FITTING AND ERECTING 251 while they definitely fix a line, the cylinder is free to expand along the dowels as it gets hot. These dowels are usually placed about midway along the cylinder length, and the expansion thus takes place in both directions from this line. In other cases, the dowels are fixed at the exhaust end of the cylinder, and the whole expansion thus takes place in the one direction viz. towards the H.P. end. It is necessary, of course, to have a fixedrpoint on the cross transverse datum line, so as to control definitely the direction of the sideways expansion. This fixed point is formed by vertical keys at both the H.P. and L.P. ends of the cylinder, set half in the bedplate and half in projections from the cylinder on the vertical centre line. These keys enable the cylinder to breathe vertically, but keep the cylinder central sideways under all conditions, and thus compel the expansion (lateral) to take place equally on both sides. The dowels should be made an easy tap- ping fit, and the holes through the cylinder feet, through which the holding-down bolts pass, should be at least J in. bigger in diameter than the bolt, so as to allow the cylinder to expand and move laterally when heated. The bolts, therefore, have to be of special construction; they are known as collar or shoulder bolts, and are shown, together with the corresponding type of stud, in the accompanying sketches. A special washer is always used under the head of the bolt or under the nut of the stud, and it will be seen that when the bolt is tightened hard down on the shoulder, and the length from shoulder to under side of head is just correct, that the cylinder, while de- finitely held down, can move or expand sideways as required. In order to get the exact length of the bolt, from shoulder to head, the bolt is tightened down with the collar in position, and the amount of slack between the collar and the head is then measured with feeler gauges, and the length from shoulder to head is then reduced by the figure obtained with the feelers, with the exception of two thousandths, which is left on in order to provide a very small clearance between the bolt head and washer, and thus allow the cylinder to expand. Before jointing up any steam-pipe,-or the valve chest to the cylinder bottom half, they should be carefully examined for any loose material, e.g. nuts, pieces of steel, borings, &c., that may have lodged in the steam passages; in the case of steam-pipes it is very advisable to either draw a heavy chain through them repeatedly, or to tap them all over the external surface with a heavy hand hammer, or to do both, in order to loosen any scale or rust that may have formed inside the pipes, due to the " weathering of the hard skin on the inside of the pipes. In particular, the steam-nozzles should Fig. 6.—Holding-down Shouldered Bolt to allow for Expansion and Contraction of Cylinder Feet relative to Bedplate x, Bedplate. 2, Cylinder feet. 3, Bolt shoulder. 4, -002 in. for sliding clearance. 5, Clearance round bolt for expansion of cylinder relative to bedplate.