The roll servo pictured is an eariler model. The screw (s) should only be installed in one of the holes. Double check with Trutrak, but I think that screw is designed to fail during some kind of  catastrophic motor locking failure so as to still have control over the aircraft. If all your shear screws threaded holes are used up you'll need to replace the round portion that the servo arm attaches to. There's also a retaining arm to protect the center screw from backing out.

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Jon Hadlich
AI Systems
(541) 815-7381