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 When tube holes are oval as at A, Fig. 46, and the plate between the holes is cracked as at B, the holes are enlarged with a “rose-bit” tool,

Every five or six years an entirely new firebox is required. This involves stripping the whole of the interior of the boiler, cutting through and knocking out all the stays, and removing the foundation ring. The boiler, when undergoing this repair, is turned upside down and the new firebox put in as in a new boiler (see Chapter II). The whole boiler will require renewal after 12 to 16 years; this is a comparatively simple matter, the old boiler being lifted away from the frames, and the new one placed on them as described in Chapter VIII.

Firebox Stays. Leaky and broken firebox stays are a constant source of trouble. Leaky copper stays, if not in too bad a condition, may be attended to in the running shed, the heads