Sessions Tambour Bim-Bam Clock Overhaul: Click Rivet Repair, Bushing, and Strike Train Diagnosis

The Sessions tambour bim-bam mantel clock — the flat-topped case style produced by the Sessions Clock Company through the 1920s and 1930s with a two-rod strike that sounds a high note followed by a low note on the half hour and bim-bam counts on each hour — is one of the most commonly encountered American mantel clocks in estate sales and antique shops today. It is also one of the most frequently found in non-running condition, not from any fundamental design weakness but from two specific mechanical vulnerabilities that affect virtually every surviving example: the Sessions click and click rivet assembly on the mainspring great wheels, which uses a thin brass rivet that loosens progressively over decades until the click slips off the ratchet wheel during winding, and the pivot holes in the brass plates, which develop the characteristic dark deposits of worn pivot material and old lubricant that signal the need for rebushing. Understanding both problems and their correct repairs — rather than the temporary patches that many previous owners have attempted — is the foundation of a Sessions tambour overhaul that will last another century.

This guide covers the complete overhaul sequence for a Sessions tambour bim-bam movement — diagnosing the click rivet failure that prevents winding, making a steel shoulder rivet on a small lathe to produce a permanent repair, reading the dark deposit evidence around pivot holes to identify bushing candidates, the functional difference between the count wheel half-hour strike and the passing bell half-hour strike used in this design and what it means for the time train, the regulating arbor assembly and the critical orientation of the cupped rate-adjustment gears, count wheel loose clip as a strike failure cause, the warning run length and its effect on strike train reliability at the end of the spring wind, and double-hit diagnosis on the high-note bim-bam hammer.

The Sessions Click and Click Rivet: The Primary Failure Mode

Why the Sessions Click Fails

The Sessions clock company's mainspring click design uses a thin click piece pivoted on a small-headed brass rivet that passes through the click and through a hole in the great wheel, with the back end of the rivet peened over to retain the click. The rivet is brass threading into a brass hole in the wheel — brass against brass — and the straight-sided rivet has no shoulder to maintain clearance between the click and the wheel face. As the rivet wears and loosens through decades of winding cycles, the click gains increasing freedom to rock on its pivot and tilt sideways rather than lying flat against the ratchet teeth. A click that has tilted sideways no longer engages the full depth of the ratchet tooth, and under mainspring load it slips off the tooth completely — allowing the spring to reverse and release its tension suddenly through the winding arbor. This is the dangerous slipping failure that Croswell warns about: the spring lets go unexpectedly during winding, driving the key or whatever is engaging the arbor in a sudden backwards rotation that has caused many injuries and damaged many movements.

The Conover repair technique — tightening the original brass rivet using a shim to maintain click clearance while the rivet is re-peened — is a temporary measure that addresses the immediate problem but not the root cause. A brass rivet in a worn brass hole, re-peened without any dimensional advantage over the original, will loosen again within a few years of regular winding cycles as the rivet head works in the worn hole. The correct permanent repair is to replace the original rivet with a steel shoulder rivet that the click pivots on a turned shoulder rather than on the rivet body — the shoulder maintains the correct click clearance without requiring a shim, the steel-in-brass contact reduces wear compared to brass-in-brass, and the larger head on the steel rivet holds the click flat against the wheel more securely than the original small-headed brass rivet.

Making and Installing a Steel Shoulder Rivet

Rivet Design and Material

The shoulder rivet for a Sessions click repair has three sections: a head large enough to hold the click flat against the wheel face without the click being able to tip sideways; a shoulder turned to the correct diameter and length to fit through the click's pivot hole with the click resting on the shoulder rather than the rivet body; and a tail section turned to a smaller diameter that passes through the wheel's rivet hole and is available for peening over on the back of the wheel. The shoulder diameter must be a close running fit in the click's pivot hole — loose enough that the click can pivot freely to engage and release the ratchet teeth, but not so loose that the click tilts. The tail section diameter must be small enough to pass through the wheel hole and to be peened over without splitting.

Cold rolled steel is the correct material for the rivet — available from metal suppliers or from any straight steel fastener of appropriate diameter that can be turned down. Common nails, carriage bolt shanks, and precision ground steel rod are all suitable starting materials. Do not use stainless steel, which work-hardens during turning and is difficult to peen. Before turning the rivet, bevel the entrance of the wheel hole on the back side with a countersink or a fine file so that the peened tail flows into the countersink and sits flush with the wheel face rather than forming a proud bump that could interfere with adjacent components. Ream the click's pivot hole to the correct diameter for the shoulder — one Bergeon reamer size larger than the hole diameter produces a running fit that allows free pivot motion without excessive play.

