Bronze Bushing Tolerance and Clearance Guide
Bushing tolerance is not just a machining note. It controls whether the bushing presses into the housing correctly, whether the shaft has enough running clearance, and whether the bearing can hold lubricant without overheating or seizing.
This guide explains the main fit and clearance questions buyers should answer before requesting a quote. It references common engineering systems such as ISO 286 for metric fits and ANSI/ASME B4.1 for inch cylindrical fits, but it does not replace the final engineering drawing or application review.
The Three Fits Buyers Must Separate
| Fit area | Why it matters |
|---|---|
| Bushing OD to housing bore | Prevents the bushing from spinning in the housing and supports heat transfer |
| Bushing ID to shaft | Provides running clearance for motion, lubrication, and thermal expansion |
| Finished bore after installation | Press-fit installation can reduce the bore, so some bushings may require post-installation sizing or reaming |
Many sourcing problems happen when buyers provide only a nominal ID and OD. For a bearing application, the shaft, housing, material, and installation method all matter.
Metric Fits: ISO 286
ISO 286 is a widely used system for metric limits and fits. It uses a letter-number notation such as H7/g6:
- Uppercase letters refer to holes.
- Lowercase letters refer to shafts.
- The number is the IT grade, which indicates tolerance width.
The ISO system includes clearance fits, transition fits, and interference fits. For bronze bushings, the practical question is usually: what housing fit keeps the bushing secure, and what shaft clearance allows the bearing to run?
Inch Fits: ANSI/ASME B4.1
For inch cylindrical parts, ANSI/ASME B4.1 is a common reference for preferred limits and fits. It includes running/sliding fits, locational clearance fits, transition fits, interference fits, and force/shrink fits.
This standard is useful when an inch drawing calls out a fit class or when a buyer needs to align a shaft and housing design with recognized inch-system terminology.
Running Clearance for Bronze Bushings
Running clearance is the space between the shaft and bearing bore during operation. It must be large enough for lubrication, thermal expansion, and small alignment errors, but not so large that the shaft becomes unstable or the bearing wears prematurely.
Clearance depends on:
- Shaft diameter and speed.
- Load and direction of load.
- Lubrication method: grease, oil, oil-impregnated material, graphite plugs, or dry-running liner.
- Bearing material, such as C93200 / SAE 660, C95400 aluminum bronze, or SAE 841 sintered bronze.
- Operating temperature.
- Shaft finish and hardness.
- Whether the bushing is pressed into a housing before final bore measurement.
The Copper Development Association has published design guidance for cast bronze bearings, including the importance of bearing geometry, lubrication, load, speed, and shaft condition. Use those engineering variables rather than a single universal clearance number.
Why Press Fit Can Change the Bore
When a sleeve bushing is pressed into a housing, the OD interference can reduce the inside diameter. The amount of change depends on wall thickness, material, housing stiffness, and fit. This is why drawings sometimes specify the final bore after installation rather than the free-state bore before installation.
If your project requires a critical running clearance, tell the supplier whether the bushing should be quoted as:
- Finished before installation.
- Machined with allowance for press-fit change.
- Finished or reamed after installation.
Lubrication and Groove Effects
Oil holes, grease grooves, annular grooves, figure-eight grooves, graphite plugs, and solid lubricant inserts can all affect bearing surface area and lubrication behavior. A groove should not be treated as a decorative feature. Its location and size should match the load zone and lubricant path.
For more detail, see the Lubrication and Grease Groove Design guide.
Practical RFQ Checklist
Before asking for a tolerance or clearance quote, send:
- Shaft diameter and tolerance.
- Housing bore and tolerance.
- Desired running clearance if already engineered.
- Bushing free-state dimensions if known.
- Finished bore requirement after installation, if applicable.
- Material grade.
- Load, speed, lubrication, and operating temperature.
- Drawing, sample photos, and quantity.
Downloadable Tolerance and Clearance Reference
A downloadable tolerance and clearance reference can be added here after your engineering table is finalized. The table should clearly state whether values are nominal guidance, drawing defaults, or application-specific recommendations.