Repairing a Broken Tooth on a Clock Count Wheel: Slot-and-Solder and Dovetail Insert Techniques

A broken or missing tooth on a clock count wheel presents a repair challenge that is more accessible than it appears at first glance — the count wheel is a relatively lightly loaded component compared to the main gear train wheels, and the tooth replacement techniques used for clock wheel repair apply directly to it. The count wheel controls the strike sequence by presenting notches to a pivoting detent lever at the correct positions in the strike cycle, and the teeth between the notches serve as the engagement surfaces for the advancing mechanism. A missing tooth at a half-hour strike position creates a gap in the wheel's outer perimeter that can cause the advancing mechanism to skip or jam at that position, potentially corrupting the strike count at and around that hour, even if the clock appears to run correctly at first inspection. Understanding the options for tooth repair — from the simple slot-and-solder method to the more refined dovetail insert — allows you to choose the appropriate approach for the specific repair and the skill level you bring to the bench.

This guide covers the complete approach to count wheel tooth repair — how the count wheel functions in the Ansonia iron case and similar American mantel clock movements, whether a clock can function with a missing count wheel tooth and when the answer is no, the slot-and-solder repair technique using a jeweler's saw and a brass strip, the dovetail insert technique for an invisible repair, sourcing flat brass strip material from hardware stores and junk movements, the correct brass hardness for this application and why annealing decisions matter, how to form flat brass from round rod stock if no flat material is available, shaping the new tooth to match the original using a tooth impression, how to practice on a scrap wheel before committing to the actual count wheel, and what typically causes count wheel teeth to break in service.

Understanding the Count Wheel in American Strike Movements

How the Count Wheel Controls the Strike Sequence

American clock movements from Ansonia, Seth Thomas, Sessions, and similar manufacturers use count wheel strike systems — the count wheel is a disk with notches cut at intervals around its perimeter that allow the strike train's detent lever to drop in at the correct position, stopping the train after the correct number of strikes for each hour position. The teeth between the notches serve as the outer perimeter against which the detent lever rides during the strike sequence — the lever slides over the teeth as the count wheel rotates, prevented from dropping into the notch until the wheel has advanced far enough for the notch to come around to the lever position. The count wheel advances one position per complete strike sequence, driven in most Ansonia iron case movements by a gear on the second arbor of the strike train rather than by a tooth-by-tooth advancing mechanism.

The tooth between the half-hour notch and the adjacent hour notch serves two functions: it provides the outer perimeter surface for the detent lever to ride on during the single half-hour strike, and it forms one of the two tooth surfaces that the advancing gear engages as it drives the count wheel from position to position. A missing tooth at this location creates a gap in the outer perimeter that may allow the detent lever to drop prematurely as it crosses the gap, producing a false notch that stops the strike at the wrong count. It also removes one engagement surface from the advancing mechanism, causing the driving gear to skip or jam at that position. The movement may appear to work correctly during initial testing if the missing tooth happens not to be in the detent lever's path during the test sequence, only revealing the problem when that specific position comes around during normal operation.

Can the Clock Work Without the Missing Tooth?

The practical answer depends on how the count wheel is driven in the specific movement. If the count wheel is advanced tooth-by-tooth by a cam or pawl mechanism — where each tooth in turn is engaged to advance the wheel one position — then the missing tooth creates a double-width gap at that position and the advancing mechanism will skip, producing an incorrect count at that hour every day. If the count wheel is driven by a gear on the strike train arbor — where the wheel turns as a continuous rotation driven from the second arbor — the missing tooth still creates a gap in the outer perimeter that may cause the detent lever to drop into the gap as a false notch, stopping the strike early at that position. In either case, leaving the tooth missing risks both incorrect strike behavior and accelerated wear on the adjacent teeth at the gap position, as the advancing mechanism or the detent lever contacts the tooth walls at the gap edge rather than at the correct engagement geometry of a full tooth. Repair is the correct course of action.

The Slot-and-Solder Method

Cutting the Receiving Slot

The slot-and-solder method is the more accessible of the two main tooth repair approaches and requires less precision in fitting than the dovetail insert, making it appropriate for clock repair technicians who have not yet developed the fine fitting skills needed for an invisible dovetail repair. Using a jeweler's saw with a fine blade, cut a straight slot at the location of the missing tooth — the slot should be exactly the width of the missing tooth and should extend from the outer perimeter of the count wheel inward to approximately the depth of the tooth roots, following the same geometry as the original tooth space. The slot walls should be perpendicular to the wheel face and parallel to each other, giving a clean rectangular cross-section that will accept a straight-sided brass strip insert.

