The old form of cushion release through bridge between ports is not acceptable. This form while leading into the exhaust passage as formerly, differs by starting from a small independent port (about 1/2-inch wide × 2-1/2 inches long) through the cylinder wall, located about 3/8 or 1/2 inch back from the cylinder head. (The exact position for affording the best action has to be determined by experiment with each different make of pump, as it depends somewhat on the extent of clearance space and on the point of closure of exhaust by piston and somewhat on the weight of reciprocating parts.) [Illustration: FIG. 2.] [Illustration: FIG. 3.] This style of cushion port makes the pump safer in case cushion valves are unskillfully left open too wide and tends to prevent a pump from pounding itself to pieces in case of a sudden release of load, as by a break in suction or delivery mains, or by a temporary admission of air to suction pipe. [Illustration: FIG. 4.] Pumps made with this form of cushion release, have given very satisfactory results, and if the ports are properly located, there will be no re-bound of piston. _c._ Cushion valves must be always provided with hand-wheels marked as per sketch, for the reason that very few men in charge of fire-pumps are found to clearly understand or to remember their use. The lettering must be very open, clear and distinct, not liable to be obscured by grease and dirt, and of a permanent character. It is desirable that spindle or wheel be so formed that a monkey wrench can get a grip to open a jammed valve. Fig. 5 shows the stem flattened for this purpose. [Illustration: FIG. 5.] _d._ The valve and stem of cushion valve must be in one piece without any swivel joint. Swivel joints are apt to come apart and make it impossible to operate the valve. 20. PISTON RODS. _a._ Piston rods for their entire length must be of solid Tobin Bronze, and the distinguishing brand of the manufacturers of this metal must be visible on at least one end of each rod. _b._ The sizes must be not less than in table below. =========+==========+===========+============+============ =Size of=| | | | =Pump.= |=500 gal.=|=750 gal.= |=1,000 gal.=|=1,500 gal.= ---------+----------+-----------+------------+------------ Diameter | | | | of rod | 2 inch. |2-1/4 inch.|2-3/8 inch. | 2-1/2 inch. ---------+----------+-----------+------------+------------ _c._ The size and form of connection of rod to piston plunger and cross-head must be such that the stress in pounds per square inch at bottom of screw thread, or at such other point of reduced area as receives the highest tensile strain, shall not exceed 8,000 lbs. per square inch, when the steam pressure acting on the piston is 80 lbs. per square inch.