I am having a difficult time making a sear spring that will last. I have used spring steel and after three or four times function firing the spring becomes fatigued.

Keith, is it a double set trigger action?

What’s your process of heat treating?  What’s the actual spring steel you are using?  I just made a spring .030 thick from 1075 and works good.

Keith, my father was a pack rat and so am I. What I have used with success is half of a broken colt trigger/bolt spring from a single-action. I just ground off the extra steel and pushed the spring into the slot.

Therein lies the truth: tempering. As I read the responses I kept thinking "when's someone gonna say how they draw the temper after initial heat/quench?

I needed to fabricate this exact spring last year, for a Pacific Ballard. I shaped it out of a hacksaw blade, heat until magnet won't attract it, oil quench. Polish scale off it (so as to accurately gauge the color shift when re-heating it, if doing such by eye), re-heat until blue/brown starting to shift to red color, oil quench again. Springiest spring you ever did see. It has withstood hundreds of cyclings now, but I hedged my bets and made two more which ride along in my shooting kit - one for me and one for someone else if a cry goes up from down the firing line.

I use to use the oil burn off method, and it worked well most of the time.  But, I had a real struggle with the method making a main spring for the sharps rifle.  You can read about the struggle in the Sharps Pistol Rifle thread.  I now use the lead for tempering, as other suggested, and have really good luck.  I feel I have much more control over the process.
Bob

If you make springs from used hacksaw or bandsaw blades, make sure they are the cheaper carbon steel type, not the more expensive bi-metal blades.
The bi-metal blades cut better and last longer than the carbon steel type, but don't work for springs.
Chuck