Repairing Clock Lantern Pinions: Replacing Trundles, Staking Tools, and Wire Selection

Clock lantern pinions — the cylindrical pinion assemblies used in many American, English, and early European clock movements where the teeth of the pinion are formed by a series of parallel round pins called trundles rather than by cut leaf teeth — are among the more satisfying repairs in clock restoration because a lantern pinion with worn, grooved, or broken trundles can be returned to like-new condition by replacing the trundles with new wire of the correct diameter and length. The repair requires no specialized machining beyond cutting wire to a consistent length, a staking tool or improvised support to crimp the shroud holes closed over the new trundles, and the judgment to know whether to remove the shroud or work with it in place. Understanding the options for each of these steps — wire material, length-cutting method, staking support, and shroud approach — allows you to select the technique that fits your available tools while producing a result that will serve the clock well for another century of service.

This guide covers the complete lantern pinion repair sequence — how to assess whether all trundles need replacement or only specific damaged ones, selecting pinion wire versus music wire versus drill rod for replacement trundles, the case for using hardened versus soft wire, cutting trundles to precise consistent lengths with and without a lathe, the debate over whether to remove the shroud before trundle replacement or work with it in place, the crow's foot tool and split stake as support methods for staking, how to crimp or knurl the shroud holes to retain new trundles without damaging the assembly, removing old trundles that are stuck or work-hardened from long service, and whether lantern pinion trundles were intended to rotate in operation or remain fixed. Whether you are working on a Sessions or Seth Thomas American movement, a Black Forest cuckoo movement, or an antique English clock with lantern pinions in the time train, the fundamental repair principles are the same.

Understanding Lantern Pinion Construction and Wear

How Lantern Pinions Work

A lantern pinion consists of two circular end plates called shrouds, held apart by a set of parallel cylindrical pins called trundles. The trundles are positioned at the correct pitch circle diameter so that as a wheel tooth contacts each trundle in sequence, the trundles act as the leaf surfaces of a conventional cut pinion, transferring the driving force from the wheel to the pinion arbor through the rigid shroud assembly. The number of trundles corresponds to the leaf count of an equivalent cut pinion, and the trundle diameter affects the effective tooth form geometry of the engagement. Lantern pinions were preferred by many clock manufacturers for their relatively simple manufacture — the trundles could be cut from drawn wire without the complex cutting operations required for cut pinion leaves — and their acceptable performance in movements that did not demand the precise tooth form accuracy of fine clock or watch movements.

Wear in lantern pinions manifests as grooves worn into the trundle surfaces where the wheel teeth repeatedly contact them. Because each trundle is contacted at the same angular position by each wheel tooth — the teeth contact the leading side of each trundle as the wheel drives the pinion — the wear groove is located on one side of each trundle and is consistent in depth and position across all trundles in a well-aligned pinion. Trundles worn to the point where the groove depth is significant relative to the wire diameter represent a real loss of effective trundle diameter at the contact zone, which changes the pinion geometry and can cause uneven power transmission and increased friction. Trundles worn completely through — broken by wear — obviously require immediate replacement, and any remaining trundles in the same pinion should be inspected for wear depth before deciding whether they can remain in service or should also be replaced.

Replace One or All Trundles?

The practical guidance from experienced clock repair professionals is to replace all trundles when any single trundle requires replacement, with new trundles cut from the same piece of wire stock. This recommendation has two justifications: trundles from the same clock have the same age and service history, so if one has worn or broken, the others are at a similar point in their wear cycle and may not survive significantly longer; and trundles cut from different pieces of wire stock may have slightly different diameters, hardnesses, or surface finishes that produce uneven contact with the wheel teeth, creating irregular power transmission through the pinion. A mixed set of old worn trundles and new replacement trundles in the same pinion is worse than either a complete set of worn originals or a complete set of new replacements, because the height difference between the worn and unworn trundles causes the wheel to rock slightly as it drives the pinion, creating vibration and accelerated wear at the transition between old and new trundles.

