16. ROCK SHAFTS, CRANKS, LINKS, ETC. _a._ Rock shafts must be either forged iron, forged steel, or cold rolled steel. Cast iron is not acceptable. The following are the minimum diameters acceptable: 500 gallon pump 1-1/2 in. 750 gallon pump 1-3/4 in. 1000 gallon pump 2 in. 1500 gallon pump 2 to 2-1/4 in. _b._ The rock shaft bearings must be bushed with bronze and the bushings pinned firmly in place. The length of each of these non-corrosive bearings must be not less than 4 inches. _c._ Rock shaft cranks, valve rod heads, valve rod links, and piston rod spools or crossheads may be wrought iron or steel forgings, or steel castings. If of a heavy, strong pattern, these parts, with the exception of valve rod links, may be of semi-steel or cast iron. _d._ The sectional area of all connections between rock shaft cranks and valve rod must be such as to give a tensile or compressive strength substantially equal to that of the valve rod. 17. VALVE MOTION LEVERS. _a._ The valve motion levers must be steel, wrought iron, or steel castings. Cast iron is not acceptable. Steel castings, if used, must be deeply stamped with the name of the makers, with letters one-eighth inch high, near the upper end of each lever, where it can easily be seen,—thus “....._Steel Castings_.” Cast-iron arms, if bulky enough to be safe against external blows, are awkward in shape. The sectional area necessary for any arm depends upon the means provided for preventing a sidewise strain on the lever, due to rotation of piston or friction of its connection to piston rod. The spool or crosshead on the piston rod should be so designed that no sidewise strain can be thus produced on the lever. _b._ The levers must have a double or bifurcated end at crosshead. The double end is less likely than a single end to put an undue strain on the lever as the rod turns, and is also likely to give trouble from lack of lubrication or from a loosening of any small parts, and has proved to be the most satisfactory arrangement. 18. VALVE MOTION STAND. _a._ The valve motion stand must be securely dowel-pinned to the yoke castings, to prevent any movement after being once adjusted. 19. CUSHION VALVES. _a._ Cushion-release valves regulating the amount of cushion steam retained at ends of stroke must be provided. _b._ The cushion release must be through an independent port as shown in Figs. 2 and 3, so located as to positively retain a certain amount of cushion steam.