Clock Loop-End Mainspring Replacements

Loop End Clock Mainspring - Used on Antique spring driven clocks. These have a loop on the end to wrap around the movements plate arbor. - Clockworks
Loop-End MainspringDescriptionDamage AssessmentMeasuring the Mainspring

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Loop-End Clock Mainspring Description

The Loop-End clock mainspring description is a long flat spring with a loop on one end. This loop gets wrapped around the outer plate pillar of the movement. In other words, this is the type of mainspring that can be seen. It is not contained in a barrel. Likewise, if the spring is in a barrel, the mainspring can't be seen. Hence, a mainspring that cannot be seen is a hole end mainspring. Hole end mainsprings are located on the hole end mainspring page. Follow the directions listed to match up your spring to a new one. Should you need assistance please feel free to email us questions. Our experienced clockmakers are happy to help.

How to know if it is broken

Use the clock key to wind the mainspring. Naturally, the spring is broken when it winds for a bit and then snaps back. Certainly, it could be wound forever and it will not stay coiled. This is how a spring reacts when it breaks. When the spring breaks, it can protrude a great deal outside of the movement. If it is a hole end mainspring it will usually stay contained within the barrel. In extreme cases, the mainspring will damage the clock case. This is not a common occurrence, but it can happen. We do not repair cases. That would need a woodworker to fix. Mainsprings can be very dangerous when they snap. It is also possible for mainsprings to cause damage to the gears in the movement. If this happens then a restoration needs to happen. Please email us for more information on this topic.

How Loop-End Clock Mainsprings come

These mainsprings come wound up with a tie wire around it. Once the movement is reassembled, make sure it is wound up all the way and cut the tie wire. As mentioned previously, treat mainsprings with respect and caution. Always wear safety glasses and gloves. Seriously, mainsprings are nothing to fool with. They can punch, whack, cut, and snap at you. All things considered, mainsprings are quite possibly the only dangerous part to work on in a clock.

Repair service offers a clock repair service. For a loop-end clock mainspring, it would be a movement restoration instead of a mainspring swap. This might be the best solution. Since mainsprings can cause a great deal of damage when they break. There could be much more damage than just a broken mainspring. A restoration will take care of any damage incurred from the break.

If you choose to go this route please email a picture of the movement. We will get back to you on the best solution. A picture is always a good start so we know exactly what you have. Once we see the movement, we can provide a quote to restore it. If the mainsprings are breaking then it is quite possible that the movement is in need of other work. This is typical of anything mechanical. Once one thing breaks, usually other parts are not far behind. Clockworks will advise what is the best and least expensive way to get the clock back up and running. Clockworks always has your best interest at heart. Occasionally movements are beyond repair. This does not happen often, however.


Clock Mainspring Break - Damage Assessment

When a mainspring breaks within a clock movement, it could cause damage. The damage could be within the movement itself or to the clock case. It is uncommon but sometimes even the sides of the wood case get damaged when a mainspring breaks.

The German movement

As for the movement itself, we need to check the gear teeth and the gear arbors. The damage will be on the lower part of the gear train if anywhere. The teeth of the gear that the mainspring is on, its a good place to start. The next gear up is also a frequent damage area. As you go up in the gear train, to the smaller wheels, damage is unlikely. The force of the breaking impact happens in the lower wheels, typically the main wheel and the next one up.

Antiques with lantern pinions

Lantern pinions are the type of gear that is made of vertical wires instead of teeth. It would be the smaller side of the two intersecting gears and would be going up the train not down. The vertical wires within the circular brass bushings act as teeth instead of having a gear. The advantage is the wire will spin within the bushings and thus decreasing friction and wear. Not all clocks have lantern pinions, this would apply to some antique American time strike units only.

Lantern pinion damage check

If the clock has lantern pinions it is possible that some pins have bent or broken off completely. It would happen on the next wheel up from the mainspring gear, or main wheel you could call it. The impact of the main wheel from the break jolts the next wheel up with such force the pinion cant withstand the impact often. These are not so easy to repair if the pin breaks, it requires the removal of the pinion and also the pinion dismantled to restore it. The gear or wheel you could say, comes out, the pinion removed via staking set. This is repaired with the same steel rod size to make perfect again. If the pin is just bent sometimes it can be straightened again with needle nose pliers for a quick fix.

Checking arbors and pivots

Damage assessment to the arbor of the next wheel up from the main wheel is also something to check. Often it will be bent from the impact of the mainspring breaking and would need to be straightened again if possible. Best to remove it and roll the arbor's long side on the table and see if it rolls straight. Also and just as important check the skinny ends of the arbor known as the pivots. These are the skinny ends of the arbor that stick into the movements outer main brass plates. If these are bent, the clock will not run. They need to be straight or the clock will have too much resistance in the gear train for the clock to function under the power of the new mainspring.

The teeth of the gear wheels

Lastly inspect the teeth of the gears on both the main wheel where the spring was attached, and the next wheel up. Bent teeth are common but not as easy to correct. Bending back and reshaping them with a needle file sometimes works, other times the wheel needs a chunk cut out and replaced with new teeth or dentures you could say i guess, soldered in place.

Wooden clock case inspection

Be sure you can get the movement out with no further damage to the clock case. Check the condition of the case to be sure it does not need attention. It may need to be reglued or nailed together more secure as the impact of the spring breaking could loosen the boards.

