1.General.      a. Primary Structure. - The primary structure of the engine mount is constructed of X-4130 chromemolybdenum steel tubes which have been heat-treated to a high tensile strength. The upper tube assemblies and the horizontal bearer tubes are heat-treated to 150,000 pounds per square inch tensile strength. The upper tube assemblies have steel fittings welded at the ends to form the joints. The horizontal bearer tubes have bolted fittings which facilitate their replacement. The lower diagonal tubes are X-4130 steel tubes heat-treated to 95,000 pounds per square inch tensile strength with fittings bolted to the tubes to permit quick disassembly. The engine mount truss and sway brace link assemblies are aluminum-alloy 14ST forgings which are heat-treated to 65,000 pounds per square inch tensile strength.      b. Nature of Design. - The engine mount was originally designed for extremely accurate jigging and close hole tolerances in order to eliminate vibration. The design of the engine mount is of such a nature that it may be easily disassembled into small units and subassemblies for quick replacements. Figure 5 and 6 show the complete disassembly of the engine mounts and call out the subassemblies in order to assist in quick identification of parts.       CAUTION: Do not, under any circumstances, apply torch heat to the engine mount tubes as the heat impairs the tensile strength of the material. When heat is required for a replacement, the assembly must be reheat-treated to the original tensile strength specified for the member. (See this section, paragraph 1.a.)

c Replacement of Damaged Parts. - Any damage to the engine mount, however slight, necessitates replacement of the damaged member. All center line of holes must be within 1/32 inch of the center line of the forgings and tubular members. See the Engine Mount Charts and figure 5 and 8 for replacement of parts. 2.Engine Mount Bushings.      Bushed holes of the engine mount make it possible to maintain the desired tolerance and close fits. However, when replacing a subassembly of the engine mount, replace bushings, only when absolutely necessary. Because of the close hole tolerances, it is improbable that any appreciable wear will develop in the bushings. 3. Elongated Holes.      If any bushed hole is slightly elongated or damaged replace the bushing and- bolt according to the Engine Mount Material Chart.

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