Selecting Replacement Trundle Wire

Wire Materials: Pivot Wire, Music Wire, and Drill Rod

Three main wire materials are used for replacement clock lantern pinion trundles: pivot wire (also called pinion wire or clock wire), music wire (piano wire), and drill rod. Each has different characteristics that affect both the ease of the repair process and the long-term durability of the result. Pivot wire intended for clock repair is the most directly appropriate material — it is sized and specified for horological applications, available in a range of diameters corresponding to common trundle sizes, and has been tested for the hardness and surface finish appropriate to clock pivot applications. However, pivot wire quality has become variable in the market, with some suppliers offering soft or inconsistently hardened wire that does not provide the wear resistance of high-quality original wire.

Music wire (piano wire) is a high-carbon steel wire drawn to precise tolerances and used for piano strings and industrial spring applications. It is available in a wide range of diameters, is consistently hard and strong, and produces excellent wear resistance in trundle applications. The hardness of music wire makes it more difficult to cut to length than soft wire — a standard wire cutter will not produce a clean square end on hard music wire, and a cutting disc or the controlled lathe setup described later is required for clean results. Gun spring wire and other spring steel wires are similar in character to music wire and are used by some clock repair professionals for the same reasons. Drill rod is another option — precision ground steel rod used for making drill blanks, available in accurate diameters with good surface finish and consistent hardness.

Hardened Versus Soft Trundles: The Wear Argument

Original trundles in American and European lantern pinion clocks were often made from relatively soft steel wire — chosen because it was inexpensive, easy to cut to length with simple tools, and adequate in service given that the wheel teeth are typically harder than the trundle wire and the contact is distributed across multiple trundles simultaneously rather than concentrated on a single point. Historical clocks with soft original trundles have provided acceptable service for a century or more, which argues that hardness requirements for lantern pinion trundles are not stringent. However, the reason trundles wear and eventually require replacement is precisely that the wheel teeth gradually abrade the softer trundle surface — harder trundles resist this abrasion more effectively and will last longer before requiring another service.

The practical position taken by most experienced clock repair professionals is that using hardened tempered wire such as music wire or quality pivot wire for replacement trundles is a sensible upgrade that does not change the clock's appearance or operating characteristics but extends the service life of the repair significantly. The modest additional difficulty in cutting hardened wire to length is a worthwhile trade-off for a repair that will not need to be repeated for another century. For clocks where exact historical fidelity is a priority — museum pieces or highly original antiques where any modification including upgrading trundle material might affect the clock's authenticity — using soft wire that matches the original material character is the more appropriate choice.

Matching the Original Trundle Diameter

The most critical dimension for replacement trundles is matching the original trundle diameter as closely as possible. The original trundle diameter was chosen to produce the correct pitch circle geometry for the specific wheel tooth form used in that clock, and using a significantly different diameter changes the effective tooth contact geometry and the meshing characteristics between wheel and pinion. Measure the original trundle diameter with a micrometer at an unworn section — away from any wear groove that would give a reduced reading — and select replacement wire that matches this measurement. Most lantern pinion trundles in American movements fall in a range that corresponds to standard pivot wire and music wire sizes available from horological and hobby suppliers, and finding a close match is straightforward in most cases.

Cutting Trundles to Precise Equal Length

Why Consistent Length Is Essential

All trundles in a lantern pinion must be exactly the same length — the distance from shroud to shroud across the pinion — or the shorter trundles will not fully engage both shroud holes, reducing their retention and allowing them to work loose during operation. The longer trundles will protrude through one shroud hole, potentially interfering with the wheel or with adjacent components, and will also cause the shroud to sit at a slight angle rather than parallel to the opposite shroud. A simple way to check trundle length consistency after cutting is to lay all the cut trundles on a flat surface and roll them gently — all trundles of exactly equal length will roll in a straight line together, while any trundle that is slightly longer or shorter will cause a visible deviation in the rolling path.

The length of replacement trundles must match the original trundles precisely — the inside measurement between the two shroud inner faces. Measure the original length using calipers before removing any trundles, and establish a reference cut length for all replacements. For small batches of trundles, a consistent cut length can be achieved by making a simple jig: a small block of material with a step or stop at the correct distance from the cutting point, against which the wire is pressed before each cut. For larger quantities or for precision matching, the lathe setup described by experienced clock repair technicians produces extremely consistent results with deviations of less than one thousandth of an inch from cut to cut.

