The correct choice also depends on nominal size, load, operating temperature, material, surface finish, manufacturing capability, inspection method, and assembly or maintenance requirements. This guide provides a practical selection workflow. For help interpreting designations such as H7/g6 and H7/p6, see our guide to ISO fit symbols and tolerance zones.
The old saying goes… No one size fits all. Your choice should be based on function first, and then assign tolerances accordingly.
1. How do you select the right ISO fit for a real application?
A reliable ISO fit selection process starts by defining the required relationship between the mating features. If movement must always be possible, select a clearance fit. If accurate location is more important and a small amount of clearance or interference is acceptable, evaluate a transition fit. If relative movement must be resisted and press or thermal assembly is acceptable, evaluate an interference fit.
Before selecting a tolerance class, document the nominal size, load direction, speed, temperature range, material pair, lubrication, surface finish, assembly method, serviceability requirements, manufacturing capability, and inspection plan.
2. What is the difference between Clearance, Transition, and Interference Fits?
ISO fits describe the dimensional relationship that can occur between two mating features across their permitted size limits:
- Clearance fit: The smallest permitted hole remains larger than the largest permitted shaft, so clearance is guaranteed throughout the allowed limits.
- Transition fit: The permitted limits can produce either a small clearance or a small interference. This type is considered when accurate location is required and either result is acceptable for the assembly.
- Interference fit: The permitted limits are selected to produce interference rather than clearance. Assembly commonly requires mechanical force, heating, cooling, or a combination of these methods.
A useful shorthand is: clearance fits permit movement, transition fits support accurate location, and interference fits resist relative movement. However, the final selection must be checked against the nominal size, operating conditions, materials, assembly method, and current tolerance tables.
3. Should one tolerance the hole, the shaft, or both? Why is H so common for holes?
Take a look at almost any technical drawing, and you will see a familiar pattern: H7/h6, H7/g6, H7/p6…
Why is H so common for holes? It’s design logic backed by manufacturing reality.
In a basic-hole system, an H hole has a lower deviation of zero, and its tolerance zone extends above the nominal size. For example, an H7 hole at a nominal diameter of 20 mm is commonly represented as 20.000 to 20.021 mm for the applicable nominal-size range. Final production limits must be verified against the current ISO 286 tables.
The basic-hole system is widely used because a standard hole class can be retained while the shaft tolerance zone is adjusted to produce the required relationship. This may simplify tooling and process planning, but it is not a universal rule. A basic-shaft system or another hole-and-shaft combination may be more appropriate for a particular design.
In real production, this is where precision CNC machining becomes critical. Even when the correct ISO fit is selected on the drawing, the final result still depends on machining strategy, material behavior, inspection method, surface finish, and how consistently the tolerance can be held across multiple parts.
The terminology, tolerance principles, and basic-hole/basic-shaft concepts are defined in ISO 286-1, while ISO 286-2 provides the standard tolerance classes and limit deviations used to establish dimensional limits.
In other words, “Let the hole stay constant. I’ll make the shaft fit the application”.
4. How does the assembly method affect tolerance selection?
Your choice of fit isn’t just about the function. It’s also about how the parts will be assembled. Let’s look at three common methods:
Manual assembly (hand-press or slide-in)
A clearance fit such as H7/g6 or H8/f7 may be considered when parts must slide together without a press. The exact choice depends on the required play, alignment, speed, lubrication, temperature, wear, and manufacturing capability.
Press-fit (mechanical force)
You need an interference fit (like H7/p6 or H7/m6). A press is used to insert one part into another, and the fit is tight enough to resist slipping under designed level loads.
Thermal assembly (heating/cooling)
For stronger interference relationships, the component containing the hole may be heated, the shaft may be cooled, or both methods may be used temporarily during assembly. Material expansion, wall thickness, coatings, available temperature difference, and the risk of permanent distortion must be evaluated before this method is specified.
Assembly force, temperature difference, edge geometry, coatings, surface finish, wall thickness, and the risk of galling or distortion should be reviewed before an interference fit is released for production.
5. How do tighter tolerances affect manufacturing cost and precision?
Tighter tolerances may give the feeling of precision and safety “the tighter the range, the better the fit and therefore the better the performance,” right? Not quite.
Tight tolerances (such as g5 or h5) require:
- More accurate machines (often slower and more expensive to maintain)
- Greater tool wear and therefore more frequent tool changes adding costs
- Added quality control steps (e.g., CMM inspection, gauge verification)
These requirements can increase machining time, process-development effort, inspection cost, scrap risk, and production lead time. The commercial impact depends on the process, material, batch size, tolerance range, and required evidence of conformity.
