Clock Bushing Guide: Selecting, Installing, and Centering Pivot Hole Bushings

Bushing worn pivot holes is one of the most consequential skills in clock repair — and one where the difference between a bushing installed correctly and one installed in the wrong position is not always immediately obvious. A clock with a bushing whose center is slightly displaced from the original pivot hole center will often still run, but with increased friction at the affected gear mesh and reduced pendulum amplitude that may leave the clock unreliable without an obvious cause. Understanding why pivot hole center preservation is the single most important criterion in bushing work — more important than the smoothness of the reamed surface, more important than the quality of the friction fit — and how to achieve it reliably whether you are working by hand or with a drill press, determines whether bushing work improves the movement or simply replaces one problem with another.

This guide covers the complete process of buying and installing clock bushings for the first time — what bushing dimensions mean and how to match them to a specific movement's plate height, KWM and related sizing systems and how the Roman numerals on supply house charts correspond to reamer sizes, why the drill press is both useful and dangerous for bushing work and how to use it correctly, the hand reaming approach and how to keep the reamer perpendicular to the plate using the movement's own pillars as guides, Jerry Kieffer's key stock guide method for achieving accurate center preservation without a milling machine, how to prepare a worn oval hole before reaming, the friction fit standard for a correctly installed bushing and how to test it, cuckoo clock plate considerations including the barrel-shaped bushing modification for thin flexible plates, and how to correct a misplaced bushing through offset reaming rather than starting over. Whether you are bushing a Sessions or Seth Thomas American movement, a Hermle grandfather clock movement, or a Regula 25 cuckoo movement, these principles apply across all brass plate clock movements.

Understanding Pivot Hole Wear and Why Bushing Is Necessary

How Pivot Holes Wear

Clock pivot holes wear oval over time because the pivot does not rotate in the center of the hole — it is pressed against the side of the hole by the driving force of the gear train. This driving force is always in the same direction for any given pivot position, because the gear mesh force has a fixed direction determined by the gear geometry and the direction of the weight or mainspring drive. The pivot therefore bears against the same portion of the hole wall on every revolution, gradually abrading the brass plate and enlarging the hole in that specific direction. A worn pivot hole is not simply a larger circle — it is an oval, with the long axis of the oval aligned with the direction of the driving force at that position in the train. This oval shape is an important diagnostic indicator of the original hole position: the original center of the circular hole is offset from the center of the oval in the direction opposite to the wear, by half the amount the hole has been enlarged in the wear direction.

The consequences of an oval pivot hole include increased play in the pivot position, which allows the pivot to move toward and away from the meshing pinion as the gear rotates. This position variation changes the tooth-to-tooth contact geometry through each rotation, producing periodic friction increases as the teeth contact deeper or shallower than the optimal depth. In severe cases, the pivot can also tilt slightly within the oversized hole, causing the gear to run slightly non-parallel to the plates and creating additional friction at the tooth-pinion mesh. The goal of bushing is to restore the pivot to its original position and provide a correctly sized round bearing surface — not simply to provide a smaller hole in approximately the right area.

Recognizing When Bushing Is Needed

The standard test for pivot hole wear is to insert the pivot and check for side play — the ability to move the pivot from side to side within the hole. A pivot hole in good condition will show essentially no side play, with the pivot fitting snugly in the hole and rotating without any rocking or lateral movement. A worn hole will show visible wobble as the pivot is moved laterally, and the amount of wobble approximately corresponds to the degree of wear. Any hole that shows clearly visible pivot wobble — more than a fraction of a millimeter of side play — is a candidate for bushing. Holes that show a tight fit but with the pivot displaced visibly toward one side of the hole rather than centered have already worn significantly and will continue to wear rapidly if not bushed, even though the wobble may not yet be dramatic.

Selecting the Right Bushings and Reamers

Bushing Dimensions: Height, OD, and ID

Clock bushings are small flanged brass cylinders with a specific outer diameter that sets the friction fit into the enlarged pivot hole, a specific inner diameter that provides the bearing surface for the pivot, and a specific height that determines how far the bushing projects above the plate surface on each side. Selecting the correct bushing for a specific application requires matching all three dimensions — outer diameter large enough to require a friction fit into the reamed hole, inner diameter that provides appropriate clearance for the specific pivot, and height that does not project so far above the plate surface that it interferes with adjacent components.

