The Timney Trigger (Model #510) consists of a right hand safety that blocks the trigger and not the sear. No screws, tapping, or drilling is required. Each trigger is geometrically identical with tolerances less than .0005". The trigger is preset from the factory at 3 lbs and adjustable from 1.5 to 4 lbs. No gunsmithing required.
We use a lot of Timney #510 triggers at the shop and they have worked great for us.