Introduction
Bushing blanks are essential for restoring worn pivot holes in mechanical clock movements. When a pivot hole becomes oval or oversized, power loss and erratic performance follow. Installing a properly sized bushing blank restores correct pivot support, improves power transmission, and extends the life of the movement. This guide explains how to choose the right bushing blank, how to install it accurately, and how to finish the repair for long-term reliability.
Understanding Bushing Blanks
What bushing blanks are
Bushing blanks are small brass cylinders used to replace worn pivot holes. They come in various outer diameters, inner diameters, and lengths.
Why bushings are needed
Worn pivot holes cause friction, power loss, and poor timekeeping. Installing a new bushing restores proper pivot support.
Types of bushing blanks
Common types include solid brass blanks, pre-sized bushings, and oversized blanks for custom fitting.
Choosing the right material
Most repairers use brass bushings because they wear predictably and are easy to machine.
When not to bush
If the pivot itself is worn or grooved, it must be polished or repaired before installing a new bushing.
Selecting the Correct Bushing Size
Measuring the pivot
Measure the pivot diameter with calipers. The bushing’s inner diameter must be broached to match this size precisely.
Choosing the outer diameter
Select a bushing with an outer diameter large enough to remove all wear when installed.
Checking plate thickness
Choose a bushing length that matches the plate thickness to avoid excessive endshake.
Allowing for broaching
Always choose a bushing with a slightly undersized inner hole so it can be broached to a perfect fit.
Matching pivot shoulder height
The bushing must not protrude beyond the plate or interfere with wheel alignment.
Installing the Bushing
Step 1: Centering the worn hole
Use a centering tool or depthing gauge to ensure the new bushing aligns with the original pivot location.
Step 2: Reaming the plate
Ream the worn hole to match the outer diameter of the bushing. The hole must be round and clean.
Step 3: Pressing in the bushing
Press the bushing into the plate using a staking set or bushing tool. It should fit snugly without distortion.
Step 4: Broaching the inner hole
Use a cutting broach to enlarge the inner hole until the pivot fits smoothly with minimal play.
Step 5: Chamfering and finishing
Chamfer the inside edge lightly to reduce friction and ensure smooth pivot entry.
Testing and Final Adjustments
Checking endshake
The arbor should move slightly back and forth without binding. Too much endshake indicates an oversized bushing.
Checking side-shake
Side-shake should be minimal but present. Excessive play reduces power; too little causes binding.
Testing wheel freedom
Spin the wheel lightly. It should rotate freely and stop gradually.
Lubricating the pivot
Apply a small drop of oil to the pivot hole after assembly. Only the pivot receives oil—never the bushing exterior.
Running a power test
Test the train under light pressure to ensure smooth operation before full reassembly.
Troubleshooting Flowcharts
If the pivot binds
Broach slightly → Check chamfer → Check pivot polish → Verify bushing alignment
If the wheel wobbles
Check side-shake → Check pivot straightness → Verify bushing depth → Inspect plate flatness
If the train loses power
Check pivot finish → Check bushing size → Check lubrication → Inspect adjacent wheels
If the bushing slips
Ream hole properly → Use larger OD bushing → Ensure tight press fit → Avoid over-reaming
If the pivot hole enlarges during broaching
Use smaller broach → Apply less pressure → Rotate evenly → Replace bushing if oversized
Common Mistakes to Avoid
Off-center bushing installation
Even slight misalignment affects depthing and causes power loss.
Over-broaching
Removing too much material creates excessive side-shake.
Failing to polish pivots first
Installing a bushing around a damaged pivot leads to rapid wear.
Using oil on levers or plates
Only pivots receive oil. Excess oil attracts dirt and causes wear.
Skipping freedom tests
Testing each wheel ensures the train runs smoothly before final assembly.
Checklist for Final Verification
• Bushing centered and secure
• Pivot fits smoothly with minimal play
• Endshake and side-shake correct
• Wheel spins freely
• Pivot lubricated properly
• Train runs smoothly under power
FAQs
How tight should a bushing fit?
It should press in firmly without distortion. A loose bushing will fail quickly.
Can I reuse old bushings?
No. Worn bushings must be replaced with new blanks.
Should I oil the bushing?
Only the pivot receives oil. The bushing exterior must remain dry.
How do I know if a hole needs a bushing?
If the pivot moves visibly in the hole or the hole is oval, a bushing is required.
Do all movements require the same bushing sizes?
No. Each movement has unique pivot diameters and plate thicknesses.
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