Smooth Broaching Clock Bushings: Tapered vs Straight Holes, Pivot Clearance, and Best Practices

Few topics in clock repair generate more genuine debate among experienced horologists than whether to smooth broach newly installed bushings, and the debate is worth understanding deeply because the choice affects pivot hole geometry in ways that influence friction, wear rate, and how long a bushing job will last before the clock needs to return to the bench. The question is not simply whether to use a smooth broach — it encompasses whether tapered holes or straight parallel holes produce better bearing conditions for clock pivots, how much clearance a pivot needs in its bushing hole, whether new bushings contain manufacturing defects that broaching corrects, and how to verify that a bushing is correctly installed before the movement is assembled. These are not trivial questions with obvious answers: experienced clockmakers with decades of practice and access to precision equipment disagree on optimal technique, which means understanding the reasoning behind each approach is more valuable than simply adopting one method without understanding why.

This guide presents the complete technical picture of bushing and broaching in clock repair — the purpose of each type of broach, the distinction between cutting broaches and smooth broaches and when each is appropriate, what happens geometrically when a tapered broach is used in a straight new bushing, the oil film bearing argument for why straight parallel holes may produce lower long-term friction than tapered holes, how to check pivot clearance without relying on the five-degree tilt rule alone, how chamfering the inside of a new bushing differs from broaching it, the use of gauge pins to size bushing holes correctly relative to pivot diameter, what the drop-between-plates test reveals about pivot fit, and when smooth broaching is genuinely warranted versus when leaving a correctly installed bushing alone is the better choice.

Understanding the Two Types of Broaches

Cutting Broaches and Their Geometry

A cutting broach is a tapered multi-faceted tool — typically five-sided — that removes metal from a hole as it is pressed and rotated through the hole, enlarging it to match the broach's diameter at the depth of insertion. The geometry of a cutting broach produces a hole that is slightly tapered in the direction of broach travel because each incremental depth of insertion contacts a slightly larger diameter of the broach. When a cutting broach is used from one side only, the result is a hole that is smallest at the plate surface where the broach entered and largest toward the opposite plate surface where the broach tip traveled deepest. When broached from both sides, the hole has a high point at the center of the bushing where the two broached regions meet, with the smallest inside diameter at that midpoint. Understanding this geometry is essential for evaluating whether broaching a bushing will improve or harm the pivot bearing condition.

The cutting action of a standard five-sided broach also has a mechanical characteristic worth noting: because the broach faces present a very negative effective rake angle to the material being cut, the cutting action involves both material removal and compression of the surface layer — a partial work-hardening effect that may slightly increase the hardness of the cut surface. This is one argument made for cutting broaches as tools that simultaneously size and surface-treat the hole. However, the work-hardening effect is modest with standard broaching technique and should not be confused with the more intentional work-hardening produced by dedicated burnishing tools.

Smooth Broaches: Purpose and Correct Use

A smooth broach is a tapered, polished tool with no cutting edges — it burnishes the surface of an existing hole rather than cutting it. Its purpose is to smooth surface irregularities, compress slight burrs, and produce a more uniform interior surface on a hole that has already been sized by a cutting broach or by the bushing installation process. Used correctly, a smooth broach is applied with a drop of oil, inserted into the hole, and given a few rotations to polish and burnish the surface without substantially changing the hole size. The key word is correctly — a smooth broach used with excessive pressure or rotated aggressively in an undersized hole can distort the bushing, change the hole geometry, or produce a tapered waisted hole where none was intended. Used with light pressure as a finishing step after cutting broach work, it can reduce surface roughness and produce a cleaner bearing surface.

One practical diagnostic use of the smooth broach that transcends the polishing debate is as an alignment indicator: when a smooth broach is inserted into a newly installed bushing and stands crooked rather than perfectly vertical, it indicates that the bushing was not installed perpendicular to the plate — a subtle installation error that a purely visual check might miss. Discovering this with a smooth broach before assembly allows the bushing to be repositioned or replaced, preventing a movement that runs with chronic friction from a misaligned bushing. This diagnostic use alone provides justification for having smooth broaches available, regardless of one's position on their use as polishing tools.

