_a. Source of pressure._ When the pressure in cylinders of compressed air or nitrogen has fallen too low to be of further use in filling pressure tanks of flame throwers, the remaining pressure may be used to blow fuel into fuel tanks if the regulator valve can reduce pressure down to 20 pounds per square inch. For precautions, see Paragraph 33. An air compressor or a hand air pump (tire pump) may be used in place of a cylinder if the latter is not available. Pressure of _no more than 15 to 20 pounds_ per square inch should be used on the fuel drums. Only a diaphragm-type regulator valve can be used safely. This valve must be capable of regulating any pressure that may be applied to it. _b. Drums._ Clean, noncorroded, steel, 55-gallon drums should be used. Drums of United States manufacture which meet requirements will be stamped ICC-5 or ICC-5A, followed by three numbers in sequence, for example, “14-55-44.” The number “14” indicates the gage of the metal; “55” indicates the capacity in gallons; and “44” indicates the year of manufacture. A steel drum of 14 gage, or heavier, is preferable, but lighter drums (of 16 or 18 gage) may be used. Drums made of gages lighter than 18 gage (20-or 22-gage) must not be used. Drums should never be moved while under pressure. _c. Connections._ The source of pressure (see _a_ above), the drum of fuel, the fuel-filling line, the air hose, and other parts, are connected as shown in Figure 30. Threaded adapters are used, as necessary, to fit lines to the drum. All threaded connections should be made tight by the use of wrenches on the joints. The drum and the pressure cylinder (if the latter is used) should be laid on their sides on the ground or a platform. The opening of the drum connected to the fuel-filling hose should be close to the ground or platform. If tank group is filled without gun group, lock coupling plug (Par 70) in tank coupling. _d. Procedure._ To fill fuel tanks: (1) Remove both the filling and safety-head plugs. (2) Inspect interior of tanks to see if clean and free from foreign matter. If not clean, flush with gasoline. (3) Place end of fuel-filling hose in either one of the two fuel-tank plug holes, using a nipple as a spout. (4) Start air compressor or pump, or open the valve on the cylinder of compressed air or nitrogen. Open regulator valve on filling line by turning handle _slowly_ until gage shows 15 to 20 pounds pressure, but no more. _Caution_: “Cracking,” or opening a cylinder valve without using the proper regulator valve (Par 39 _a_), may result in explosive pressure in the drums. (5) Both tanks must be filled to within 2 inches of their tops. Close valve on fuel-filling hose to halt flow at this level.