(1) Before removing the filling plug, the safety-head plug, or an unbroken safety head, operate the fuel valve until any pressure which may have accumulated in the fuel tanks is eliminated. If the coupling plug is in the tank coupling, very slightly loosen the threads of either the filling plug or the safety-head plug, using the 1-3/4-inch wrench, to eliminate pressure in the fuel tanks. Keep face and eyes away from the threads. (2) The plug-retainer assemblies should not be lifted completely out of the tanks unless required. (3) If either the rod or the rod and chain breaks from one of the plugs and falls into the tank, upend the tank group to permit removal of the parts. (4) To replace burst safety head, unscrew lock nut and deflector tube. (Fig 11) Using wrench (Fig 41), unscrew safety head. Never disassemble the safety head. _c. Installation of plugs._ The filling plug, safety-head plug, and safety head are screwed in by hand and then tightened with wrenches. No substitution will be made for the safety head, which is manufactured to burst at the safe limit of pressure. The plug threads and seats should be cleaned with a cloth (Fig 29) before installing plugs. Screw in deflector tube, using hand pressure. The tube outlet should face to the rear and at a 45-degree angle to operator’s left shoulder. (Fig 18) Replace lock nut and tighten with wrench. (Use wrench on lock nut, not on deflector tube.) _d. Maintenance of plugs._ Replace safety head if damaged or blown. Never repair safety head or use an improvised head. 70. TANK COUPLING. _a. Description and functioning._ This quick-connecting coupling (Fig 42) connects and locks the fuel hose or the coupling plug to the tank group. The coupling cams, lock, and washer provide a secure and tight joint. The tank coupling makes possible rapid replacement of emptied tank groups with filled and charged tank groups in the field. No tools are needed for this operation. [Illustration: Fig 42. Tank coupling and end of fuel-hose assembly.] _b. Removal._ (1) To remove the tank coupling from the hose connector, apply a wrench and unscrew. [Illustration: Fig 43. Closing cams of tank coupling to connect gun and tank group. This is done before locking. (See below.)] [Illustration: Fig 44. Closing lock of tank coupling to secure gun to tank group. This also provides a fuel-tight seal.] (2) To disconnect the tank coupling from the fuel hose or the coupling plug: (_a_) Release pressure from fuel tanks by operating the fuel valve or by opening very slightly the filling plug. (_b_) Using hands, pivot the coupling lock back on the coupling body. (_c_) Using hands, pivot the two coupling cams back on the coupling. (_d_) Slide out the fuel hose or the tank coupling. (_e_) If coupling washer is to be removed, pry out with a screw driver. _c. Installation of tank coupling._ Proceed as follows: