Mainsprings – should they be replaced every time a clock is serviced?

A short while ago I received a letter from one of my readers. The writer asked if I knew the size of a Junghans B11 mantel clock spring. That was it. No mention that the spring was weak or broken, just one sentence with no explanation.

My reply: “I do not have the mainspring size”.

I went on to say: “I rarely if ever replace mainsprings in a German clock or any clock for that matter, especially ones that were made over 100 years ago. I do not measure the springs unless I plan to replace them. The reason is that the steel used for the mainsprings at the time the clock was made is of higher quality than the mainsprings sold today.”

Junghans movement work
Junghans B11 movement with top plate removed

And then I said, “you don’t mention if the mainspring is broken or not. If you suspect it is weak, it may not the mainspring but there may be friction losses up the train preventing the clock from running its designed cycle, so, the reason for the poor performance must be found, plus the movement needs to be cleaned and bushed, if necessary.”

Junghans Corner feet finished
Junghans bracket clock with B11 movement

The reply stunned me. “That’s the strangest thing I’ve ever heard come from a clock repair person. You cannot expect a 100-year-old mainspring to still have the proper power. ANY proper clock repair person would be replacing mainsprings. It’s crucial to proper service.” I replied that if the customer requests that the springs be replaced or if they are broken, they are replaced.

He replied back, “No it’s because it is needed, bud. Spring steel loses its power after so many years. Springs also become set. I have countless pictures showing this and there is irrefutable evidence of this from clock masters worldwide. Sorry but you not replacing mainsprings is doing clock repair a disservice.”

“Bud”, you say!

I refused to be drawn into a protracted argument that would resolve nothing and the correspondence ended at that point but that got my blood boiling.

My take on mainspring replacement

Way back in clock repair circles and I am talking 15 to 25 years ago it was a general rule to replace all “set” mainsprings and the cost was passed on to the customer. And what is the definition of “set”? A real “set” spring is a defective spring that can no longer power the clock. Few mainsprings actually fall into that category, in my view.

Demonstration of the cleaning of a mainspring
Demonstration of the cleaning of a mainspring

But if we acknowledge that mainsprings can become set, a lot of good old original springs will be replaced and then in a few weeks the original problem resurfaces, and it was not the set mainspring after all.

Sessions mainsprings are cleaned and ready to be re-installed in an American mantel clock movement

Some clock repair persons replace the mainsprings as a matter of course because the additional cost of a spring is nothing (or very little) compared to the cost to disassemble the movement and redo the installation if something goes wrong after servicing. If you are in the business of clock repair perhaps and that is your standard practice, there is nothing fundamentally wrong with that approach. It is insurance against a potential future problem.

A broken mainspring that cannot be repaired

Unless the mainspring is broken the mainspring is probably good if the clock runs for the period it is supposed to run when and only if the rest of the clock is in good order.

If there are fractures or cracks along the spring, yes replacement is prudent and necessary but if the fracture is very close to the outer end loop, it can sometimes be repaired. 

Any cracking or serious rust pitting along the spring and I call it defective and replace it but surface rust is rarely anything to be concerned about.

Furthermore, the poor quality of some new springs makes the decision even more difficult today. But if a new spring is required I avoid anything from India or Asia.

When a clock fails to run a lot of people, especially beginners, assume that if everything else looks good to them that the mainspring must be “weak” or “set”, but unless the spring is actually broken or defective, it is probably fine. Mainsprings generally do what they are supposed to do and often take the blame for non-running clock movements

Mainspring servicing
Using an Olie Baker spring winder to servide a mainspring

Many years ago manufacturers installed springs that were more powerful than they needed to be so that the movement could power through inevitable wear over time. If the main wheel teeth are burring out it is generally the result of a replacement mainspring that is too powerful. Re-using original springs if the spring is in good shape is the better course of action. New more powerful springs may make things worse.

I have been acquiring junk American movements and using them for spare parts and a supply of old loop mainsprings. Usually, the cost of old movements is lower than the cost of new mainsprings.

Final thoughts

I am a collector and repair my own clocks. I do not generally view mainspring replacement as necessary unless issues such as those I mentioned above raise their ugly head.

Many people write to me for advice on their clock issue and I am happy to provide what answers I can. They are kind-hearted folks looking for solutions to their antique or vintage clock problem but every now and again I get tested by somebody who thinks they know everything.

There will be as many opinions as there are clockmakers. However, I believe I echo the view of most clock repairers today regarding mainspring replacement, and perhaps dear letter writer, it is you that is doing clock repair a disservice.


4 thoughts on “Mainsprings – should they be replaced every time a clock is serviced?

  1. The person who wrote to you seems to know enough, or has done “their own research” however, they didn’t seem knowledgeable enough to search for the very simple formula to calculate a new mainspring themselves. There are free calculators for this on the web, and the formula has existed for over a century. For what it’s worth it’s a hot debate. Old springs are not always better quality as new ones, and some CAN pose a risks of sudden failure (which could then break pivots or pinions), so there are some arguments to be made for changing them. Generally I would go by how they look/feel. A set spring is generally obvious. They will barely uncoil when released. I only replace springs when they appear bad, or are obviously broken, the wrong size, etc.


  2. Really interesting piece. Technical matters aside, I would always proceed with caution when ever anyone who doesn’t know me addresses me as ‘bud’. Perhaps that’s just me being very English!


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