Lathe and Dremel Cutoff Setup for Consistent Length

A precise and efficient method for cutting large numbers of trundles to identical length uses a Dremel rotary tool with a thin cutoff wheel mounted in the lathe spindle, with the wire stock held in a collet in the tool post. The wire is advanced through the collet after each cut by a measured distance controlled by the lathe carriage handwheel calibration, and a stop limits the travel to ensure each cut produces the same length. The Dremel cutoff wheel produces a clean, square cut end on both the harvested trundle and the remaining stock with no deformation of the wire at the cut point — unlike snipping with wire cutters, which crushes and distorts the cut ends of hardened wire and requires additional finishing to produce clean square ends. This setup is ideal for clock repair work requiring a batch of identical precision-cut components, and the technique can be adapted to other materials and lengths by adjusting the collet position and carriage stop.

For clock repair technicians without a lathe, a Dremel cutoff wheel used freehand can still produce good results with a simple wooden jig that holds the wire at the correct length while the wheel makes the cut. The critical requirement is that the wire be held perpendicular to the cutting disc and that the disc approach the wire at right angles to produce a square cut end rather than an angled one. Mark each cut position on the wire with a permanent marker before cutting, using a reference trundle placed alongside the stock as the measurement guide. After cutting, check each trundle's length against the reference before proceeding to installation — small discrepancies caught at this stage are much easier to address than after the trundle has been staked into the shroud.

Removing Old Trundles

Pushing Trundles Out Without Removing the Shroud

When working with the shroud in place — the preferred approach of many experienced clock repair professionals — old trundles are removed by pushing them out through the bottom shroud hole using a fine punch or pin, driving them upward through the knurled or crimped retention at the top shroud hole. Most soft-wire trundles in American clock movements can be pushed out this way without difficulty — the knurled retention at the top provides minimal resistance to a controlled punch from below, and the trundle exits cleanly through the top without disturbing the bottom hole. The bottom hole, which typically receives the staking or crimping that retains the trundle, is left undisturbed by this approach, providing a clean hole for the new trundle.

When old trundles are made of harder wire and resist direct pushing, a small drill bit held in a pin vise — not a power drill — can be used to gently open the top retention slightly before pushing. Use a drill bit that is slightly smaller than the trundle diameter so that the drill removes the knurled or crimped retention material without enlarging the hole beyond the trundle diameter. Be cautious with this approach because removing too much material from the hole edge makes it harder to re-crimp over the new trundle, potentially leaving a less secure retention than the original. For particularly stubborn trundles made of genuinely hard tempered wire, cutting through the trundle with a thin Dremel cutoff wheel near the bottom shroud — being careful not to damage adjacent trundles — creates two sections that can be individually extracted through the top hole. After cutting, the bottom hole may need to be cleaned using the pointed end of a piece of pivot wire ground to a spade-drill profile, as described later.

Shroud Removal: When and Why

Some clock repair technicians remove the shroud — the top end plate of the lantern pinion — before replacing trundles, arguing that this provides easier access and allows a more thorough inspection of the shroud holes and arbor condition before reinstallation. The counterargument, held by other experienced practitioners, is that removing the shroud and pressing it back into position invariably produces a looser friction fit than the original press fit — the shroud will have slightly less resistance on the arbor after reinstallation than it had originally, which can allow it to shift position under load in the lower parts of the train where the driving torque is highest. For this reason, many experienced clock repair professionals never remove the shroud and work entirely through the shroud holes, replacing trundles by pushing out the old ones and inserting new ones through the existing openings.

When the shroud must be removed — because the old trundles cannot be extracted without it, or because the arbor requires inspection or service — it can be pressed back onto the arbor after replacement is complete and the fit re-established by careful controlled pressing. The concern about reduced friction fit applies most strongly in the lower train positions where driving torques are highest; in the upper train where loads are lighter, a modest reduction in shroud fit tightness is less likely to cause problems in service. Evaluate the specific location of the lantern pinion being repaired and the condition of the shroud fit before deciding whether to remove the shroud — if the fit can be restored to adequate tightness after reinstallation, the access benefits of removal may justify the approach.