For clock repair technicians without a jeweler's saw, a Dremel rotary tool with a thin cutting disc can cut the slot, though achieving perfectly straight, parallel walls requires careful control and practice on scrap material first. The slot must be clean and square — any taper or irregularity in the slot walls will create a gap between the insert and the wheel that the solder must bridge, weakening the repair and making it more visible. Clean the slot with a fine file after sawing to remove any burrs and to ensure the walls are exactly parallel and the bottom is flat for consistent insert seating.

Preparing and Soldering the Brass Strip Insert

Select a piece of flat brass stock that is slightly thicker than the count wheel — the insert should protrude slightly beyond both faces of the wheel before final dressing, so that it can be filed flush without creating a depression that catches the detent lever. The width of the strip should match the tooth slot width as closely as possible — a snug fit with minimal gap between insert and slot walls produces the strongest joint and the most invisible repair after polishing. Cut the strip slightly longer than needed and trim to length after fitting, rather than trying to achieve the exact length before insertion.

Flux the slot and the insert thoroughly before soldering — flux is essential for good solder adhesion on brass and its omission is the most common cause of a solder joint that appears set but has poor adhesion. Place small chips of soft solder at the joint and heat the count wheel from the opposite face using an alcohol lamp or small torch — the heat source applied to the back of the wheel allows the solder to flow from the heat toward the joint through capillary action, producing a fully filled joint rather than a surface bead. Do not apply heat directly to the solder or the joint face, as this produces surface melting before the full joint is up to temperature and results in poor penetration. After soldering, wash the flux residue immediately with hot water and a stiff brush — most fluxes are mildly acidic and will corrode the brass if left in contact. After washing and drying, file the insert flush with the wheel face on both sides, shape the tooth profile to match adjacent teeth, and polish to a consistent finish.

The Dovetail Insert Technique

Cutting the Dovetail Receiver

The dovetail insert technique produces a mechanically stronger and more visually invisible repair than the slot-and-solder method because the dovetail geometry locks the insert against both the pulling and pushing forces that the advancing mechanism applies to the tooth, rather than relying solely on the solder joint for mechanical retention. The dovetail receiver is cut into the count wheel at the missing tooth location using a jeweler's saw and fine files to produce a trapezoidal slot — wider at the back of the wheel than at the outer perimeter — that accepts a correspondingly tapered insert. The insert is pressed into the dovetail receiver from the outer perimeter, with the narrow end entering first, and drives inward until the tapered sides are in firm contact with the dovetail walls.

Cutting a clean dovetail receiver requires more skill and patience than cutting a straight slot, and first-time practitioners should practice on scrap wheels from junk movements before attempting the repair on the actual count wheel. The dovetail angle does not need to be extreme — a modest taper of five to ten degrees per side is sufficient to produce the mechanical locking effect — but the taper must be consistent from the outer face to the inner face of the wheel. Use a fine triangular file to achieve the taper after sawing the initial slot, checking the taper frequently with the insert blank to ensure the fit is converging toward the correct seating position rather than binding at one face or the other.

Fitting the Insert and Peening

Shape the dovetail insert from flat brass stock to match the receiver geometry — the insert should be a snug sliding fit that requires light pressing to drive home but does not require mallet blows that would risk splitting the wheel at the narrow dovetail root. Drive the insert to its final seated position, verify that it is flush with both wheel faces, and flux and solder the joint as described for the slot method. After soldering and cleaning, use a small hammer or punch to lightly peen the insert in place — the peening compresses and spreads the brass slightly, filling any remaining gaps in the dovetail walls and obscuring the solder line. Peening also work-hardens the surface of the insert, improving its wear resistance at the tooth contact surfaces.

After peening, heat the wheel briefly with an alcohol lamp to relieve any stress from the peening operation — this heating does not need to reach a dull red color, any brief heating above room temperature is sufficient to relieve the work-hardening stress introduced by the light peening. File and sand the insert flush with both wheel faces, and file the tooth profile to match adjacent teeth. A piece of Rodico, modeling clay, or similar impression material pressed over a good tooth on the same wheel before beginning the repair provides a template for the correct tooth profile to target when filing the repaired tooth — the impression preserves the tooth shape for reference throughout the filing process.

