_d._ Piston rod nuts, in both steam and water ends, must be tightly fitted, and preferably of a finer thread than the United States Standard. This is to avoid as much as possible the unnecessary weakening of the rod at the bottom of the thread, and to reduce the tendency of the nut to work loose. In practice 8 threads per inch has been found to give good satisfaction. _e._ In addition to a tightly fitting nut, some reliable device must be provided, in both steam and water ends, for absolutely preventing these nuts from working off. [Illustration: FIG. 6.] Fig. 6 shows one form of such a locking device and illustrates the kind of security desired. This device combines the advantage of a taper key and a split pin, and the elongated key-slot gives sufficient leeway to always insure that the key can be driven up tight against the nut and thus prevent it from even starting to work off. Other methods will be approved in writing, if found satisfactory. 21. VALVE RODS. _a._ Valve Rods for their entire length must be of solid Tobin Bronze, with sizes not less than in table below. =========+==========+===========+============+============ =Size of=| | | | =Pump.= |=500 gal.=|=750 gal.= |=1,000 gal.=|=1,500 gal.= ---------+----------+-----------+------------+------------ Diameter | | | | of rod | 1 inch. |1-1/8 inch.| 1-1/8 inch.| 1-1/4 inch. ---------+----------+-----------+------------+------------ _b._ The net area of valve-rod at its smallest section subject to tensile stress, must not be smaller than at bottom of U. S. standard screw thread on rod of diameter given above. The construction of this rod as affecting lost motion at slide valve is specified under Article 15. 22. STUFFING BOXES. [Illustration: FIG. 7.] _a._ All six stuffing boxes must be bushed at the bottom with a brass ring with suitable neck and flange, and the follower or gland must be either of solid brass, or be lined with a brass shell 3/16-inch thick, having a flange next the packing, as shown in the sketch. The bottom of stuffing boxes and the end of glands should taper slightly towards the center as per sketch. _b._ These glands should be strong enough to withstand considerable abuse, so as not to break from the unfair treatment of unskilled men. 23. PRESSURE GAUGE. _a._ A pressure gauge of the Lane double tube spring pattern with 5 inch case must be provided and attached to the steam chest inside the throttle valve. The dial of gauge should be scaled to indicate pressures up to 120 lbs. and be marked “STEAM.” This kind of gauge is used on locomotives and is the best for withstanding the vibration which causes fire-pump gauges to be often unreliable. Moreover, this double spring is safer against freezing. 24. DRAIN COCKS.