Installation Procedure

Remove the old brass rivet by grinding or filing its peened back end flush with the wheel face, then supporting the click over a hole in a piece of scrap steel and driving the rivet out with a punch. Clean any burrs from the wheel hole with a reamer or fine file. Insert the new steel shoulder rivet from the front of the wheel, with the head on the front and the tail passing through the wheel from front to back. Hold the rivet head against a hard flat surface and peen the tail on the back of the wheel using a ball peen hammer, working progressively around the tail circumference to spread the material evenly into the countersink. After peening, check that the click pivots freely on the shoulder — it should swing completely from engaged to disengaged without any binding — and that it cannot tilt sideways on the shoulder. Verify by engaging the click with the ratchet teeth and pressing it firmly from the side — the click should remain flat without tilting. If any tipping is detected, a light additional peen on the back will reduce the click's axial freedom without affecting its rotational freedom.

Steel vs Brass Click Spring Wire

The factory click spring on Sessions movements is a fine brass wire that provides the return force pushing the click into engagement with the ratchet teeth. Brass wire click springs are adequate in service but fatigue over time from the millions of flex cycles corresponding to every tooth of the ratchet during winding, and can break without warning after many decades of use. Replacing the brass wire with steel spring wire of a smaller diameter produces a more durable spring that will maintain its tension for longer, at the cost of slightly more work force required to release the click during winding. If the movement will be routinely serviced, a brass wire replacement matches the original; if the movement will be in daily use for many years between services, steel wire is the better choice. Either material is installed by bending the wire to the original spring profile and anchoring one end under the rivet head while the free end presses against the click.

Reading Pivot Poop: Identifying Bushing Candidates

What Pivot Poop Indicates

The dark deposits around pivot holes in clock plates — visible as black or dark brown smudges in a roughly circular pattern centered on each pivot hole — are the accumulated result of worn pivot material and degraded lubricant being ground together over years of operation. When a pivot hole wears oval from the load direction of wheel-to-pinion engagement pressing the arbor consistently toward one side, the pivot and the brass hole wall contact each other beyond the oil film, and metal-to-metal contact grinds tiny amounts of brass from the hole wall and iron from the steel pivot into a fine metallic powder that mixes with the lubricant residue. This mixture oxidizes to the characteristic dark color and migrates outward from the hole, depositing on the plate face in the visible smudge pattern. The presence of this deposit is reliable evidence of significant pivot hole wear — not necessarily so severe that the clock cannot run, but severe enough that the pivot is running on metal rather than on an oil film, accelerating further wear.

The severity of bushing need can be roughly assessed from the deposit size and darkness: a faint, light-colored smudge indicates early wear that can be monitored; a prominent dark smudge occupying a centimeter or more of plate surface indicates wear that has been ongoing for some time and requires bushing before reassembly. In the Sessions tambour movement, pivot poop is most commonly found at the third wheel of the strike side and at the third wheel and escape wheel of the time side — the positions identified in this thread as showing the most prominent deposits. These positions carry the highest loads in their respective trains and develop oval wear first, typically requiring bushing before the lower-load positions in the same movement.

The Passing Bell Half-Hour Strike

How Sessions Bim-Bam Clocks Handle the Half Hour

Most count wheel struck clock movements use the count wheel to control both the hour strike and the half-hour single strike — the count wheel has notches for both, and the strike train activates for both hour and half-hour sequences. The Sessions tambour bim-bam, however, uses a passing bell system for the half-hour: the half-hour hammer lift is produced by a dedicated cam on the center shaft (the shaft that carries the motion work) rather than by releasing the strike train. A small tab on this cam lifts the high-note hammer once as the minute hand passes the six o'clock position, and the hammer drops back under its own weight to produce the single note. The strike train does not run at all during the half-hour.

This design has both advantages and a specific limitation. The advantage is that the strike train wheels run only for the hour sequences, halving the wear on the strike train components relative to a design that activates for both half and full hours. The limitation is that the half-hour note is powered directly by the time train through the center shaft cam, creating a periodic loading on the time train that does not exist in designs where the strike train handles its own power. This loading is typically small — lifting a single light hammer requires little energy — but it is real and can theoretically affect rate accuracy slightly in a very precisely regulated clock. For a Sessions tambour mantel clock, this effect is negligible in practice.

