The bores should be set as near true as possible, tested by a spirit-level rested on the bore and placed as near true as can be judged with the length of the bore, and a plumb rule may be applied to the end faces where the cover joint comes. Then a straight-edge should be applied, as in [Fig. 2511], in which s is the straight-edge, and c and d the two cylinder ends.
The method of testing is shown in [Fig. 2511], where the straight-edge s is shown in three positions, marked respectively 1, 2, and 3 at one end, and f, g, and h at the other.
The first test should be made by simply placing the straight-edge across the two cylinder faces, as at g 3; and when the cylinders are set apparently true and the spirit-level applied to the respective bores shows them true, greater accuracy may be secured by placing the straight-edge in position 1 h, being pressed firmly to its cylinder face with end 1 above the other cylinder face. Then, while end h is held firmly to its cylinder, let end 1 lower until it passes entirely over the face of cylinder c, whose face it should just touch; if on meeting c the straight-edge strikes it or does not meet it, further adjustment of the cylinder positions is necessary. Next place the straight-edge in position 2, pressing end f firmly against cylinder d, and passing the other end entirely over the end of cylinder c, which it should just touch, and no more. It will then be necessary to repeat this process, pressing the straight-edge against cylinder c and testing the other end with cylinder d, and the cylinders thus set will be (if the end faces are true, as they should be, and usually are) more truly aligned than is possible by the use of the spirit-level. This method also brings the end faces of the cylinders in the same plane, so that each piston head will travel central in the length of the cylinder bore, approaching the cylinder covers equally, and therefore keeps the clearance equal. Incidentally, also, this secures accuracy in the cross-head traverse on the guide bars (supposing these bars to be bolted to the cylinder cover). The holes for bolting the cylinders together may then be reamed and the bolts driven in and screwed up.
Fig. 2512.
To guide the tap when tapping the cylinder cover and steam-chest holes the guide stand s, shown in [Fig. 2512], should be employed. It is bolted to the cylinder face by the bolt b, which passes through a slot in the stand.
The tap t is inserted through the two arms of the stand and its end inserted in the hole to be tapped when bolt b is tightened up.
The stem of the tap should be of slightly larger diameter than the tap thread, so as to fit in the holes of the guide or stand.
When, however, one end of the guide bars is carried on the cylinder cover, it is necessary when setting that cover to be marked for the drilling, to so set it that the seats for the guide bar ends shall be horizontally level when the cylinder is on the engine; and when setting the bores of the cylinder in line to mark the holes for bolting the cylinders together or to the saddle, this point should also be looked to, as if these seats are not in line the faces of the guide bars will not be in line, and will not, therefore, bed fair to the cross-head guide unless the error is in some way corrected.