August 12, 2013
It doesn't look like much. A flat piece of spring steel slit in two places to form three legs which are bent individually to adjust tension against the relevant fire control parts. Sear springs don't usually wear out and you probably don't need one in your emergency stash of spare parts but they may occasionally require adjustment to prevent malfunctions. Let's look at the function of this spring and its relationship to the 1911 trigger mechanism.
Viewing the spring from the rear the longest leg should be to your left. This leg applies pressure directly to the sear causing it to bear against the hammer until the trigger pushes it out of engagement. Insufficient sear spring tension can cause the hammer to follow the slide down as it moves forward during loading or firing. A pistolsmith doing a trigger job may adjust sear pressure a bit but minimum tension should never be set less than one pound.
The center sear spring leg serves two functions, applying pressure to both the disconnector and trigger. Spring pressure against the angled bottom surface of the disconnector causes it to snap up into a notch in the bottom of the slide when the slide is in battery connecting the sear and trigger so the pistol may be fired. You can see how the disconnector works by assembling your pistol without the grip safety and cycling the slide, watching it move down disconnecting the trigger as the slide starts moving to the rear. The disconnector prevents the pistol from firing more than one round with each pull of the trigger. Insufficient spring pressure may cause a malfunction where the pistol simply won't fire because the disconnector doesn't return to "fire" position.
A second function of the center leg is as a trigger return spring, causing the trigger to return forward to firing position as pressure is released after a shot is fired so the disconnector can reset as described above. Obviously, this leg is adjusted to regulate trigger pull weight as part of a standard trigger job but tension should never be less than ½ pound. Insufficient tension may not return the trigger fully forward allowing the disconnector to reset.
The right leg of the sear spring applies pressure to the grip safety causing it to block the trigger when the shooter's grip is released. Insufficient tension de-activates the grip safety.
One sear spring variation should be noted. Clark Custom Guns markets a four leg spring that has the center leg split into two parts so the disconnect and trigger return functions can be adjusted individually, a feature useful for very light triggers in competition guns.
A standard sear spring (left) and a Clark four-legged spring (right)
Left Sear Spring
The left sear spring leg applies pressure to the bottom left corner of the sear. Grip safety, thumb safety and hammer have been removed for clarity.
The Center Leg
The center leg runs the disconnector and acts as a trigger return spring.