a special “tube-expander” is used. This consists of a casing which will enter the tube, and in it are slots through which a number of small rollers protrude slightly, the axes of the rollers being nearly parallel to the axis of the tube. An internal conical mandril can be pushed in by the operator so that the cone gradually forces the rollers outwards against the inside wall of the tube, and as the tool revolves the rollers roll the metal of the tube against that of the tube-plate, thus making a tight joint. In some cases a thin soft copper ring or ferrule is placed between the outside of the tube and the hole in the tube-plate, the ferrule being practically squeezed between the tube and the plate. The outer ends of the tubes project into the firebox about ⅜ in. and are afterwards “beaded” or worked over against the face of the tube-plate.
Safety Valves. These and other steam valves wear slightly on the valves and seats, so that they leak and have to be “ground in.” A little fine emery and oil is sprinkled on the valve and its seat, and the valve is rotated backwards and forwards until both are ground together to a perfect seating, which can be seen from the dark colour all round their bearing faces.
Frames. Frames occasionally give way by breaking from the inside corners of the recesses into which the driving hornblocks fit, as at A in Fig. 17. The alternate thrust and pull due to the steam action is felt most severely at these
