Measurement of Mainsprings for 8-Day American Antique Clocks, June 2010

June 12, 2010 | By More

Over the past three years, I’ve posted about mainsprings for antique American clocks. I started with a post on June 12, 2007 about a mainspring from Merritt’s Antiques.

Most clock parts distributors offer a “standard” American loop end 8-day mainspring 3/4 inch wide by 0.018 inch thick and 96 inches long. These springs are too strong! They may cause severe wear to the clock’s mainwheel teeth. A thinner mainspring about 0.0165 inch thick should be used instead. (Regarding mainspring replacement, I believe that the original mainsprings should be kept in an American antique clock unless a spring is broken, damaged or too strong. Notice that I said “too strong” – the old mainsprings are almost never too weak!)

Several clock parts suppliers offer the thinner mainsprings for American clocks, typically 3/4 x 0.0165 x 96 inches. Clock repairers have noted inconsistencies in these springs – the thickness and strength can vary from batch to batch. I recently bought springs from 3 suppliers to test. For each spring, I measured its thickness, and its diameter when first removed from its retainer. On some springs I also measured the diameter after being fully wound and unwound 2 times.

The measurements are given below, with the mainsprings placed in categories based on my recommendations. At times, a supplier will ship mainsprings from a different source. The mainspring types pictured below show the appearance of the springs that I measured.

Recommended Standard Length Springs – 96 Inches Long

Timesavers 18790, Specified as 3/4 x 0.0165 x 96 Inches


Sample 1) 0.0165 inch thick, opens to 10″ (this spring has some rust on the outside and will be exchanged for another.)

Sample 2) 0.0162 inch thick, opens to 11.5″

R & M Imports 77.308, Specified as 3/4 x 0.0165 x 96 Inches


Sample 1) 0.0160″, opens to 11″, 9.5″

Sample 2) 0.0160″, opens to 11.5″

Sample 3) 0.0160″, opens to 11″

Sample 4) 0.0158″, opens to 13″

I have been using this spring from R & M for years. Typical batches from 4 – 5 years ago measured 0.0165 inch thick. There was a batch about 2 – 3 years ago that had brittle inner ends that broke easily. Current springs seem fine in this regard.

R & M Imports 83065, Specified as 3/4 x 0.0165 x 96 Inches

Sample 1) 0.0160″, opens to 11″, 9.5″

This sample is identical to the above spring (R & M 77.308) and costs over twice as much, so there is no reason to buy it!

Merritt’s Antiques P-1496, Specified as 3/4 x 0.0165 x 96 Inches

New springs purchased May 2010:

Sample 1) 0.0159″, opens to 12″, 11″

Sample 2) 0.0161″, opens to 12″, 10″

I first used this mainspring 4 – 5 or more years ago. Some batches have been significantly thinner than others. A batch that that I purchased in October 2008 is packaged the same as this current batch. The springs  have the following measurements:

Sample 1) 0.0165″, opens to 11″

Sample 2) 0.0165″, opens to 11″

Sample 3) 0.0158″, opens to 11″

Sample 4) 0.0165″, opens to 9″, 8″

Sample 5) 0.0165″, opens to 11″

Sample 6) 0.0156″, opens to 11″

An older batch with plastic inner wrap had thinner springs:

Sample 1) 0.0157″, opens to 11″

Sample 2) 0.0155″, opens to 11″

Another old batch (probably 4 – 5 years old) marked “Anchor” on the plastic inner wrap contained mainsprings that were very thin:

Sample 1) 0.0135″, opens to 10″

Sample 2) 0.0140″, opens to 12″

Around June 2007, Merritt’s was shipping a different mainspring under this part number. It was the same as the R & M 77.308 and the Timesavers 18790.

Recommended Long Springs

Timesavers 15959, Specified as 3/4 x 0.0165 x 120 Inches

Sample 1) 0.0160″, opens to 12″

Sample 2) 0.0168″, opens to 13″

120 inch long mainsprings are sometimes recommended for more uniform timekeeping over a one week running period. I have had excellent results with the 96 inch long springs, however, and recommend them for most movements.

Springs That Are Very Strong

Merritt’s Antiques MS309, Specified as 3/4 x 0.0165 x 96 Inches

Sample 1) 0.0158″, opens to 15″, 13″

This is the type of spring made in Germany that is very springy. The package has a yellow label saying “Beco Technic Germany”. They are tempered to a straw color instead of blue like many other springs. I don’t like to use these in average American antique clocks, as they provide more power than necessary.

Merritt’s Antiques MS310, Specified as 3/4 x 0.0165 x 108 Inches

Sample 1) 0.0152″, opens to 21″, 18″

This spring is by Beco Technic Germany just as the above spring. It is intended for the Seth Thomas No. 89 movement. It may be stronger than necessary, and I prefer to use springs in the first or second categories above instead.

Springs That Are Very Weak

Merritt’s Antiques P-1956, Specified as 3/4 x 0.0165 x 96 Inches

Sample 1) 0.0165″, opens to 6″

Sample 2) 0.0165″, opens to 6″

Springs in this batch open out to only 6 inches when released from their clamp. They are not very springy and are too weak for most American antique clocks.

The springs in my first batch two years ago were excellent! They opened out to 11 – 12 inches when released from their clamps, and provided plenty of power. I used these springs in several clocks. But batches received last fall and later have been very weak. So I don’t routinely use these springs anymore.

Other Springs

Timesavers 29515, Specified as 3/4 x 0.016 x 108 Inches

Sample 1) 0.0153″, opens to 28″, 20.5″

This spring seems way too powerful. Also, the surface is not smooth and the spring has a rough action.

Conclusions and Recommendations

For American antique clocks, I keep original mainsprings in the clock unless there is a good reason to change them. For replacements, I prefer springs from the first category (Recommended Standard Length Springs – 96 Inches Long). Many typical Seth Thomas, Ingraham, Gilbert, Sessions and New Haven clocks can use these springs.

Ingraham mantel clocks with deadbeat (or almost-deadbeat) escapement can use a somewhat thinner spring for the time side of the movement.

Some Waterbury and Ansonia movements made from the 1880’s through the 1920’s originally had mainsprings 0.0145″ to 0.016″ thick. It is important to select the proper thinner spring for these clocks.

It is best to measure and label new mainsprings when you receive them. After a time you can build up a stock of normal, thicker and thinner springs, so you can choose the proper mainspring when a mainspring does need replacing.

I believe that routine mainspring replacement should be avoided in antique American clocks.

Thanks to Len Lataille for encouragement to measure these mainsprings.


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Category: American Clock Mainsprings, American Clock Repair

Last updated: August 31, 2011

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