In this operation let it be noted that the close fit of the piston to the cylinder bore and of the gland to the stuffing box is alone depended upon as a guide whereby to so set the guide bars that the axial line of the piston rod and its plane of motion shall be in line with the centre of the crank shaft.
Fig. 2521.
Suppose, however, that the piston head is a new one, and the gland is worn a loose fit to the stuffing box, then setting the bar to the gland would produce the result shown in [Fig. 2521], in which the dotted line a a is a line or cord stretched axially true with the cylinder bore, and coincident with the centre of the pillow block at b. The gland being a loose fit permits the piston rod to fall below its proper level, and the surface of the bars, if set by the gland only, without using the spirit-level, would be set true to the full line c c, and therefore out of true line. If the bars are set by spirit-level true to the length of the cylinder bore, the gland becomes useless as a guide to set the bars by. It is a not uncommon practice, when the gland has play, to insert in the stuffing-box bore, at the bottom, a piece of tin or sheet brass, equal in thickness to one-half the amount to which the gland is too small, or to insert a similar piece beneath the piston head if it is too small. As a rule, however, there will be at least as much play between the piston rod and the gland bore as between the gland and the stuffing-box bore; hence, if there is any play, it is better to discard the use of the gland altogether.
The proper method of setting guide bars by a stretched line is as follows:—
Fig. 2522.