The Case for Straight Parallel Pivot Holes

Oil Film Bearing Theory in Clock Movements

The argument for preferring straight parallel pivot holes over tapered broached holes rests on bearing theory: in a correctly sized straight pivot hole with adequate lubrication, the rotating pivot can be supported by a continuous oil film rather than resting in metal-to-metal contact with the bushing wall. When a clock is running and the pivot rotates in its hole with correct clearance and adequate oil, the rotation of the pivot relative to the hole creates a hydrodynamic wedge of oil that slightly lifts the pivot away from direct contact with the brass, reducing friction dramatically compared to what would occur with metal-to-metal contact. This oil film bearing effect — well established in larger engineering bearings — has been demonstrated under magnification in clock movements at the upper half of the movement where gravitational load is lightest and the pivot clearance most favorable.

A tapered pivot hole, whether tapered from one side or hourglass-shaped from broaching both sides, concentrates the pivot's bearing contact at the narrowest point of the taper — a small contact zone where all the pivot's load is concentrated on a limited area of bushing surface. At this contact point, oil film bearing conditions are very difficult to sustain because the concentrated load squeezes oil out of the contact zone. The result is effectively metal-to-metal contact at the tightest point of the taper, producing more friction and faster local wear than would occur with the same pivot in a straight parallel hole of correct diameter. This is the theoretical basis for the position that tapered broaching, even when the resulting hole diameter is nominally correct, may produce worse long-term bearing conditions than a straight reamed hole of the same diameter.

Straight Hole Sizing Using Gauge Pins

Producing a straight parallel pivot hole to the correct diameter requires matching the hole size to the pivot diameter with appropriate clearance — approximately four to five percent larger than the pivot diameter, which is the relationship typically found in factory-original pivot holes on well-made clock movements. The practical method for achieving this without a precision reamer for every possible size is to use gauge pins — precision-ground cylindrical pins available in graduated diameter increments — to first measure the original pivot hole size and then reproduce that size in the new bushing.

The technique begins by finding the gauge pin that fits snugly in the original pivot hole — the pin that is just small enough to insert but locks when pushed in slightly further. This pin's known diameter is then used to select a reamer of that same diameter, which is used to finish the bushing hole to the correct size. If the pivot has been reduced in diameter through polishing work on worn grooves, the pivot hole is reduced by the same percentage — maintaining the original clearance ratio rather than restoring the original absolute diameter. This approach produces consistently correct pivot-to-hole clearances across different movement types and pivot sizes, without relying on subjective assessments of how much clearance looks or feels right.

Why Hand-Held Broaches Always Produce More Taper Than Their Design Angle

A fundamental geometric limitation of hand-held broach work is that it is virtually impossible to hold a broach exactly perpendicular to a plate during broaching. Any deviation from perpendicular — even a fraction of a degree — causes the broach to cut more on one side of the hole than the other, producing a greater taper than the broach's own design angle would create. This effect is cumulative: if the broach enters even slightly off-axis at the beginning of the cut, the cut deepens the off-axis tendency rather than correcting it. The result is a hole that has a taper component from the broach's inherent geometry plus an additional taper component from the hand-holding error, which is different and variable for every broaching operation. This unpredictability is one of the primary arguments for using machine-held reamers of the correct diameter rather than hand-held broaches for sizing bushing holes.

When Broaching New Bushings Is Warranted

Correcting Undersized Bushing IDs

The most straightforward justification for using a cutting broach on a new bushing is when the bushing's inside diameter is slightly undersized for the pivot it must accommodate — the common situation where the next available bushing size up would be too large in outside diameter for the plate thickness, but the chosen bushing's bore is just too small to accept the pivot with correct clearance. In this case, a cutting broach is the appropriate tool for opening the bore to the correct clearance, and the result is a tapered hole that is at least sized correctly at the point of pivot contact. Following the cutting broach with a smooth broach and oil produces a cleaner surface on the newly cut bore.

