Couple of notes about drilling springs...
Just wanted to pass on a few tips about cirkut springs and this probably applies to a lot of other springs as well.
I have repaired dozens of cirkut springs over the years. The broken springs usually break on the ends and can be shortened without much change to the overall performance. Here is a good way to do it, Heat a couple of inches with a small torch to remove the temper and then drill your hole being careful to make it perfect, i.e. no burrs or roughness. You will need to hand file the hole to make it pear shaped to fit over the screw head and not pop off. The loss of temper helps when the hole is for the inside connection as the shaft it goes on needs a few wraps that a full tension spring cannot do.
Another very neat way to drill a hole through a spring and not remove a few inches of temper is to use a blank piece of steel the size of the hole you wish to make or a bit smaller before you drill the hole with a regular drill. This has to be done in a drill press.
Let me explain, the blank steel rod is used like a drill, only as you bear down on the spring it gets really hot but only at the spot you are drilling the hole. You do this until you see color (red) on the spring and then it has the temper removed only where the hole is going. Replace the steel rod with your drill and make the hole. and you've done it.
Another quick note about cirkut springs, They actually made different thickness springs so there was more power but less running time. A normal spring takes about 15 full turns to wind and the "power" spring uses about half that to tightly wind. Quite frankly, I don't like the heavier springs. A well tuned cirkut motor doesn't need that. The more important issue is proper lubrication, NOT OIL, use dry graphite on the spring.
All for now