Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
If you are an M1A owner, you may experience the frustration of a loose rear sight. It seems like you are constantly adding elevation to maintain your zero. The problem is that the elevation knob is slipping down a notch each time a shot is fired. A spring washer is supposed to maintain tension so that the tooth on the elevation drum won't slip out of engagement with the serrations in the receiver. Over time, the washer may fatigue and lose tension or the screw may back out.
This situation is usually easy to rectify. You'll need a pair of pliers and screwdrivers. First, grasp the elevation knob with the pliers and hold it firmly so it can't turn. You may want to pad the jaws of the pliers with a strip of leather or similar material to prevent marring the elevation scale .
Tighten the elevation knob screw. Now move to the windage knob. Turn the slotted nut clockwise a half-turn until you feel a click. Attempt to turn the windage knob with normal finger pressure. If you can't turn it without a pair of pliers, back off the nut a half-turn and try again. The goal is to tighten it as much as possible while still being able to adjust the windage knob.
If you have adjusted both screws correctly and the sight still won't hold elevation the problem is most likely a worn-out spring washer or the tooth on the elevation drum is rounded off. In either case, you will have to replace the elevation pinion assembly.
Grasp the elevation knob with a pair of pliers to keep it from turning and tighten the screw. Pad the pliers to prevent scratching.
Turn the windage knob nut clockwise a half-turn at a time until you can't move it and then back it off a half-turn.