Brass Material Selection and Hardness

Why Material Hardness Matters for Count Wheel Teeth

Count wheel teeth present a specific material challenge because the tooth must be hard enough to resist the pushing and pulling loads from the advancing mechanism without bending, but not so hard and brittle that it is prone to cracking at the narrow root — the same failure mode that broke the original tooth. This is the same design compromise the original manufacturer faced, and the best way to honor it is to use brass from an old clock movement that has already demonstrated its properties through years of service. Brass from junk movements will typically be in the half-hard condition appropriate for this application — soft enough to file and shape without difficulty, hard enough to resist permanent deformation under normal strike train loads.

Brass strips from hardware stores and hobby suppliers (sold under the K&S brand in the U.S. and available at hobby stores and hardware stores near the scale modeling or metal stock sections) are typically in a half-hard condition that is appropriate for count wheel tooth repair. These strips are available in a range of thicknesses and widths, and the cost is modest for small quantities. Old keys from a locksmith — miscut keys that the locksmith cannot use — are another source of flat brass in a useful hardness range, and locksmiths often provide these at no cost or minimal charge since the miscuts are otherwise scrap.

Avoiding Brittle Brass from Work Hardening

Brass becomes work-hardened — harder and more brittle — when it is deformed by hammering, bending, or rolling without being annealed between deformation steps. Brass that has been hammered flat from round rod stock will be significantly more brittle than the same amount of brass in the original rod form, making it less suitable for a slender count wheel tooth that must flex slightly under load without cracking. If flat brass must be produced from round rod stock, the work-hardening from hammering can be relieved by annealing — heating the brass to a dull red and allowing it to cool naturally. Unlike steel, brass does not require controlled cooling rate for annealing — it can be allowed to air cool, quenched in water, or cooled in any other way without affecting the annealing result, since the annealing temperature for brass is well below the temperatures where cooling rate matters.

Avoid using brass that has been heavily work-hardened without annealing for the tooth insert, as the increased brittleness significantly reduces the tooth's resistance to the type of accidental impact that probably broke the original tooth. Full-hard brass — the hardest commercial temper — is both more prone to cracking at the tooth root and more difficult to file and shape to the correct tooth profile. Half-hard or soft brass from a junk movement, or half-hard commercial strip from a hobby supplier, provides the best combination of machinability and service durability for this application.

Sourcing Flat Brass Stock

Flat brass strip is available from several sources at modest cost for the small quantities needed for clock tooth repair. K&S metal strips at hobby stores are available in multiple thicknesses from approximately 0.016 inches upward and in widths from a quarter inch to several inches, typically in eight to twelve inch lengths at two to five dollars per strip. Hardware stores with a metal stock section carry similar strips. Online metal suppliers like onlinemetals.com sell sheet brass by the square foot at twelve dollars and up depending on thickness — expensive for small quantities but providing a lifelong supply for occasional repair work. Old clock movements provide a free source of appropriately aged and tempered brass in multiple thicknesses from wheel blanks, dial washers, and other flat components. The most practical approach for a technician who does occasional clock repair is to maintain a small collection of flat brass pieces from junk movements in various thicknesses, supplemented by K&S strips for specific thicknesses not available from junk movements.

Practicing on Scrap Before the Repair

Why Practice Matters

Cutting a dovetail receiver into an actual count wheel is a commitment — an incorrectly cut slot that is too wide, too deep, or at the wrong angle cannot be easily corrected without removing more material than intended. Practicing the complete repair sequence — cutting the slot or dovetail, shaping the insert, fitting, soldering, peening, and filing the tooth profile — on a scrap wheel from a junk movement develops the specific muscle memory and judgment needed for a successful repair without the cost of ruining the actual count wheel. The scrap wheel repair also reveals any tools that need adjustment or substitution before the actual repair is attempted, and provides a reference for how the final result should look at each stage.

Use the same brass material for the practice repair as will be used for the actual repair, and complete the practice repair to the point of filing the final tooth profile. Examine the practice result critically — is the insert securely retained, is the joint invisible after polishing, does the tooth profile match adjacent teeth? The quality standard for the actual repair should be set by what the practice repair reveals is achievable, not by an abstract ideal. A first dovetail repair that is detectable on close inspection but invisible in normal use is a good outcome for a beginner and sets a baseline for improvement on subsequent repairs.