Count Wheel Groove Pattern for Passing Bell Movements

Because the half-hour is not handled by the count wheel in this passing bell design, the count wheel has notches only for the hour positions — twelve deep notches corresponding to the twelve hours, without the additional shallower notches for half-hour positions that would be present in a full count wheel design. This count wheel pattern is distinctive and can be used to identify a passing bell movement from a full count wheel movement: a count wheel with twelve notches only, separated by equal-length arcs between them, is a passing bell design; a count wheel with both twelve deeper notches and twelve shallower notches between them is a full count wheel design that handles both hour and half-hour strikes through the strike train.

The Regulating Gear Assembly

Cupped Gears and Correct Orientation

The rate regulating mechanism in the Sessions tambour movement uses two small bevel gears — one on the regulating arbor (the square-ended arbor turned by the key to raise or lower the pendulum) and one on the suspension assembly that actually moves the pendulum bob's attachment point. These gears are cupped — their tooth faces are curved inward rather than flat — which gives them a specific correct orientation when assembled: the concave faces of both gears must face each other, not face away from each other. Installing one or both gears with the concave face facing outward (the convex face toward the mating gear) places the gear teeth in a configuration where they will not mesh correctly and where tightening the suspension assembly screws locks the gears in a bound, immovable state. This is one of the most common reassembly mistakes on Sessions movements — the cupped gears look similar from either direction until they are installed and mated, at which point the bound configuration becomes immediately apparent but the cause is not obvious without knowing to look for the gear orientation.

The correct assembly is confirmed when the regulating arbor turns freely after the suspension assembly is fully tightened to the plate — the gears mesh smoothly and the suspension assembly translates the arbor rotation into a linear movement of the pendulum attachment point. If the assembly locks up on tightening, flip the suspension assembly 180 degrees before reinstalling — this reverses the orientation of one gear relative to the other and typically produces the correct meshing geometry. Check the free rotation of the regulating arbor after every reassembly of the suspension system, as this is the most sensitive indicator of correct gear orientation.

Count Wheel Clip Loosening and Strike Failure

The Loose Count Wheel Clip Problem

The count wheel in Sessions movements is retained on its driving arbor by a small clip — a thin brass or spring-steel ring that fits into a groove on the arbor and bears against the back face of the count wheel, preventing it from sliding off axially. This clip can loosen over time, allowing the count wheel to wobble on its arbor as the strike train runs. A count wheel that wobbles may intermittently disengage from the driving pinion that advances it by one position per strike sequence, causing the strike to lose count synchronization or to produce only a single strike regardless of the hour. The wobble may be intermittent and apparently random, making diagnosis difficult — the clock strikes correctly most of the time but produces occasional wrong counts or single-strike failures without apparent cause.

The repair requires disassembling the movement to access the clip from behind the wheel, which typically means separating the plates. Once accessible, the clip is tightened by increasing its tension against the arbor groove — either by imparting more curvature to the clip so it grips the groove more firmly, or by replacing the clip with a new one of the correct size. After tightening, verify that the count wheel is held firmly with no axial play before reassembling — the wheel should not tilt or wobble when pressed from either side with a fingertip. If the wheel continues to wobble after clip tightening, check whether the arbor groove has worn, which can be corrected by fitting a slightly thicker clip or by adding a thin washer between the clip and the wheel face.

Warning Run Length and Strike Train Reliability

Why Warning Run Length Matters at Low Spring Power

The warning run — the brief rotation of the strike train between the warning release and the actual strike that allows the warning lever to take position before the train fires — must be of a specific arc length that is long enough to allow the warning lever to engage reliably but short enough that the strike train builds adequate speed before the first hammer lift. A warning run that is too long causes the strike train to begin lifting the hammer before it has built momentum, which can cause the train to stall at the end of the wind cycle when the spring is at its weakest. At full wind, the spring has ample power to overcome the hammer lift from a standing start; at the end of the week when the spring is nearly run down, the available torque may be insufficient to lift the hammer if the train must begin the lift immediately from rest at the warning position.

The warning run length is controlled by the angular position of the stop pin relative to the stop lever when the count finger drops into the count wheel notch. To shorten an excessively long warning run, the warning wheel must be unmeshed from its adjacent wheel, rotated to a position where the stop pin is closer to the stop lever at the moment the count finger drops, and remeshed. This is a delicate adjustment that requires partially separating the plates to access the warning wheel — a procedure that is easier to perform during the initial assembly of the movement after cleaning than as a separate later operation. Once correctly set, the warning run length should require minimal further adjustment over the movement's service life.