Staking and Retaining New Trundles

The Crow's Foot Tool

The crow's foot is a commercial or shop-made tool that supports the lantern pinion assembly during the staking operation, with a notched edge against which each trundle is positioned while the shroud hole is crimped over the trundle end with a punch. The design keeps the shroud from distorting inward toward the trundles during staking, which would reduce the clearance between trundles and potentially cause them to bind. To use a crow's foot, mount the pinion in the tool with the wheel down and the collet on top. Position the trundle to be staked at the edge of the tool's support notch, and apply the stake to the shroud hole with a gentle tap. Work around the pinion one trundle at a time, rotating the assembly to present each trundle to the support notch before staking.

Crow's foot tools are available commercially from clock supply houses in brass or steel construction, with steel providing better durability under repeated staking operations. The commercial tools are modestly priced and suitable for occasional use. For clock repair technicians who do frequent lantern pinion work, a shop-made crow's foot from scrap steel stock is a worthwhile project — steel is harder than the commercial brass versions and will not deform at the notch edge under staking force. The notch must be sized to support the specific shroud diameter being worked — most commercial tools provide one or two sizes appropriate for common American clock lantern pinions, while a shop-made tool can be made with multiple notch sizes to accommodate different pinion assemblies.

The Split Stake: 360-Degree Support

The split stake is an alternative lantern pinion support tool that provides 360-degree support to the shroud rather than the edge support of a crow's foot. It consists of two flat metal bars of appropriate thickness, held together side by side with a series of holes drilled along the seam between the bars to accommodate different lantern pinion sizes. To use the split stake, the bars are separated at one end to allow the pinion arbor to be inserted between them with the pinion resting in the appropriately sized hole at the seam, and then the bars are brought back together and held by a latch or rubber band to support the pinion. The full circular support of the hole provides better resistance to shroud distortion during staking than the single-point support of a crow's foot notch, particularly for pinions with thin or worn shrouds that might distort under less-supported staking operations.

Making a split stake requires only basic metalworking — two strips of steel or stainless steel approximately 6 inches long and half an inch wide, with a series of holes drilled at the seam between them in sizes to match common pinion diameters. A bridge piece at one end and a latch or rubber band at the other holds the two strips together during use. The simplicity of this construction makes it an ideal shop-made tool for clock repair technicians who work frequently with American lantern pinion movements, and the ability to drill additional holes as new sizes are encountered makes it easily expandable. Steel or stainless steel is preferred over brass for durability — the staking operations will eventually deform softer material at the hole edges, reducing the tool's effectiveness.

Staking Methods: Crimping, Knurling, and Flattening

The original retention of trundles in lantern pinion shroud holes was achieved in different ways by different manufacturers and at different periods: some shroud holes show a straight-line crimp across the hole diameter at the trundle end, some show a knurled pattern around the hole circumference, and some simply press the trundle through a hole that is slightly smaller than the trundle diameter, relying on the interference fit for retention. Reproducing the original retention method is the most aesthetically correct approach for a restoration-quality repair, but functionally any of these methods will adequately retain a well-fitted trundle in a correctly sized hole.

A straight-line crimp can be applied with a modified needle-nose pliers — ground to produce a narrow, flat-faced jaw that contacts the shroud surface just at the hole edge — or with a purpose-ground punch and staking tool. The knurled retention seen on many original lantern pinions can be reproduced with a knurling tool in a lathe, giving a result indistinguishable from the original. For clock repair technicians without a lathe, a series of light punch marks around the hole perimeter approximates the retention effect of a knurl without the regular pattern — adequate for retention purposes even if not visually identical to the original. Slightly flattening one end of each trundle before insertion — as one technique described in the literature — provides retention by a different mechanism, preventing the flattened end from withdrawing through the shroud hole after installation, though this departs from the original manufacturing approach and is not necessary if the hole crimp or knurl retention is properly executed.

