Angles
Lemme try to clear it up.
The "Clearance" angle is commonly referred to as a breakaway, or escape angle. To better understand its function, let me explain the mechanics of the sear/hammer hook interface as the trigger is pulled, just as the sear is rolling out of engagement with the hooks.
The sear's movement describes an arc. A very short arc, but an arc. The edge of the sear that sits at the bottom of the hammer hooks follows this arc as the sear rotates around the axis of the sear pin. As it rotates, the edge pushes up on the hooks, actually serving to move the hammer further rearward....or overcock it...for lack of a better description.
The breakaway angle on the sear reduces the amount of "lift" that this edge provides, and allows the sear to escape from the hooks a little easier and a little smoother.
Many years ago, a common practice was to put an escape angle on the hooks themselves. The angle didn't involve the entire length of the hooks, but was right on the tips...for maybe the last .010 inch, and it served much the same purpose...except that the "breakaway" occurred at a later point in the sear's rotation, while the escape angle on the sear itself provides the benefits of the breakaway as soon as the sear moves.
As the triggersmiths began to trim the hooks ever-shorter in the quest for crisper and cleaner trigger actions, the practice of putting the escape angle on the sear began to show up more often because a heavier escape angle could be applied to the sear with hooks shortened to .020 inch and even less.
Stoning an .010 inch deep angle on the tips of .018 inch hooks leaves precious little engagement surface, that...as the edge of the sear began to wear...became rather "iffy". Stoning it on the sear still provides engagement toward the tips of the hooks, making hammer follow and unexpected full-auto events much less likely.
This doesn't mean that it's safe practice to put a heavy escape angle on any old sear and go. Any time you start dinkin' around with sear to hook engagement without making sure that the angles are right, you're engaging in a risky crapshoot in which the odds are all against you. A light escape angle on the sear is *usually* okay, as long as the width of the angle doesn't exceed 1/4th the width of the remaining primary angle. I say usually because it greatly depends on the length of the hammer hooks...the angle of the hooks themselves...(Whether undersquare/captive or square relative to the hammer. Undersquare hooks are not safe with any sear angle)...and whether the sear primary angle agrees with the hammer hook angle...so the operative word "usually" is a disclaimer to let you know that you can wind up with a very unsafe and unreliable gun if you're not careful. To err on the side of caution, I strongly advise that any escape angle done on a sear without seeing to the details carefully be held to a very light one...something on the order of 90/10 instead of the above maximum of 75/25, and stoning the hooks to a minimum of .025 inch...or just enough to equalize the hooks' lengths.
When you start getting below about .023 inch for the hooks...and you use a 50/50 escape angle on the sear...you're treading on thin ice unless everything else is made right. There's a lot that you can do to improve a rough trigger without crossing the "Line of Departure" in which you've created a situation that's mainly a matter of "WHEN" and not "IF" it will turn on you.
Open a caliper to .025 inch and you can see just how tiny that engagement surface is...and proceed with extreme caution.