Double-Hit Diagnosis on the High-Note Hammer

A double hit on the high-note hammer — where the hammer strikes the rod twice in quick succession rather than once — indicates that the hammer is not resting high enough above the tone rod when at rest, allowing the hammer to contact the rod once on its downward fall and then rebound back onto the rod a second time before the strike train moves the hammer to the fully lifted position for the next note. The standard clearance between the resting hammer and the tone rod is approximately 3mm — enough that the hammer's downward fall produces a single clean strike and the hammer returns to its rest position above the rod without a second contact. Adjusting the hammer rest position upward by bending the hammer pivot mounting point slightly, or by adjusting the hammer's position on its arbor if it is adjustable, increases the resting clearance and eliminates the double hit.

FAQs

Why does the Sessions tambour clock slip when I try to wind it?

The most common cause of winding slip on a Sessions tambour clock is a worn click rivet — the brass rivet that pivots the click has loosened over years of use, allowing the click to tilt sideways away from its correct flat position against the ratchet teeth. A tilted click does not fully engage the ratchet teeth and slips off them under the mainspring's load when winding pressure is applied. The correct repair is to replace the worn brass rivet with a steel shoulder rivet turned on a small lathe — the shoulder maintains correct click clearance, the larger head holds the click flat, and steel-against-brass wears less than the original brass-against-brass contact. Temporary re-peening of the original brass rivet provides only a short-term fix that will loosen again within a few years.

What does the dark deposit around a Sessions clock pivot hole mean?

The dark deposit around a pivot hole — called pivot poop — is the accumulated mixture of worn metal particles from the pivot and hole walls, degraded lubricant, and oxidation products. It is reliable evidence that the pivot hole has worn oval from the consistent load direction of wheel-to-pinion engagement, and that the pivot is running on metal rather than on an oil film. Pivot holes showing this deposit should be bushed before the movement is reassembled — installing a correctly sized brass bushing centered on the original hole position restores the correct bearing geometry and allows the pivot to run with oil lubrication rather than metal contact.

How does the Sessions bim-bam half-hour strike work?

The Sessions tambour bim-bam uses a passing bell system for the half-hour — a dedicated cam on the center shaft lifts the high-note hammer once as the minute hand passes the six o'clock position. The strike train does not run at all during the half-hour; only the time train provides power for the half-hour note through the center shaft cam. This is why the count wheel on these movements has only twelve notches (for the twelve hours) rather than twenty-four — the half-hour is not counted or controlled by the count wheel. The passing bell system reduces wear on the strike train but adds a small periodic load to the time train at each half-hour passage.

Why does the regulating arbor lock up when I reinstall the suspension assembly?

The rate adjustment uses two cupped (concave-faced) bevel gears — one on the regulating arbor and one on the suspension assembly. Both gears must be assembled with their concave faces toward each other. If either gear is installed with its convex face toward the mating gear, tightening the suspension assembly screws locks the gears in a non-meshing bound condition. If the regulating arbor locks up on tightening, flip the suspension assembly 180 degrees — this reverses the orientation of one gear and should produce the correct face-to-face meshing configuration. Always check that the regulating arbor turns freely after tightening the suspension assembly fully.

What causes occasional wrong hour counts or single strikes on a Sessions tambour clock?

Intermittent wrong strike counts or single-strike failures on an otherwise functional Sessions tambour movement are most commonly caused by a loose count wheel clip — the small spring clip that retains the count wheel on its driving arbor. A loose clip allows the count wheel to wobble, intermittently disengaging from the advancing pinion and losing count synchronization. The repair requires disassembling the movement to access the clip and increase its tension against the arbor groove. After tightening, verify that the count wheel has no axial wobble before reassembly.

How do I fix a double hit on the bim-bam hammer?

A double hit occurs when the hammer rests too close to the tone rod — close enough that after the hammer falls and strikes, it rebounds back onto the rod for a second contact before being lifted again by the cam. The standard resting clearance between hammer and rod should be approximately 3mm. Bend the hammer pivot mounting slightly to raise the hammer's rest position, or adjust the hammer on its arbor if the position is adjustable. After adjustment, verify that the hammer produces a single clean strike at all positions in the hour count, including the last strike before the train stops.

Does the pendulum bob weight matter on a Sessions tambour clock?

Not critically. The pendulum bob provides resistance that allows the escapement to function correctly — without any bob, a recoil escapement will race and a deadbeat will behave erratically. For a recoil escapement like that used in the Sessions tambour, a heavier bob produces a more stable beat and slightly more power absorption that reduces the escapement's sensitivity to minor power variations. Within the typical range of 1.5 to 4 ounces that the pendulum bobs commonly found with these clocks represent, any bob will allow the clock to run. Use the branded Sessions bob if available for historical correctness; use any bob of a similar size if the original is missing.