Patience is the watchword with a thumb safety. A few file strokes, test fit. A few more strokes, test it again. And so on. As others have said, remove the grip safety so you can watch the fit. You know you're getting close when you can insert the safety shank when you thumb the hammer back past the cocked position, but now when it's at the normal cocked position.
When I think the fit is right, I perform three tests. First, I cock the hammer, apply the safety, and release it. If the hammer moves forward at all, there's still a bit too much material. Second, I engage the thumb safety (keeping the grip safety removed) and try to pull the trigger. Watch for any movement whatsoever. Movement is bad ju-ju. The old military spec allowed for a slight amount, but now with modified sear geometries, shorter hammer hooks (often cut at negative angles), etc, you want no movement.
If it passes the "no sear movement" test, I go on to the "click test." Cock the hammer. Apply the thumb safety. Give the trigger a good, hard pull, with the safety on. Now, de-activate the safety, but do not pull the trigger. Hold the hammer-end of the gun up to your ear, and slowly thumb the hammer back past cock. If you hear a little "tink" sound, that's the sear resetting because the TS has allowed some sear movement. Repeat this half a dozen times to be sure. I've seen a bunch of factory Kimbers and Colts that would fail this test, either consistently, or every two or three times it was performed.
DISCLAIMER: I'm not a professionally trained gunsmith. I'm learning slowly by reading and listening a whole lot to knowledgeable people.
Do not mistake precedent for justification.
Doubt is an unpleasant condition, but certainty is absurd. Voltaire
Last edited by crazymoose; 02-11-2012 at 04:50..