Clock Loop-End Mainspring Measuring
Loop end mainsprings are measured by width, thickness, and length. These are not always easy measurements to get, and the mainspring would need to be removed from the clock. It can be a frustrating puzzle to do, and a dangerous one also. But if your learning clock repair or have time on your hands so to speak, here are some guidelines.
Always wear gloves and safety glasses when working with mainsprings. They can cut or punch the person working on the clock so please treat them with respect. It is not uncommon for the mainspring to break or shatter while working with them. Just treat them with caution and respect while protect your hands and eyes. Be aware if a mainspring is wound up or stretched way out to measure it, it has a lot of snapping power. These things can hurt when they smack your hand or fingers, and I would not want to even think about what it can do to an eye.
Getting the thickness and width
Items needed for this measurement is a tape measure and a digital caliper, these are the best things to use. To get the width of the mainspring is easy, just use the digital caliper in inches and see how wide it is first. When you have the width, find that width section first on the below chart. Next is the thickness, again using the digital caliper. Write these two measurements down and again refer to the chart and narrow down the section even more. Now you have a narrow section of the chart of sizes your working with and only need the length.
Measuring the loop-end mainspring length
The length is the hardest to get the measurement on. What we do here is put the end of the spring in a vise and stretch it out. We pull the spring straight with a tape measure beside it as we unwind it. Of course the tension of the spring is such it wants to wind back up in a violent manner. When the spring is let go it winds back up with a snap. This is the sort of stuff that I was cautioning about earlier, if no gloves it can cut. If there is a cat sitting by the vise and you let go, the cat may not like you anymore. The length is the hardest to get but close enough is ok. The length has not much to do with the function of the clock, only the time in which the clock will run. For example if the mainspring substantially shorter, it may not run the full 8 days.
The most common size loop end mainspring for antique 1900s through 1945 American units is CML304. This size was sort of the industry standard when Seth Thomas, New Haven, Gilbert, Sessions was in full swing with the 8 day time strike units in mass production.