As my title suggests, I did a Mantle clock mainspring replacement. I picked up a clock with Cuckoo Mfg. Co. movement 341-020.
The pendulum spring pivot was broken and clock was dirty. Cleaned it up (no disassembly) replaced the spring pivot adjusted escapement and got it to run for a full 24 hours and the time-mainspring broke (while it was running).

The closest match was a Clockworks 178.
However, the old spring was .015" thick, the CMH-178 is .011 thick.

Now the clock will not run longer than a few minutes.

Is the new spring too weak?

Hi Dennis,

If the width of the new spring is the same as the old spring, the new spring, being almost a third thinner will definitely have less power and IMHO is causing your clock to stop running, the spring just doesn't have enough power to keep the pendulum swinging. My 2 cents for what it's worth.

Norm

I think this is what you need:
Timesavers