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.
Fig 40. Safety-head plug assembly (cross section).
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])