Today I am going to show you 2 different methods to resize the center coil of a watch mainspring from a that’s too big for your barrel arbor, so you can get a perfect fit every time.

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In my video, How to Size Any Watch Mainspring to a Barrel, I went over the formula to calculate the replacement size for any watch barrel. One issue that will come up is the center coil being to large for the barrel arbor. This can easily be fixed, so let’s get into it.

Breaking the Watch Mainsprings Inner Coil

See if this sounds familiar. You needed a new mainspring and ordered a generic replacement. When you went to install it, you found out the inner coil is too big for the arbor so your new mainspring won’t stay hooked.

Naturally you try squeezing it to make it smaller and end up breaking it off right by the hook hole.

You can order a new mainspring, but you’ll have the exact same problem again. This is where understanding a little bit about tempering metal comes in handy.

Tempering Steel Makes it Softer

The problem and the reason the inner coil is breaking is because it’s too brittle to bend. We need to temper the coil to soften the metal a little.

Tempering is a process of reheating the hardened metal to a specific temperature to make the metal softer.   The Higher the temperature the softer the steel becomes.

Secure the Barrel Arbor with shellac

In order to help shape the coil after the coil has been tempered, I like to shellac the arbor to a bench block to hold it so that  can shape the coil around it. 

Arbor shellaced to Bench Block
Arbor shellacked to Bench Block

Once the Coil has been shaped then it’s just a matter of removing the arbor from the block and cleaning it up in some denatured alcohol.

How Should the Inner Coil Fit to the Arbor ?

When installing a mainspring, you want the coil to fit tight to the arbor so that when the spring is fully unwound the spring doesn’t become unhooked. 

Tempering the Inner Mainspring Coils

Soldering Iron Method

The first way to temper the coil is with a soldering iron. This soldering iron allows me to pinpoint the heat right on the inner coil of the mainspring. This is really ideal for two reasons.

First, since this soldering iron has a digital temperature display, I know exactly how much heat is being applied to the spring. In this case it’s 440 degrees

Tempering with Soldering Iron
Tempering with Soldering Iron

And secondly it’s better for the springs performance to limit the tempering to the part of the spring that actually needs to be manipulated. You want to leave as much of the spring unchanged as possible.

I leave the mainspring on the soldering iron for several minutes until I get the color I’m looking for.

Tempering Colors

I’m going to keep it on until the coil to turns to a light straw color.

Tempering changes the color of the steel as it gets hotter.  At first it starts turning to a very pale yellow then turns to the light straw color we are looking for.  As the steel gets hotter it turns to a dark straw r brown color.

Steel Tempering Colors
Steel Tempering Colors

Once you get to about 540 degrees it starts turning purple all the way to that  dark blue that we look for when bluing screws at around 590 degrees.

As the temperature continues to increase then the dark blue color starts to lighten again to a medium then lighter blue all the way to grey.

Re-Forming the Coil

Once the coil has been softened, I use a pair of round nose pliers to hold the end of the inner coil of the mainspring for a watch. Try clamping down right where the hole for the hook is. The pliers will not only help shape the inner coil, but they also prevent the coil from getting crushed.

I use a set of Dumont 00 tweezers because they are stiff enough for shaping the coil.

When I shape the coil, I find that by placing the tweezer as close as I can to the hook hole that I can reach. Then by pulling the tweezer across the spring,  the spring will form to the rounded jaw of the pliers. You may have to do this several times to get it to form to the jaws of the pliers. If it doesn’t you need to temper it again.

What I am going for on the reshaping is to get the coil a little smaller than the arbor. Then I can  finish up the shaping on the actual arbor itself.

Tempering with a Spirit Lamp

Sometimes you will run into a situation where the coil is a lot larger than the arbor, its really thick or maybe you don’t have a soldering iron.   The second method you can use is by tempering the coil with a spirit lamp.

I use denatured alcohol in mine because it burns clean and doesn’t leave any black suet behind.

Tempering Watch Mainspring with Spirit Lamp
Tempering Watch Mainspring with Spirit Lamp

You do need to be aware that the spirit lamp gets up well over 1000 degrees. This is way more than you need for tempering so you only want to put it in the flame for a couple seconds at a time.

Using a Balance Tack

Another tool you might have that you can use to form the coil is a balance tack. It is not as easy to use as the round nose pliers, it will work

It takes a little bit to maneuver the coil around so can work on it, but you use the same method of squeezing and pulling like I did before.