Air carrier Technician reported during an installation of an APU on a B777-200; they found damage to both the left aft mount attached to the aircraft and the APU mount.

Narrative

During installation of an APU into Aircraft X; both the left aft mount attached to the aircraft and the Apu mount received damage. After reviewing the damage it was clear what caused it; but during installation it was not easily seen how to avoid it. Maintenance Manual 49-11-00-400-802 part J-step 3 (c) 1-5 gives instructions on how to tighten the nuts once the Apu has been hoisted into place on the mounting bolts. During step 5 while torquing the nuts; the flange on the Apu aft mount cone bolt (attached to aircraft) was snapped off. The torque was set to 400 inch lbs and the process was being supervised by Quality Control. While torquing there was very little resistance; and any felt was assumed to be the weight of the APU resting on the nuts. What had occurred was before torquing; the flange on the cone bolt was not properly aligned with the mating surface on the APU mount. It appears that normally when the APU is hoisted to the bolts; the flange(which rotates freely); makes contact with a fin on the APU mount; which forces the flange into the correct orientation and self aligns. This particular bolt did not move freely without the aid of a tool; and the flange got caught on the fin on the APU mount(instead of rotating into correct orientation). When torque was applied to the nut the flange was bent to the point of breaking and it also left indentations on the APU mount. The Maintenance Manual does not give clear instructions to make sure the flange is lined up properly before torquing the nut. In hindsight it is easy to look at the mount and flange and see how it should be installed but during hoisting and torquing from below the APU; it is not.Suggestions: Add additional instructions in the Aircraft Maintenance Manual to beware of the flange orientation before torquing; making sure the bolt has aligned itself properly.