The plate height dimension is particularly important and is the first thing to verify before ordering bushings. Most American clock movements have plates approximately 1.9 millimeters thick, and the standard bushing height of 1.9 mm is designed so that the bushing ends up approximately flush with both plate surfaces after installation. The Regula 25 cuckoo movement and similar lightweight cuckoo movements have plates significantly thinner than 1.9 mm — approximately 1.4 mm or less — and using a standard 1.9 mm bushing in these thin plates causes the bushing to project substantially above both plate surfaces, where it may interfere with wheels, cocks, or other components that pass close to the plate. For thin-plate movements, bushings with the correct shorter height — 1.4 mm for Regula-type plates — are the appropriate choice. Alternatively, a standard 1.9 mm bushing can be ground down to the correct height, but this requires a flat surface and careful work to maintain the bushing's perpendicularity while grinding.

KWM and Stubs Sizing Systems

Supply house bushing charts list multiple dimensions simultaneously — outer diameter in millimeters, inner diameter in millimeters, height in millimeters, and the reamer size required to prepare the hole for that specific bushing's outer diameter. The reamer size may be expressed in millimeters, in the KWM (Kiefer-Weiss-Mahler) system commonly used for American and German clock bushings, or in Stubs wire gauge numbers used for broaches made to the older English system. The Roman numerals that appear on some bushing charts correspond to specific KWM reamer sizes — the KWM system designates each reamer by a Roman numeral designation that corresponds to a specific outer diameter range. The correct approach is to select the bushing by matching its inner diameter to the pivot that will run in it, then read the corresponding reamer designation from the chart to know which reamer is needed to prepare the hole for that specific bushing's outer diameter.

A practical approach to simplifying bushing selection for a beginner is to make a gauge plate — a strip of brass or aluminum approximately the same thickness as a movement plate — drilled with a series of progressively larger pivot-sized holes and bushed with a graduated set of the most common bushing sizes. Marking each bushed hole with the bushing size, then checking a worn pivot against the gauge plate to find the bushed hole that provides the correct fit, reduces bushing selection to a physical comparison rather than a dimensional calculation. This gauge plate also provides practice with the reaming and installation process before applying the skills to an actual movement, allowing mistakes to be made on scrap material rather than on the clock being repaired.

Drill Press Versus Hand Reaming

The Drill Press: Useful But Requiring Preparation

The drill press is useful for clock bushing work but requires more preparation than simply chucking the reamer and lowering it into the worn hole. The two primary risks with drill press bushing are reamer wander — the reamer following the existing worn oval hole rather than cutting to the original center — and over-reaming, where the drill press continues to cut after the reamer has passed through the parallel-sided portion of the bushing hole, enlarging the hole beyond the reamer's nominal diameter and preventing an adequate friction fit. Both risks can be managed with correct technique, but they cannot be ignored.

Reamer wander is addressed by preparing the worn hole before reaming — enlarging it enough that the oval shape is eliminated and the hole is approximately circular, centered as closely as possible over the original pivot center position. This preparation can be done by carefully filing the hole toward the original center, removing brass from the side of the hole opposite the wear direction until the hole is approximately round and centered correctly. Once the prepared hole is approximately round and correctly positioned, the reamer will cut toward the existing hole geometry rather than wandering toward the worn-out side. Without this preparation, the reamer will follow the path of least resistance, which is the existing worn oval, and cut the bushing hole in the wrong position. Over-reaming is managed by turning the drill press by hand rather than running it under power, or by using a slow feed and stopping immediately when the reamer breaks through rather than continuing to lower the quill after breakthrough.

Hand Reaming: Perpendicularity Is the Challenge

Hand reaming — working the KWM reamer through the plate entirely by hand without any power tool — avoids the over-reaming risk and gives excellent tactile feedback about the reamer's progress, but introduces the challenge of keeping the reamer perpendicular to the plate throughout the reaming process. A reamer that tilts as it cuts enlarges the hole unevenly, producing a hole that is conical rather than cylindrical, which results in a bushing that is not perpendicular to the plate and that will not provide a correctly aligned bearing for the pivot. The movement's own pillar posts — the threaded posts that hold the plates together — can serve as a guide for reamer perpendicularity when the plate still has its pillars attached: hold the reamer parallel to a nearby pillar while reaming, using the pillar as a visual reference for the correct perpendicular direction.

For movements where the pillars have been removed before reaming or are not conveniently located near the hole being bushed, a drill press can be used to establish perpendicularity without being used for cutting — chuck the reamer in the drill press, lower it until it just contacts the prepared hole, and then use the drill press's perpendicular quill as a guide while turning the reamer by hand to avoid over-reaming. This hybrid approach combines the perpendicularity control of the drill press with the tactile control of hand reaming, avoiding the risks of both pure approaches while gaining the benefits of each.