Post-Installation Finishing and Verification

Filing and Smoothing Trundle Ends

After all trundles are installed and staked, the ends of the trundles at both shroud faces must be verified to be flush with or slightly below the shroud surface — no trundle should protrude beyond the shroud surface, as protruding ends can catch on the wheel teeth during meshing or on adjacent components during the clock's operation. Check each trundle end with a flat-edged tool or a fingernail dragged across the shroud face — any trundle that catches indicates a protruding end that needs to be filed flush. Use a fine flat file to dress down any protruding ends, working carefully to avoid filing the shroud surface itself, and check frequently to avoid removing more length than necessary. After filing, the trundle ends should be slightly rounded rather than sharp-edged — a few light strokes with a fine file across the face of each trundle end removes the sharp edge and produces a small chamfer that reduces the risk of the trundle end catching during operation.

After completing the finishing work, test the lantern pinion manually by holding the arbor and rotating the pinion while pressing it against a smooth surface to simulate wheel tooth contact. The rotation should feel smooth and even, with no catching or binding at any rotational position. Any roughness at a specific position indicates either a trundle that is not fully seated, an end that is not properly finished, or a shroud hole that was not correctly closed over the new trundle. Address any problem found in this manual test before installing the pinion in the movement — problems are far easier to correct at this stage than after the movement is assembled.

Do Lantern Pinion Trundles Rotate in Operation?

Whether lantern pinion trundles were intended to rotate during clock operation — functioning as true rolling elements that reduce sliding friction between wheel tooth and trundle surface — or remain stationary while the wheel tooth slides across them is a question that generates genuine discussion among clock repair practitioners. The evidence from inspected trundles tells both stories: some trundles show wear grooves on one side only, indicating that they were stationary during operation and the wheel tooth slid across the same position each time; others show even wear around their full circumference, indicating that they did rotate during operation. Both conditions can exist in trundles from the same clock, suggesting that fit tightness rather than design intent determines whether a given trundle rotates.

The functional answer is that lantern pinion clocks operated successfully for a century or more with both stationary and rotating trundles, and the clock will run correctly with new trundles regardless of whether they rotate or remain fixed. Some authorities suggest that slightly loose-fitting trundles that are free to rotate actually improve the meshing characteristics of the pinion by allowing slight self-alignment between trundle and wheel tooth contact geometry — a possible benefit but one that has not been formally demonstrated. Others suggest that rotating trundles distribute wear more evenly around the trundle circumference, extending service life. The practical recommendation is to match the original trundle fit as closely as possible — snug but not binding — and not to over-engineer either the fit tightness or the freedom of rotation for a component that has historically provided reliable service across a wide range of fit conditions.

Lantern Pinions in American Clock Movements

Common Applications in American Movements

Lantern pinions appear throughout American clock movements from the major manufacturers — Sessions, Seth Thomas, Ansonia, Waterbury, Gilbert, Ingraham, and others — typically in the time train between the great wheel and the second wheel, and sometimes continuing through the train to the escape wheel pinion depending on the specific movement design. The fly pinion in the strike train on many American movements is also a lantern pinion design. American lantern pinion movements are among the most common subjects for lantern pinion repair in clock restoration work, and the materials and techniques described in this guide are directly applicable to these movements. The trundle wire sizes used in most American clock lantern pinions correspond to standard pivot wire and music wire sizes available from clock supply houses, making material sourcing straightforward.

On the strike side of American movements, the fly pinion experiences different wear patterns than the time train pinions because the fly operates only during the strike sequence rather than continuously. However, when fly pinion trundles do wear — typically showing deep grooves from the repeated rapid engagement during strike operation — the wear can be severe because the strike train operates at higher speed than the time train and with greater inertia from the fly mass behind it. A worn fly pinion trundle can catch during the strike sequence and cause the clock to run erratically during striking or, in severe cases, cause the strike train to stop mid-sequence. Inspecting the fly pinion trundles during a service that reveals worn time train trundles is good practice, as both sets may have reached similar levels of wear simultaneously.

FAQs

What is a lantern pinion and how does it wear?

