Clock Pivot Bushing Wear Tolerance

Clock movements run on small steel pivots (the cylindrical ends of each arbor) turning in pivot holes drilled through the front and back plates. When the holes wear oval — almost always toward the direction of force — the gears mesh poorly, the train binds, and the clock stops working. The fix is “re-bushing”: pressing a small brass bushing (essentially a tiny tube) into the worn hole to restore the original tight fit.

The wear tolerance test:

A clock pivot hole should be only slightly larger than the pivot itself — typically 0.05 to 0.10 mm clearance for most domestic clocks. When the hole reaches roughly 1.10× the pivot diameter, replacement is generally recommended. By 1.25× or worse, the clock is unreliable.

Worked example:

• Original pivot: 1.5 mm diameter
• Measured hole: 1.7 mm
• Ratio: 1.7 / 1.5 = 1.13 (just above the 1.10 threshold)
• Recommendation: re-bush is appropriate; clock will work better with a fresh bushing

Bushing selection:

After confirming a hole needs re-bushing, choose a bushing with:

• Inside diameter slightly larger than the pivot (typically pivot + 0.05 mm)
• Outside diameter such that the bushing fits the cleaned-out (reamed) hole
• Wall thickness adequate for support (usually 0.4-1.0 mm wall)

Standard Bergeon and KWM bushing sizes go in 0.05 mm increments and are sold by inside diameter. A common size for a 1.5 mm pivot is “1.55 mm ID, 2.5 mm OD.”

The re-bushing process (overview, not a tutorial):

1. Remove the worn movement plates from the rest of the movement
2. Ream the worn hole to a clean round shape, slightly larger than the bushing OD
3. Press the bushing in flush with the plate, using a bushing press tool
4. Burnish the new bushing’s inside hole to size with a smoothing broach to fit the pivot
5. Verify the pivot rotates smoothly with no binding and no detectable side play

Common errors:

• Bushing too tight on pivot: causes friction, accelerated wear, eventual stoppage
• Bushing too loose: same problem you started with
• Off-center bushing: causes mesh issues with adjacent gears, often requiring re-bushing AGAIN at the proper position
• Drilling instead of reaming: leaves rough hole walls and uneven bushing fit

When re-bushing is NOT the answer:

• Pivot itself is worn or scored: polish the pivot first; if heavily worn, replace the arbor
• Bent arbor: straighten before re-bushing, otherwise the new bushing will wear at the same angle
• Cracked plate around the hole: the plate may need patching or replacement; bushing alone won’t help

For high-grade clocks (carriage clocks, antique English bracket clocks, fine French regulators), bushing work should be done by an experienced restorer. The plates are often soft brass that distorts under improper pressure, and the original bushings may have been precisely jeweled or hardened. For run-of-the-mill domestic clocks (mantel, kitchen, anniversary), DIY re-bushing with quality tools is achievable for a careful amateur.

Required tools for a starter bushing job: bushing assortment ($60), reamers in matching sizes ($80), bushing press ($30-200), smoothing broach set ($40), magnification (loupe or microscope). Total starter cost roughly $200-400 — pays for itself in 3-4 jobs at typical clock-shop prices.