Preserving the Original Pivot Hole Center

Why Center Preservation Is the First Priority

The original pivot hole center determines the gear mesh depth between the wheel at that position and the pinion of the next wheel in the train. This depth was established at the clock's manufacture to produce the correct tooth contact geometry — too deep and the teeth bind; too shallow and they slip. When a bushing is installed with its center displaced from the original pivot hole center, the pivot runs at a position that alters this mesh depth by the amount of displacement. For most American movements, which are manufactured with relatively generous tolerance ranges, small displacements of a few hundredths of a millimeter may not produce noticeable problems. For precision movements, or for specific positions where the geometry is critical — such as the gathering pallet position where even a small displacement can prevent the rack from being gathered correctly — any displacement can be immediately problematic.

The practical implication is that center preservation is not something to aim for when convenient but an absolute requirement for every bushing, approached with the same care regardless of how worn the specific hole appears. A bushing installed slightly off-center may run acceptably for months before the marginal gear mesh depth produces enough friction to stop the clock, at which point the cause is not obvious because the bushing itself looks correct. Developing the habit of center preservation from the first bushing — not after experiencing the consequences of misplaced bushings — prevents this class of failure entirely.

Jerry Kieffer's Key Stock Guide Method

For clock repair technicians who do not have access to a milling machine — which is the ideal tool for accurately locating and reaming a bushing hole to any desired center position — the key stock guide method provides an inexpensive and effective alternative that constrains the reamer to cut at the correct center without requiring sophisticated machinery. The method uses a piece of square steel key stock — the flat bar used for machine key applications, available at hardware stores in standard sizes — with a guide hole drilled precisely through its center on an accurate drill press. The key stock is clamped over the clock plate with the guide hole aligned as closely as possible over the original pivot position, and the reamer is run through the guide hole to cut the bushing hole. The guide hole constrains the reamer from wandering laterally, ensuring that it cuts at the position determined by the guide hole alignment rather than following the existing worn oval.

For the KWM 2.7mm reamer, a Number 36 drill bit (approximately 2.65mm) produces a guide hole that is slightly smaller than the reamer diameter, providing the snug fit needed to constrain the reamer without so much clearance that the reamer can wander within the guide hole. The key stock guide hole will enlarge slightly after several uses as the reamer cuts into it with each pass, reducing its constraining effectiveness — drilling multiple guide holes in the same piece of key stock before beginning, or replacing the key stock periodically, maintains the guide accuracy throughout a bushing session. The total cost of key stock and the drill bit is typically under five dollars, making this method accessible to clock repair technicians at any equipment level.

Filing the Hole to Restore Center Before Reaming

The traditional hand-tool approach to center restoration before reaming is to file the worn oval hole toward the original center — removing brass from the side of the hole opposite the wear direction to shift the hole's center back toward the original position. This requires identifying the wear direction from the oval's long axis, and then filing on the opposite side to create a more circular hole centered closer to the original position. The filing must be done carefully with a fine round or oval needle file, removing material in very small increments and checking the hole shape frequently against a known-round reference. After preparation, the hole should be approximately circular and its center should be as close to the original pivot position as the filing accuracy allows.

The limitation of the filing approach is that it requires skill and patience to achieve accurate center restoration, and the result depends on the technician's ability to judge the hole center by eye and feel while filing. A beginner is more likely to achieve acceptable center preservation with the key stock guide method — which constrains the reamer mechanically — than with the filing approach, which relies entirely on developed skill. As skill develops through experience with the key stock method, the filing preparation approach becomes increasingly reliable as an additional centering step before the guided reaming. These approaches are not mutually exclusive — filing to preliminary center followed by guided reaming with key stock provides better center accuracy than either method alone.

Installing the Bushing and Verifying the Friction Fit

The Friction Fit Standard

A correctly installed bushing must fit tightly enough in the reamed hole that it cannot be pushed out during normal clock operation, but not so tightly that pressing it in damages the plate or deforms the bushing's pivot hole. The test for correct friction fit is straightforward: press the bushing into the reamed hole by hand or with a staking tool applying controlled pressure. If the bushing slides in with no resistance, the hole is too large and the bushing will not stay in place. If the bushing cannot be started into the hole without damaging either the bushing or the plate, the hole is too small and further reaming is needed. The correct fit is firm resistance to entry — the bushing can be pressed in with moderate, steady pressure but will not slide in under its own weight or fall out when the plate is inverted.