When choosing between a bushing that is slightly undersized in bore and one that is slightly oversized in bore for a given pivot, neither option is ideal but the slightly undersized bore is generally preferable as a starting point because the bore can be opened up to the correct clearance while the outside diameter remains compatible with the plate. An oversized bore cannot be reduced without replacing the bushing entirely. This is why cutting broach sizing work is a routine part of hand bushing that uses standard-size bushing stock rather than custom-sized replacement bushings matched to each individual pivot hole.

Detecting and Correcting Dog-Boned Bushing Holes

A dog-boned hole — one that is slightly oval or elongated rather than round — can sometimes occur in a new bushing from manufacturing variation or from slight movement of the bushing during the pressing operation. Visual inspection and even loupe examination may not reveal a subtle oval bore, but inserting a smooth broach lightly and holding it up to a light source while rotating it will show whether the broach sits consistently vertical or tilts differently at different rotational positions, indicating an out-of-round bore. Addressing a dog-boned bore before assembly prevents the rocking motion that a slightly oval hole allows — an oval bore allows the pivot to shift laterally within the hole as it rotates, creating a hammering action that accelerates both pivot and bore wear far beyond what a correctly round hole produces.

Chamfering Versus Broaching: An Important Distinction

Chamfering the inside edge of a new bushing — creating a slight bevel at the entrance of the hole on both sides — is a separate operation from broaching and serves a different purpose. The chamfer provides clearance for the shoulder area of the pivot where the pivot transitions to the larger diameter of the arbor. Without a chamfer, the sharp square edge of the bushing bore may contact the pivot shoulder during normal operation, creating a binding point that imposes friction independent of the pivot-to-bore clearance in the bearing zone. A chamfer removes this edge contact without changing the bearing geometry of the hole itself. Many clock repair professionals who avoid broaching new bushing bores as a routine practice still chamfer new bushings as a standard step, recognizing that the chamfer addresses a real mechanical problem while leaving the bore geometry intact.

Testing Pivot Fit Before Assembly

The Drop-Between-Plates Test

After installing a bushing and sizing the bore, a practical bench test confirms that the pivot fits correctly before the movement is fully assembled. With the arbor's pivot in the bushing hole, the wheel should rotate freely and coast down smoothly when given a gentle spin — behaving as though it is running in a well-lubricated bearing. If the wheel stops abruptly after a partial rotation, the pivot fit is too tight and the bore needs opening by a small increment. If the wheel spins indefinitely with the lightest touch and rattles audibly in the hole, the bore is too large and the bushing must be replaced. The characteristic of a correctly fitted pivot is free rotation with gradual deceleration, like a wheel on a well-maintained axle — neither the abrupt stop of a tight fit nor the sloppy rattle of an oversized bore.

An alternative version of this test checks the whole plate assembly by placing the movement with the escape wheel at top position and allowing it to drop a short distance before catching it. A movement with correct pivot fits and properly running train will coast freely for a perceptible period after the catch — the inertia of the gear train carries it forward without the friction of tight pivots arresting it immediately. This whole-movement drop test catches problems that individual pivot inspection misses, because it evaluates all pivots simultaneously under the actual assembled plate geometry rather than one at a time in isolation. Testing on a movement stand with the weights briefly attached and then removed while observing the pendulum amplitude also reveals the net effect of all pivot conditions on actual movement power output.

Pivot Tilt Testing for Clearance Verification

An alternative to numerical clearance specification is the pivot tilt test: with the pivot in the bushing hole and the opposite plate held in position over the opposite pivot without allowing that pivot to enter its hole, tilt the arbor and observe how far the pivot can tilt relative to the bushing hole before contacting the hole edge. Two things should be observed: the pivot should tilt at least one pivot diameter from the edge of the hole before the pivot shoulder contacts the bushing, and the pivot should tilt the same amount in all directions around the 360-degree arc. The first observation confirms that adequate clearance exists for running; the second confirms that the bore is round and the bushing is installed perpendicular to the plate. If the tilt is different at different positions around the arc — more to one side than another — the bore is either oval or the bushing is installed off-axis, both of which require correction before assembly.

