625. Q.—­What description of piston is used?

A.—­The piston is packed with hemp, but the junk ring is made of malleable iron, as cast iron junk rings have been found liable to break:  there are four plugs screwed into the cylinder cover, which, when removed, permit a box key to be introduced, to screw down the piston packing.  The screws in the junk ring are each provided with a small ratchet, cut in a washer fixed upon the head, to prevent the screw from turning back; and the number of clicks given by these ratchets, in tightening up the bolts, enables the engineer to know when they have all been tightened equally.  In more recent engines, and especially in those of large size, Messrs. Penn employ for the piston packing a single metallic ring with tongue piece and indented plate behind the joint; and this ring is packed behind with hemp squeezed by the junk ring as in ordinary hemp-packed pistons.

626. Q.—­Will you describe the construction of the cap for connecting the piston rod with the crank pin?

A.—­The cap for attaching the piston rod to the crank pin, is formed altogether of brass, which brass serves to form the bearing of the crank pin.  The external diameter of the socket by which this cap is attached to the piston rod is 3-5/16 inches.  The diameter of the crank pin is 3 inches, and the length of the crank pin bearing 3-7/8 inches.  The thickness of the brass around the crank pin bearing is 1 inch, and the upper portion of the brass is secured to the lower portion, by means of lugs, which are of such a depth that the perpendicular section through the centre of the bearing has a square outline measuring 7 inches in the horizontal direction, 3-7/8 inches from the centre of the pin to the level of the top of the lugs, and 2-1/2 inches from the centre of the pin to the level of the bottom of the lugs.  The width of the lugs is 2 inches, and the bolts passing through them are 1-1/4 inch in diameter.  The bolts are tapped into the lower portion of the cap, and are fitted very accurately by scraping where they pass through the upper portion, so as to act as steady pins in preventing the cover of the crank pin bearing from being worked sideways by the alternate thrust on each side.  The distance between the centres of the bolts is 5 inches, and in the centre of the cover, where the lugs, continued in the form of a web, meet one another, an oil cup 1-5/8 inch in diameter, 1-1/8 inch high, and provided with an internal pipe, is cast upon the cover, to contain oil for the lubrication of the crank pin bearing.  The depth of the cutter for attaching the cap to the piston rod is 1-1/4 inch and its thickness is 3/8ths of an inch.

627. Q.—­Will you describe the means by which the air pump rod is connected with the crank which works the air pump?

[Illustration:  Fig. 50.  AIR PUMP CONNECTING ROD AND CROSS HEAD.  Messrs. Penn.]