After installation, the bushing should project equally above both faces of the plate when the plate height and bushing height are correctly matched. Any projection above the plate surface must be verified not to interfere with adjacent components — wheels, bridges, or cocks that pass close to the plate surface may contact a bushing that projects even a fraction of a millimeter higher than expected. Check clearance with the movement assembled before declaring a bushing correctly installed. The pivot hole in the installed bushing should be chamfered lightly at both faces — a brief contact with a small twist drill slightly larger than the pivot diameter at each face, applied by hand without power, removes the sharp edge that would otherwise abrade the pivot shoulder during operation.

Testing the Installation

After all bushings are installed, test the movement on a movement stand or by hand-powering the train before reassembling the case. The train should spin freely with no position-dependent binding or friction increases. Any position in the train that shows a periodic binding — a slight resistance that occurs at the same rotational position on every revolution — indicates that a bushing at that position is either displaced from the original center, causing a mesh depth error, or that the bushing is tilted, causing the pivot to bind at one point in its rotation. Identify which wheel's rotation coincides with the binding frequency and inspect the bushings at that wheel's pivot positions for tilt or displacement. Correct the problem before installing the movement in the case, because position-dependent binding found at this stage requires bushing removal and replacement rather than external adjustment.

Special Situations: Thin Plates and Cuckoo Movements

Barrel-Shaped Bushings for Cuckoo and Thin-Plate Movements

Cuckoo clock movements — including the Regula 25 and other common German cuckoo calibers — use plates that are significantly thinner and more flexible than the plates of standard American or European mantel clock movements. This thinness creates a specific problem with standard flanged bushings: when the plates flex slightly under load — which thin plates do to a greater degree than thick ones — the movement pillars can pull the plates slightly closer or farther apart, and if a standard bushing has been installed flush with both plate surfaces, the flexing plates will bind against the bushing flange rather than flexing freely. This binding adds friction to the pivot bearing at exactly the moments when the train is under the highest load, which is precisely when the least additional friction is wanted.

The solution for thin-plate movements is to chamfer the bushing on both faces after installation — lightly beveling each end of the bushing cylinder so that the bushing presents a gradually tapering surface to the plate rather than a sharp right-angle edge. This chamfer allows the thin plate to flex slightly around the bushing without catching on the bushing's edge, maintaining free pivot rotation through the plate's flexing cycle. The chamfering is done with a small file or countersink applied very lightly to each face of the installed bushing — just enough to remove the sharp edge and create a small bevel, not enough to weaken the bushing or create excessive clearance at the plate surface. After chamfering, verify that the pivot hole remains the correct size and that the pivot runs freely through the chamfered bushing before declaring the installation complete.

Correcting a Misplaced Bushing

When a bushing has been installed with its center displaced from the original pivot position — either because the reaming wandered into the worn area or because the center was not correctly identified before reaming — the options are to remove the bushing and start over, to install a new bushing with the hole offset from center, or in mild cases to observe whether the displacement actually affects the movement's operation before taking corrective action. Removing the installed bushing requires a punch sized to fit the bushing's outer diameter, driven carefully from the inside of the plate to push the bushing out without damaging the plate. After removal, the hole will be slightly larger than before, but a bushing with a larger outer diameter can be installed and its center located correctly, provided the new bushing's larger outer diameter does not cause it to overlap into an adjacent hole or feature.

The offset bushing approach installs a new bushing whose hole is intentionally drilled off-center — with the hole displaced from the bushing's axis in the direction needed to correct the pivot position — so that when the bushing's outer diameter is centered in the reamed hole, the pivot hole center is at the correct original position. This approach requires either purchasing offset bushings from a horological supply house or modifying a standard bushing by drilling the pivot hole at an offset position before installation. It is a more advanced technique but produces a correct result without requiring the plate to be reworked to a larger hole than necessary for a correctly centered standard bushing.

Sourcing Practice Materials

Finding Donor Movements for Practice

Developing bushing skill before applying it to a clock that matters requires access to practice plates — old movement plates or complete junk movements that can absorb the inevitable beginner mistakes without consequence. Several reliable sources for these practice materials include local clock repair shops, which typically accumulate a bone pile of dead movements that cannot be economically repaired and which they may provide at low or no cost; the eBay platform, where searching for terms like "clock movement lot" and sorting by total price including shipping can surface batches of inexpensive movements; Facebook Marketplace and similar local platforms where old clocks that no longer run are frequently offered at low prices; and NAWCC chapter members who often have excess movements accumulated over decades of collecting and who may share practice material with members who are learning.

