Here we have a complete listing of both hole end clock mainsprings as well as the loop end. The hole end mainspring is the kind that is in a barrel and you cannot see the clock mainspring unless you open the barrel cap to see it. The loop end mainspring is the kind you can see right away when you look at the movement as it is not in an enclosure. It is called a loop end as it has a loop on the outer end that goes over the pillar of the movement. Both the hole end and loop end clock mainsprings have a hole end in the inner end of the spring where it will attach to the winding arbor.
There is a special tool required to change clock mainsprings, called a mainspring let down key set. This tool eliminates the possibility of the repairman getting injured and is described in the Clock Help pages. You really do not want to get whacked by one of the clock mainsprings, they can hurt. This is why the tension of the spring is released before separating the clock plates, or attempting to remove the spring in anyway. Without the tension of the spring being released, the spring will unwind it self very fast in a uncontrollable fashion that can harm the repair person and the rest of the clock movement.
In order to swap a mainspring you would need to measure the approximate length, the thickness and the width of the mainspring you now have. This requires the broken mainspring to come out of the clock movement, and out of the barrel if it has one. If the mainspring is contained in a casing, called a barrel, then it’s a hole end mainspring. If you can see the mainspring in the clock then it’s a loop end mainspring.
You will use a micrometer or caliper to measure the thickness of the spring steel, then measure how wide the spring is, then try to get the length the best you can. The length is the trickiest measurement to get, usually it’s done by clamping one end in a vise and stretching it out and measuring section by section and then adding it up. So the length is the one measurement that does not have to be perfect. If you have the thickness and width correct and you find it on the chart, but the length is not perfect it is still ok. Best to get one a little shorter than longer if using a barrel as a longer spring may not fit in the container. So now if the mainspring you install is a little shorter than the original the only thing that will happen is it will not run quiet as long. Instead of 8 days on a full wind it may run 7 ½ days or something. Sometimes this is all that can be done if the mainspring is not in production at the exact same size as the one needed.
If all this is confusing or troublesome, then you can just send the movement, pendulum, and hands into us for a repair. However when we do a repair we not only fix the mainspring issue but we test each gear and arbor for damage that often occurs during mainspring breakage. We also rebush any worn pivot holes and put the movement through the cleaning and testing process. If you would like a quote for this work before sending it in, please email some pics of the movement to firstname.lastname@example.org and request a quote.
Handle with 4 chucks covers about every key size. Put the chuck into the handle and then release the mainspring in a slow controlled manner.
Item #CT25-- $38
Set of 4 mainspring clamps keep the clock mainsprings in the wound position to keep them under control. Best to get two of these sets for paired sized mainsprings.
Item #CT2-- $11
Used to release the power in the wound up clock mainsprings. Key goes into the chuck and the chuck goes into the handle so you can let down the power of the spring. Just another way to release the mainspring power.
Item #CT26-- $29
Winder for loop end clock mainsprings, to be used with the mainspring clamps next item over.
Item #CT1-- $10
This fits most American 8 day clocks 3/4 x 0.18 x 96" or 5/8 x 0.18 x 96"
This fits most 31 day Korean clocks 3/4 x .165 x 170