The five-degree tilt rule — commonly cited as the target clearance — is a useful starting point but needs contextual adjustment. The same physical clearance produces a larger tilt angle in a thin plate than in a thick plate, so a movement with thin plates will show more apparent tilt than one with thick plates at the same actual clearance dimension. Additionally, a bushing with a chamfered entrance will show more apparent tilt than one without chamfering at the same true bore clearance, because the chamfer effectively shortens the bearing length. These variables mean that five degrees is a guideline to be interpreted intelligently rather than a hard specification that applies equally in all circumstances.

Checking New Bushings for Manufacturing Defects

New bushings from reputable suppliers are generally machined to good tolerances, but no manufacturing process produces perfectly defect-free parts in every unit. Common defects in new bushings include slight bore ovality from the drawing process, minor burrs at the bore entrance from the cutting operation, slight wall thickness variations that cause the bushing to sit at a microscopic angle when pressed into the plate, and surface roughness in the bore that may feel acceptable to a fingertip but shows significant tool marks under magnification. Inspecting new bushings under a loupe before installation costs only a few seconds and catches the occasional defective bushing before it is pressed into the plate and must be drilled out — a more destructive and time-consuming correction. Rejecting one defective bushing in twenty is better than installing all twenty and discovering the problem only after the movement has been running for several months.

Bushing Hole Surface Quality and Long-Term Wear

Surface Roughness and Bearing Wear Rate

The relationship between pivot hole surface roughness and bearing wear rate is a real factor in clock movement longevity, even if the specific numbers are difficult to quantify for the slow speeds and thin plates of horological movements. A smoother bore surface means that the asperities — microscopic high points on the metal surface — are smaller and more closely spaced, producing more uniform load distribution across the bearing contact area and allowing a thinner oil film to provide adequate separation between pivot and bore. A rough bore surface means that the load is concentrated on fewer, higher asperities, increasing the stress at each contact point and accelerating the local wear that enlarges the bore over time.

The counterintuitive finding from precision machining research — that ultra-smooth surfaces are not always ideal for plain bearings and that a small degree of surface texture can help retain lubricant in the contact zone — adds nuance to the goal of maximum smoothness. In practice, the relevant range for clock pivot holes runs from obviously rough (freshly drilled or poorly broached) to very smooth (well-burnished or reamed), and within this range, smoother is reliably better. The academic finding about optimal micro-roughness applies at levels of surface finish far beyond what is achieved in routine clock repair work, so the practical guidance remains: produce the smoothest bore surface practically achievable with available tools, whether that means using a smooth broach on a freshly cut bore or using a reamer that already produces a smooth surface as part of the cutting action.

Steel Wool Deburring as an Alternative Finishing Step

An alternative finishing step for new bushing bores that avoids the tapered geometry of broach work is deburring with 0000-grade steel wool on a finely tapered peg wood stick. The steel wool wrapped around the peg wood tip is inserted into the bore and rotated gently, polishing the bore surface and removing any minor burrs or sharp edges at the bore entrance without introducing the tapered geometry of a broach tool. This technique produces a smoother bore without changing the cylindrical form established by the bushing manufacturing process or the pressing operation. After deburring, the bore is thoroughly cleaned to remove all steel wool fibers before the pivot is installed — loose steel wool fibers in a pivot bore are highly abrasive and must be completely eliminated before assembly.

Practical Approach for Different Skill Levels and Equipment

Hand Bushing Without a Machine

Clock repair enthusiasts working without a bushing machine or precision mill can achieve good bushing results through careful hand technique, appropriate bushing selection, and systematic clearance testing. The key principles are: select the bushing size whose outside diameter correctly spans the worn hole with appropriate metal remaining around the press fit, use a hand bushing tool or staking set to press the bushing concentrically and perpendicular to the plate, use a cutting broach sparingly and carefully only when the bore needs slight enlargement, chamfer both sides of the bore to eliminate shoulder binding, and verify pivot fit with the tilt test before assembling. This sequence, applied consistently, produces reliable results that will serve the movement well through a normal service interval even without machine-level precision.

