exhaust ports (p) Fig. 20, emerge must also be marked off, a template in which holes are cut for the ports being used for this purpose. The edges of the ports were formerly slotted out, but they are now generally milled out by a milling machine. It should be added that the cylinders shown in Fig. 19 are cast separately, the two halves being bolted together to form one pair. One of the flanges is shown at f, Fig. 19, and in this case these flanges have also to be marked off for planing, the essential point being that the dimensions g, g from the face of the finished flange to the centre of the cylinder on each side are exactly equal.
In many works much of the planing of the various outer surfaces is done first, using the scribed lines as a guide, and then the cylinders are set down on the bed of a boring machine on one of the planed surfaces and bored. The centre line of the bore of the cylinder is the datum line to which all other dimensions refer, and therefore some works consider it preferable to bore the cylinders before planing them. Fig. 21 shows a double boring machine for the purpose of boring two inside cylinders at once. The two heavy revolving boring bars are passed through the cylinders, the latter being bolted to the grooves in the bed of the machine. On each bar there is a disc carrying three cutting tools, and the two projecting arms carry tools for facing the ends of the cylinders, on to which the covers have to be fitted. Each cylinder is
