(5) _Diffusion-pipe assembly._ This T-shaped tubing carries compressed air or nitrogen from the pressure regulator to each of the fuel tanks. A flared tube connection and elbow connect the stem of the T to the pressure regulator. The horizontal tubes of the T extend into the fuel tanks and are welded to the fuel tank walls. Within the fuel tanks these tubes are perforated with holes which permit ready escape of the compressed nitrogen or air into the fuel tanks when the pressure-tank valve is open. _b. Removal and installation._ The tank connector, hose connector, diffusion-pipe assembly, and the two fuel tanks are welded together and cannot be disassembled from each other. No attempt should be made to remove any of these parts or assemblies. _c. Maintenance._ Other than cleaning (Pars 51 _d_ and 55 _e_), repainting, and tightening of threaded joints, no repairs will be attempted by the first or second echelon on the fuel tanks, tank and hose connectors, or diffusion-pipe assembly. Emergency repairs may be made only by the third or fourth echelon. No attempt should be made to weld or patch any part of the fuel tanks. 69. FILLING AND SAFETY-HEAD PLUG ASSEMBLIES. _a. Description and functioning._ (1) _Filling-plug assembly._ (Fig 39) This assembly fits into the 1-3/8-inch threaded opening at the top of either one of the fuel tanks. It permits filling and cleaning of the tanks, and seals the opening when the tank is not being filled or cleaned. The assembly includes the filling plug proper and a plug-retainer assembly. The latter is a metal rod which hangs from the plug on a metal chain. The rod and chain prevent accidental loss of the plug. (2) _Safety-head plug assembly._ (Figs 39 and 40) This assembly is screwed into the threaded opening on top of either fuel tank. It serves the same functions as the filling-plug assembly and moreover protects the firer and other personnel. It includes: (_a_) _Safety-head plug._ This plug is similar to the filling plug except for the threaded hole which receives the safety head. (_b_) _Safety head._ This metal head screws into the safety-head plug. It includes a soft metal diaphragm which bursts when the pressure in the fuel tanks exceeds 500 pounds per square inch. It prevents the building up of dangerous pressures in the fuel tanks. [Illustration: Fig 40. Safety-head plug assembly (cross section).] [Illustration: Fig 41. Unscrewing safety head from safety-head plug, using wrench.] (_c_) _Deflector tube._ This short, curved piece of 1/8-inch pipe deflects fuel and pressure away from the firer if the safety head bursts. A lock nut holds the tube in position. (Par 12 _j_) (_d_) _Plug-retainer assembly._ This assembly consists of a metal rod and chain which hang from the plug and prevent accidental loss of the plug when filling or inspecting. _b. Removal of plugs._