which when made are marked with centre punch dots. Centre strips a of thin iron are placed diametrically across the bores. The centres at both ends are found with compasses, and marked on the strips. The cylinders are then levelled by using a straight edge and spirit level across the centres, the necessary adjustments of level being
_Steam_Locomotive_Construction_and_Maintenance_Fig20.png)
Fig. 20.—Marking-Off
Cylinder Port Faces.
made by the wedges. The centres are tested by the scribing block c which has the underside of its foot a perfectly plane surface; the needle pointer of this is shown on the centre line, which is drawn by it all round the casting. All lines such as d and e for surfaces which require planing are marked. Vertical centre and other vertical lines are drawn with the blade of a T-square having its broad edge on the table. The bore of the cylinder and all holes are drawn with compasses and centre popped. The dimensions are taken from the drawings, or in the case of standard engines will be measured from gauges specially made for the cylinders of different classes of engines. The whole work must be checked carefully to see that there is sufficient material on all surfaces for planing and boring to finished sizes.
The face into which the steam ports (s) and
