Measuring Hole End Mainsprings

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About Hole End Mainsprings

The type of springs that are wound in a brass barrel enclosure is known as a Hole End Mainspring.

First, measure the Hole End Mainspring by taking the spring out of the brass clock movement barrel.

Then the mainspring has to come out of the brass barrel in order to measure it. There is no other way to do this task. This sounds easy but carefully read the following text on how to do this safely.


WARNING: Always wear safety glasses and gloves when working with hole end mainsprings. They can, and will be, extremely dangerous to work on. Removal can be violent so please be cautious and know that springs always store power when it is in a coil for so long.

The spring can be broken, or off the hook from the center, and still have an extreme amount of power in the spring. Of course, it can whack, cut and may even shatter, into pieces. So, severe damage to fingers and eyes is a real possibility so protective gear is mandatory.

Let us do it instead

Clockworks offers a mainspring replacement service so send us the entire barrel and avoid the hassle altogether. We will use special tools to safely remove the spring, measure it, and swap it out for the new one.

Measuring Hole End Mainsprings includes the thickness, the length and also the width. Then a comparison of these measurements to the mainspring chart will show the appropriate spring to install. If sending in the barrel, please leave the mainspring in the barrel.

Do not remove it. This is because we need to know which way the clock mainspring gets wound inside the barrel. If the spring is out of the barrel the direction will not be known. Of course this is important in order to replicate the same direction with the new one.

Measuring Hole End Mainsprings

Measuring is done to get the size of the new mainspring to match up to the old one. Measure the width first, then measure the thickness with a micrometer. Then the final measurement is the approximate length. Take all those numbers and compare them to the chart.


Width of Hole End Mainsprings is done first. Measure this in inches or millimeters with a digital micrometer. Then see which spring matches the best in width to the old one.


Once the width of the clock spring is written down and you are in the right location of the chart, continue with the next measurement. The thickness of the Hole End Mainspring is done next.

This measurement can be in inches or millimeters with a digital micrometer. Take this measurement and compare it to the chart along with the width measurement. Then see which spring comes closest to the old spring.


Measuring the length can be the hardest part of this. Close enough is sufficient for this. If the spring is way too short it only means the clock will not run for 8 days.

So more frequent winding will need to be done. It is not easy to uncoil this spring to get the length. What we end up doing is putting one end in a vise and stretching it out with a tape measure. Of course when you let go it snaps back quickly and wildly.


This is just the width of the inside of the clock barrel. As we said before, the barrel is what houses the Hole End Mainsprings.

The barrel inside diameter is in the list because it can help determine the size of the spring that can fit into that space.

Measuring a Hole End Mainspring

If there is no match for the Hole end Mainspring

Sometimes there just is no match for a mainspring and another size has to be put in. It's a tough choice at times what to substitute with in this situation. As far as the width goes it's best to go with a skinnier one instead of wider. The spring that is too wide will make the barrel cap not go on, or stay on.

The thickness is pretty important to hopefully get right. If it is too thick it will put up a fight getting in the barrel and may not want to go in. If it is too thin it may not run the clock. The length is the duration the clock will run such as 8 days or less if it's too short.

Clock Mainspring
Clock Mainspring
Measuring the Clock Mainspring
Measuring the Clock Mainspring
The content of this website is copyright by Clockworks and written by James Stoudenmire in year 2022

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2 years ago

It’s marked 20 on barrel is that the size spring I need

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