates, due to occasional shortness of water, causes burning of the plates. Expansion and contraction cause frequent breakages of the stays which unite the roof and sides to the firebox casing, and bulging-in of the plates may then result. The presence of defective side stays is detected by tapping the heads with a light hammer, when an experienced man can tell them by the sound. Stay heads are generally burnt away by the flame.

All these defects, of which some are sure to be found, would become dangerous if left, and for this reason the firebox is thoroughly examined at frequent regular periods. The high pressures used in modern locomotives, from 170 to 225 lbs. per sq. in., increase the tendency to firebox troubles, and some engineers take advantage of the benefit due to super-heating by reducing the pressure to 160 lbs. per sq. in.

To repair cracked plates, patches of various sizes and shapes are used. It is essential in patching to cut away the defective portion of the plate first. This, leaves an oval or rectangular