What Causes Count Wheel Teeth to Break

Common Causes of Tooth Breakage

Count wheel teeth break in service through several mechanisms. The most common is mechanical impact from a previous failed repair attempt — someone has tried to adjust the strike mechanism with the clock running or under tension, and a tool has slipped and struck the count wheel tooth, breaking it cleanly from the outer edge. This cause is suggested by a clean, straight break across the tooth root rather than the jagged fracture that fatigue failure produces. A second common cause is over-wound or stuck strike trains — if the strike train is wound under full spring tension and then jammed by a mechanical obstruction, the forcing of the count wheel against the detent can break a tooth at the root. A third cause is simply old brass that has become work-hardened over decades of operation and has reached the end of its fatigue life — these breaks typically show evidence of the gradual crack propagation (a curved fracture face with a different texture at the crack initiation zone versus the final fracture zone) rather than the clean single-event fracture of impact damage.

FAQs

Is the tooth repair technique the same for a count wheel as for a regular gear wheel?

Yes — the fundamental techniques of slot-and-solder and dovetail insert apply equally to count wheels and to regular gear wheels. The count wheel's tooth geometry may differ from gear wheel teeth — count wheel teeth are typically narrower and more evenly spaced than gear teeth, and the tooth profile may be more rectangular than the involute or cycloidal profile of clock wheel teeth — but the repair procedure of cutting a receiving slot, fitting a brass insert, soldering, and filing the tooth profile applies to both. The main consideration specific to count wheels is verifying that the repair tooth sits at the correct angular position relative to the adjacent notch, so that the strike count at that position remains accurate.

Can an Ansonia iron case clock run correctly with a missing count wheel tooth?

Probably not reliably. The Ansonia iron case count wheel is driven by a gear on the second strike arbor rather than a tooth-by-tooth advancing mechanism, but the missing tooth still creates a gap in the outer perimeter that the detent lever will encounter as the wheel rotates. This gap may act as a false notch, causing the lever to drop prematurely and stop the strike at the wrong count at that position. Additionally, the advancing gear engages the count wheel teeth to drive the wheel forward, and a missing tooth creates an irregular engagement that can cause skipping or jamming at that position. Test the clock through a full twelve-hour cycle before concluding it runs correctly with the tooth missing — a problem at a specific position may not be apparent during brief initial testing.

What brass should I use for the replacement tooth?

Half-hard brass from an old clock movement is the best choice — it matches the hardness and properties of the original count wheel material, is already aged and stabilized, and is freely available from junk movements. K&S brass strips from hobby stores are a good commercial alternative in multiple thicknesses. Avoid brass that has been hammered flat from round stock without annealing, as the work-hardening from hammering increases brittleness and makes the tooth more prone to the same failure that broke the original. Do not use full-hard brass, which is both more brittle and harder to file to the correct tooth profile than half-hard material.

Which repair method is better: slot-and-solder or dovetail insert?

The dovetail insert is mechanically stronger because its geometry locks the insert against both pushing and pulling forces rather than relying entirely on the solder joint. It also produces a more invisible result when correctly executed. However, it requires more skill and patience to cut the dovetail receiver and fit the insert without gaps. The slot-and-solder method is more accessible for beginners and produces a functional repair on the lightly loaded count wheel even if the visual result is less refined. For a first repair, practice the dovetail technique on scrap before attempting it on the actual count wheel, and use the slot-and-solder method if the dovetail practice results are not satisfactory.

Where can I find flat brass for clock tooth repair?

K&S metal strips at hobby stores are the most convenient source — available in multiple thicknesses and widths in short lengths at modest cost. Hardware stores with metal stock sections carry similar strips. Old clock movements from junk lots provide a free source in multiple thicknesses from wheel blanks, dial washers, and flat levers. Old keys from a locksmith — miscut keys they cannot use — are another practical source of appropriately hard flat brass at low or no cost. Online metal suppliers provide flat brass by the sheet at lower cost per unit area but at a higher minimum purchase than most occasional repair work justifies.

How do I shape the new tooth to match the original profile?

Before beginning any repair work, press a piece of Rodico, modeling clay, or similar impression material firmly over one or more good teeth on the count wheel to capture an impression of the correct tooth profile. This impression serves as a template throughout the filing process — compare the tooth being shaped against the impression at each stage of filing to guide the shape toward the correct profile. File the tooth profile using a fine file, working from both sides alternately rather than completing one face before starting the other, and check against the template frequently. When the tooth profile matches the impression closely and the tooth height is correct relative to adjacent teeth, progress to fine sandpaper for the final surface finish.