To much information

If this is not in the realm of your interest in learning about and experimenting with, please send us the movement instead. We will take care of everything for a fee. Please email a pic of what is going on to [email protected] so we can quote.


Clock Loop-End Mainspring Measuring

Loop end mainsprings are measured by width, thickness, and length. These are not always easy measurements to get, and the mainspring would need to be removed from the clock. It can be a frustrating puzzle to do, and a dangerous one also. But if your learning clock repair or have time on your hands so to speak, here are some guidelines.

Caution !!

Always wear gloves and safety glasses when working with mainsprings. They can cut or punch the person working on the clock so please treat them with respect. It is not uncommon for the mainspring to break or shatter while working with them. Just treat them with caution and respect while protect your hands and eyes. Be aware if a mainspring is wound up or stretched way out to measure it, it has a lot of snapping power. These things can hurt when they smack your hand or fingers, or eye.

Getting the thickness and width

Items needed for this measurement is a tape measure and a digital caliper, these are the best things to use. To get the width of the mainspring is easy, just use the digital caliper in inches and see how wide it is first. Look on the chart below for the width column. Next is the thickness, again using the digital caliper in inch or mm. Write these two measurements down and again refer to the chart and narrow down the section even more. Now you have a narrow section of the chart of sizes your working with and only need the length.

Measuring the loop-end mainspring length

The length is the hardest to get the measurement on. What we do here is put the end of the spring in a vise and stretch it out. We pull the spring straight with a tape measure beside it as we unwind it. Of course the tension of the spring is such it wants to wind back up in a violent manner. When the spring is let go it winds back up with a snap. If there is a cat sitting by the vise and you let go, the cat may not like you anymore. The length is the hardest to get but close enough is ok. The length has not much to do with the function of the clock, only the time in which the clock will run. For example if the mainspring substantially shorter, it may not run the full 8 days.

Alternate way of measuring the length

It is possible to make the mainspring straight by just putting bends into the spring to uncoil it. Start from the end and every 6 inches put a bend with pliers in the opposite direction of the coil to make it straight. The mainspring will eventually all be straight and the length can be measured.

The most common size loop end mainspring for antique 1900s through 1945 American units is CML304. This size was sort of the industry standard when Seth Thomas, New Haven, Gilbert, Sessions was in full swing with the 8 day time strike units in mass production.

Loop End Mainspring
LESPRING: $15.00$50.00
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Loop End Mainsprings Measurements to item #

WIDTH inch | mmTHICKNESS inch | mmLENGTH inchItem #
1/8" 3.2.010" | 0.2519"CML 245
9/64" 3.6.009" | 0.2325"CML 246
5/32" 4.009" | 0.2326"CML 247
3/16" 4.8.012" 0.331"CML 252
3/16" 4.8.016" 0.4154"CML 253
13/64" 5.0105" 0.2625 1/2"CML 254
7/32 5.6.011" 0.2824"CML 255
7/32 5.6.017" 0.4330"CML 256
1/4" 6.4.010" 0.2526"CML 257
1/4" 6.4.012" 0.3141"CML 259
1/4" 6.4.013" | 0.3324"CML 260
1/4" 6.4.016" | 0.4154"CML 261
9/32" 7.009" | 0.2330"CML 262
9/32" 7.012" | 0.327"CML 263
9/32" 7.012" | 0.330"CML 264
9/32" 7.017" 0.4348"CML 265
19/64" 7.5.015" 0.3839"CML 266
5/16" 8.009" 0.2320"CML 267
5/16" 8.009" 0.2336"CML 268
5/16 8.010" 0.2530"CML 269
5/16 8.013" 0.3324"CML 270
5/16" 8.013" 0.3330"CML 272
5/16".013" 0.3330"CML 272
5/16" 8.015" 0.3842"CML 273
5/16" 8.016" | 0.4154"CML 274
5/16" 8.016" | 0.4172"CML 275
5/16" 8.017" | 0.4342"CML 276
5/16" 8.020" | 0.5146"CML 277
3/8" 9.5.011" 0.2851"CML 279
3/8" 9.5.013" 0.3334"CML 280
3/8" 9.5.014" 0.3648"CML 281
3/8" 9.5.014" 0.3660"CML 282
3/8 9.5.015" 0.3853"CML 283
3/8 9.5.016" 0.4160"CML 284
3/8" 9.5.017" 0.4396"CML 285
3/8" 9.5.019" 0.4848"CML 286
7/16" 11.014" 0.3672"CML 287
7/16" 11.018" | 0.4648"CML 288
7/16" 11.018" | 0.4660"CML 289
1/2" 12.7.015" | 0.3884"CML 290
1/2" 12.7.016" | 0.4166"CML 291
1/2" 12.7.018" | 0.4660"CML 292
1/2" 12.7.018" 0.4696"CML 293
6/16" 14.3.015" 0.3878"CML 294
9/16" 14.3.018" 0.4696"CML 295
5/8" 16.013" 0.3369"CML 296
5/8 16.018" 0.4696"CML 297
11/16 17.5.015" 0.38108"CML 298
11/16" 17.5.018" 0.4696"CML 299
3/4" 19.012" 0.3172"CML 300
3/4" 19.014" 0.36108"CML 301
3/4" 19.016" | 0.4178"CML 302
3/4" 19.017" | 0.43120"CML 303
3/4" 19.018" | 0.5696"CML 304
13/16" 20.6.018" | 0.4696"CML 305
7/8" 22.2.018" 0.4696"CML 306
1" 25.4.018" 0.4696"CML 307