Starting practice on plates from movements that are plentiful and inexpensive — common American mantel clock movements from Sessions, Seth Thomas, or similar manufacturers — is preferable to beginning on rare or unusual movements where a practice mistake might damage something difficult to replace. The Regula 25 plate mentioned in the thread, while providing authentic cuckoo movement practice, is thin enough that it presents an additional challenge beyond the basic bushing skill being developed. Beginning practice on plates of standard 1.9mm thickness with standard pivot hole sizes and standard bushing types — mastering the basic process on familiar material — before advancing to the thin-plate modifications required for cuckoo work is the more reliable learning path.

FAQs

What is the most important factor in installing a clock bushing?

Preserving the original pivot hole center is the single most important factor, more important than the smoothness of the reamed surface or the quality of the friction fit. A bushing installed with its center at the correct original position restores proper gear mesh depth and allows the movement to run with the correct power transmission at every position in the train. A bushing installed even a fraction of a millimeter away from the original center alters the gear mesh depth at that position, potentially causing friction increases, reduced pendulum amplitude, or intermittent stopping that can be very difficult to diagnose afterward. Every other aspect of bushing technique is secondary to achieving correct center placement.

Should I use a drill press or hand ream clock pivot holes?

Both methods can produce acceptable results with correct technique. The drill press risks over-reaming if continued past the point of breakthrough and requires the hole to be prepared by filing to the correct center before the reamer is applied, because the reamer will follow the existing worn oval if the hole has not been pre-centered. Hand reaming provides better tactile control and eliminates over-reaming risk, but requires careful attention to keeping the reamer perpendicular to the plate throughout the operation. A practical compromise is to use the drill press to establish perpendicularity by chucking the reamer and lowering it into contact with the prepared hole, then turning the reamer by hand while using the drill press quill as a perpendicularity guide.

Why does the bushing need to match the plate height?

Clock bushings are designed so that a bushing whose height matches the plate thickness will sit approximately flush with both plate surfaces after installation. A bushing that is taller than the plate thickness will project above one or both plate surfaces, potentially interfering with wheels, cocks, or bridges that pass close to the plate. A bushing shorter than the plate thickness will be recessed below the surface, reducing the bearing length and potentially allowing the pivot to contact the plate material at the edge of the bushing hole. Standard American movements use approximately 1.9mm plate thickness and corresponding bushing heights. Cuckoo clock movements with thinner plates require shorter bushings, typically around 1.4mm height.

What is the key stock guide method for clock bushing?

The key stock guide method uses a piece of square steel bar with a guide hole drilled precisely through its center, clamped over the clock plate with the guide hole aligned over the original pivot position. The bushing reamer is run through the guide hole, which constrains it from wandering laterally and ensures it cuts at the position determined by the guide hole alignment rather than following the existing worn oval hole. A Number 36 drill bit produces a guide hole slightly smaller than the KWM 2.7mm reamer, providing effective constraint at minimal cost — the key stock and drill bit together typically cost under five dollars. The guide hole enlarges slightly with use and should be redone periodically to maintain accuracy.

Why do cuckoo clock bushings need to be chamfered?

Cuckoo clock movements have thinner, more flexible plates than standard American or European mantel clock movements. When the thin plates flex slightly under load, they can bind against the sharp edge of a standard bushing that sits flush with both plate surfaces, adding friction to the pivot bearing at exactly the moments of highest train load. Chamfering the bushing — lightly beveling each face of the installed bushing with a file or countersink — creates a gradually tapering surface that allows the plate to flex around the bushing edge without catching, maintaining free pivot rotation through the flexing cycle. This is described as producing a barrel-shaped profile in horological literature, and it is standard practice for any thin-plate movement that shows flexing under normal operating loads.

What should I do if I install a bushing in the wrong position?

First, test the movement before assuming the misplacement is a problem — many American movements tolerate small center displacements because they are manufactured with generous tolerances. If the movement runs cleanly with no position-dependent binding, the displacement may be within acceptable range. If there is binding or the clock runs with reduced amplitude, the bushing must be corrected. Remove the bushing by driving it out from the inside with an appropriately sized punch, assess how far the center has moved, and install a replacement bushing using improved center-finding technique. In cases where significant correction is needed without enlarging the hole further, an offset bushing with the pivot hole drilled off-center can be used to place the pivot hole at the original center position while the bushing's outer diameter is centered in the existing reamed hole.