A lantern pinion is a type of clock pinion where the teeth are formed by parallel cylindrical pins called trundles, held between two circular end plates called shrouds, rather than by cut leaf teeth. Wear occurs where the wheel teeth repeatedly contact the same position on each trundle's surface, creating a groove that deepens over years of service. Trundles worn to a significant groove depth change the effective tooth form geometry and can cause increased friction and irregular power transmission. Trundles worn completely through — broken by the accumulated groove depth — require immediate replacement. When any trundle shows significant wear or breakage, all trundles in the pinion should be replaced simultaneously with new wire cut from the same stock.

What wire should I use for replacement clock lantern pinion trundles?

Quality pivot wire, music wire (piano wire), or drill rod in the diameter matching the original trundles are all suitable materials. Hardened tempered wire is preferred over soft wire because it resists the wear from wheel tooth contact more effectively and will last longer before requiring another service. Music wire is a practical choice because it is available in precise diameter increments, is consistently hard, and provides excellent wear resistance. Soft steel wire was used in many original movements because it was inexpensive and easier to cut, and historical clocks with soft original trundles have provided acceptable service for over a century — but a service upgrade to harder wire is appropriate when replacing trundles in a movement that will continue in regular use.

Should I remove the shroud to replace trundles?

Most experienced clock repair professionals recommend leaving the shroud in place and working through the existing shroud holes rather than removing and reinstalling the shroud. Removing the shroud and pressing it back onto the arbor typically produces a looser friction fit than the original pressed position, which can allow the shroud to shift under load particularly in the lower train where driving torque is highest. Old trundles can be pushed out through the top shroud hole from below using a fine punch in most cases. For stubborn trundles, a small drill bit in a pin vise can open the top retention slightly before pushing. Remove the shroud only when access through the holes is genuinely impossible or when other service work requires it.

How do I cut trundles to consistent length?

All trundles must be exactly the same length for correct pinion function. Measure the original trundle length with calipers before removing any trundles and establish this as your target cut length. For precise cutting, a lathe setup using a thin cutoff wheel in the spindle with wire stock held in a tool post collet produces consistent results within a thousandth of an inch per cut. Without a lathe, a Dremel cutoff wheel used with a jig or stop that positions each cut at the same distance produces good results for hardened wire that cannot be cleanly cut with wire cutters. After cutting, check all trundles for length consistency by laying them on a flat surface and rolling them — equal-length trundles roll in a straight line without deviation.

What is a crow's foot tool and how is it used?

A crow's foot is a support tool used during the staking of lantern pinion trundles that prevents the shroud from distorting inward toward the trundles as the shroud hole is crimped closed over each trundle end. It has a notched edge against which each trundle is positioned while the staking punch is applied to the adjacent shroud hole from above. To use it, mount the pinion in the crow's foot with the wheel down and the collet on top, position each trundle to be staked at the edge notch, and apply light stake blows one trundle at a time. Commercial crow's foot tools are available from clock supply houses. A split stake that provides full 360-degree circular support to the shroud is an alternative that some practitioners prefer for better distortion resistance.

Can lantern pinion trundles rotate in operation?

Historical evidence from inspected trundles shows both stationary and rotating wear patterns — some trundles from the same clock show single-side grooves indicating they did not rotate, others show even circumferential wear indicating they did. Both conditions appear to produce acceptable clock function over the long term. Whether a given trundle rotates depends primarily on the tightness of its fit in the shroud holes rather than on design intent. Clock repair professionals generally recommend fitting replacement trundles snugly but not binding — neither so tight that they cannot rotate at all nor so loose that they rattle in the holes — and accepting that some may rotate and others may not depending on the specific hole tolerances achieved during installation.

How do I prevent the trundle ends from catching after installation?

After staking, verify that all trundle ends are flush with or slightly below the shroud surface by dragging a flat tool or fingernail across the shroud face. Any trundle end that catches indicates a protrusion that needs to be filed flush with a fine flat file. After filing flush, a few light strokes of the file across each trundle end remove the sharp cut edge and produce a slight chamfer that reduces the risk of catching during wheel engagement. Test the completed pinion by holding the arbor and rotating the pinion while pressing it against a smooth surface — smooth, even rotation throughout confirms that all ends are correctly finished and that no trundle is binding.