The most common error in hand bushing is placing the new bushing off-center relative to the worn hole — pressing the bushing in a position that does not restore the correct arbor center location. This error causes the gear mesh between the newly bushed wheel and its neighbors to change, producing friction that did not exist in the original movement. Correct centering requires accurately locating the original hole center before pressing, which is the primary function of bushing machines and the primary challenge of hand bushing. A dial indicator or magnified observation of the original hole relative to the surrounding plate surface guides centering in hand work, and checking the gear mesh in both the affected wheel's neighbors after bushing confirms whether centering was successful.

When to Broach and When to Leave the Bushing Alone

A practical decision framework for smooth broaching new bushings: broach when the smooth broach reveals a misaligned bushing by standing crooked in the hole, broach when the pivot fit is slightly tight and the bore needs a small size adjustment, broach when the bore shows detectable roughness under magnification that needs smoothing, and chamfer whenever the pivot shoulder might contact the bore entrance. Leave the bushing unbroached when it is installed correctly perpendicular to the plate, when the bore shows acceptable smoothness under inspection, and when the pivot fit is correct as verified by the drop test and tilt test. This approach uses broaching as a correction and quality-verification tool rather than as a routine post-installation step applied regardless of whether the bushing needs it.

Bushing in Context: The Complete Movement Service Sequence

Bushing as Part of a Systematic Service

Bushing work does not occur in isolation — it is one step in a complete movement service that includes cleaning, pivot polishing, mainspring inspection and lubrication, escapement adjustment, and reassembly. The quality of the bushing work contributes to the overall service result, but its contribution is limited if other aspects of the service are poorly executed. A perfectly fitted bushing in a pivot hole that still has an unpolished, grooved pivot will wear faster than an adequately fitted bushing with a well-polished pivot. A correctly centered bushing that is subsequently assembled with incorrect plate alignment will experience side loading that nullifies the centering work. Bushing quality is necessary but not sufficient for a good service outcome — it must be accompanied by careful work throughout the complete service sequence.

The interaction between bushing work and mainspring service is worth noting specifically: a movement that returns from service with tight, newly bushed pivot holes will experience higher initial friction than a movement with broken-in pivot holes, and this higher initial friction temporarily reduces the power margin available to the escapement. If the mainspring has been correctly lubricated with mainspring grease and installed using a mainspring winder that does not kink or damage the spring during installation, the mainspring will deliver its full rated output and the reduced power margin from new bushing friction will not prevent the movement from running. If the mainspring is damaged, unlubricated, or incorrectly tensioned from improper winding, the combination of new bushing friction and inadequate mainspring output may stop the movement — leading to a false diagnosis that the bushing work caused the problem when the actual cause is the mainspring condition.

Running In and Initial Wear

All newly bushed pivot holes have a brief running-in period during which the microscopic high points on the bore surface are worn flat by contact with the pivot, after which the friction stabilizes at its long-term running level. This running-in wear is normal and expected — it is not a sign that the bushing work was inadequate. The volume of metal removed during running-in is small relative to the total bushing life, and a clock that is tested on a movement stand for forty-eight to seventy-two hours after service will have completed most of its running-in before being returned to the customer. Testing for this period also reveals any burrs that were not completely removed during the bushing process, which can cause a clock to run correctly for weeks and then stop as a burr works loose and lodges in a pivot hole. A clock that passes a seventy-two hour movement stand test and is then installed in its case and set correctly is highly likely to provide reliable service through the normal service interval.

FAQs

Should I smooth broach every new bushing I install?

Not necessarily. The decision depends on what the broach reveals when used diagnostically. Insert the smooth broach with oil and check whether it stands perpendicular to the plate — if it tilts, the bushing is misaligned and should be repositioned or replaced, not simply broached. If the broach stands perpendicular and the pivot fit is correct as verified by the drop test and tilt test, the bushing may be left as installed. Broach when: the bore needs slight enlargement, the broach reveals misalignment, the surface shows detectable roughness, or you want to chamfer the bore entrance. Leave alone when the bushing is correctly installed, correctly sized, and passes clearance testing.

What is the difference between a cutting broach and a smooth broach?

A cutting broach has faceted edges that remove metal from the hole as the tool is pressed and rotated through it, enlarging the hole to match the broach diameter at the depth of insertion. A smooth broach has no cutting edges — it is a polished tapered tool that burnishes the hole surface by compressing and polishing the metal without substantially removing it. Cutting broaches are used to size undersized bushing bores; smooth broaches are used as finishing tools after cutting broach work, or diagnostically to check bushing alignment and bore surface quality.

Why might a tapered pivot hole cause more friction than a straight one?

A tapered hole concentrates the pivot's bearing contact at the narrowest point of the taper rather than distributing it across the full length of the bore. This concentrated contact zone makes it harder for an oil film to sustain between pivot and bore, increasing the likelihood of metal-to-metal contact at the high point. A straight parallel hole of the correct diameter distributes the pivot load more evenly across the bore length and provides better conditions for the pivot to be partially supported by a continuous oil film, reducing metal-to-metal contact and long-term friction. This difference may be most significant in the upper half of a movement where pivot loads are light enough for oil film conditions to develop.

How much clearance should a pivot have in its bushing hole?

Factory clock movements typically show pivot holes approximately four to five percent larger in diameter than the pivot they accommodate. A practical verification method is the tilt test: with the pivot in the hole and the opposite plate held without engaging the opposite pivot, the arbor should tilt at least one pivot diameter from the edge of the hole in all directions around the 360-degree arc. Equal tilt in all directions confirms a round hole correctly aligned with the plate. Significantly less tilt indicates a tight fit; obviously excessive tilt or audible rattling indicates an oversized bore. The five-degree tilt rule is a useful guideline but must be interpreted relative to plate thickness and bore chamfering conditions.

Can I use a gauge pin to size a bushing hole correctly?

Yes — gauge pins are an effective tool for establishing correct bushing hole size. Find the gauge pin that just locks in the original pivot hole to establish the original hole diameter. Select a reamer of that diameter to finish the new bushing bore to the same size. If the pivot has been reduced in diameter through polishing, reduce the target bore diameter by the same percentage to maintain the original clearance ratio. This method produces consistently correct clearances without relying on subjective tilt assessment, and it works in both machine and hand bushing contexts.

What is the drop test for pivot fit?

With the pivot in the bushing hole and the arbor oriented vertically, give the wheel a gentle spin and observe how it decelerates. A correctly fitted pivot spins freely with gradual deceleration over several revolutions — smooth and consistent like a well-balanced wheel on a clean axle. An overly tight fit produces abrupt deceleration or stops within one or two turns. An oversized bore produces a loose rattling motion where the pivot moves laterally within the hole as the wheel rotates. The whole-movement drop test extends this to the complete train: hold the movement with escape wheel at top, drop it a short distance and catch it, and observe whether the train coasts freely — a correctly fitted movement will show perceptible coasting inertia rather than immediately arresting.

Should I chamfer the inside of a new bushing bore?

Yes — a slight chamfer at both entrances of the bore removes the sharp square edge that can contact the pivot shoulder where the pivot transitions to the larger arbor diameter. Without this chamfer, the square bore edge may bind against the shoulder under axial load, creating friction that is independent of the pivot-to-bore clearance in the bearing zone. Chamfering is a distinct operation from broaching: it targets only the entrance edges and does not change the cylindrical geometry of the bore itself. Many experienced clock repair professionals who avoid routine smooth broaching of new bushing bores still chamfer as a standard practice, correctly identifying the chamfer as addressing a real mechanical problem at no cost to the bore geometry.