Let’s put it this way:
Tight tolerances where they matter→ essential.
Tight tolerances everywhere → expensive and overkill.
For parts that require reliable mating fits, controlled bore or shaft dimensions, and repeatable assembly performance, tight-tolerance CNC machining should be reviewed early in the design stage rather than after production problems appear.
If a bracket hole provides only bolt clearance and does not locate the assembly, an H7 requirement may be unnecessarily restrictive. The appropriate looser tolerance must still be selected from the fastener standard, functional clearance, tolerance stack-up, manufacturing process, and drawing requirements.
6. What other factors affect tolerance selection?
Tolerancing isn’t just about size other technical realities matter as well:
Material
- Aluminum expands more than steel. Therefore a press-fit in aluminum needs to consider thermal expansion during the design process.
- Plastics can change dimension through creep, moisture absorption, temperature, residual stress, and long-term loading. A fit that works immediately after machining or molding may behave differently during assembly or service.
Surface Finish
- Surface roughness, waviness, coatings, and plating thickness can change actual contact conditions and assembly behavior.
- For press or shrink fits, the surface condition can affect insertion force, galling risk, wear, and the effective interference remaining after assembly.
Manufacturing Method
- A turned shaft using a lathe may have different tolerance capabilities compared to a precision-ground shaft.
- Some tolerances may not even be possible or practical when using low-cost manufacturing methods.
For shafts, pins, sleeves, bushings, and other rotational components, CNC turning services can be evaluated against the required fit, surface finish, material, and inspection method.
These factors must be considered alongside fit type during the design process, not after.
Inspection Method
The measuring equipment, temperature condition, datum strategy, sampling plan, and acceptance rule must be appropriate for the specified tolerance. When a measured value is close to a specification limit, measurement uncertainty can affect the conformity decision. ISO 14253-1 provides decision rules for verifying conformity or nonconformity while accounting for measurement uncertainty.
7. What are the most common mistakes engineers make when choosing fit?
Even experienced engineers and designers fall into these traps:
Over tolerancing
Specifying unnecessarily tight tolerances out of habit or misinformation, without functional justification.
Result: Higher costs, longer lead times, greater part rejections.
Assuming parts from different suppliers will match or be consistent
Different production facilities may have different process capabilities, inspection equipment, measurement methods, and default general tolerances. If the mating dimensions and acceptance requirements are not explicitly defined, components produced in separate facilities may fail to assemble consistently.
Ignoring the assembly method
Designing a fit that theoretically works however is impossible to assemble with available tools or fixtures.
“Copy and Paste” from past drawings
Reusing a fit from a previous drawing without checking the current nominal size, loads, materials, process capability, assembly method, and service conditions can create unnecessary cost or functional failure.
The lesson: Every fit choice should answer this question.
“What does this part need to do, and how will it be made and assembled?”
Learn more about our CNC machining services.
8. How can one ensure parts fit when multiple suppliers are involved?
When mating components are produced in different facilities, interchangeability depends on shared drawing and inspection requirements rather than nominal size alone. Process capability, datum interpretation, surface condition, measurement methods, and acceptance criteria must be aligned before production.
To ensure global interchangeability:
- Specify the nominal size and the hole-and-shaft tolerance classes explicitly, such as H7/g6.
- For critical assemblies, include the resulting limit dimensions or required clearance/interference range on the controlled drawing.
- Define relevant datums, GD&T requirements, material condition, surface finish, coatings, and operating temperature.
- State whether the fit must guarantee clearance, permit a transition relationship, or guarantee interference.
- Agree on inspection equipment, measurement locations, sampling frequency, and acceptance rules.
- Confirm that both components are evaluated together as part of the assembly tolerance stack-up.
Also, communicate clearly. If it’s a transition fit, state as such. If it must be a press-fit without fail, specify the required interference value.
Interchangeability comes from a shared dimensional and inspection definition, not simply from assigning the same nominal diameter to two components.
Interchangeability isn’t just about drawing quality; it’s about shared understanding.
9. Which ISO fits are commonly evaluated for different applications?
The examples below are starting points, not universal design rules. The required fit can change with nominal size, load direction, speed, temperature, lubrication, material, wall thickness, surface finish, assembly method, and maintenance requirements. Final limits must be verified against the current ISO 286 tables and component-specific engineering requirements.
Sliding or Rotating Shaft in a Plain Bore
H7/g6 or H8/f7 may be evaluated when guaranteed clearance is required. The acceptable choice depends on allowable play, speed, lubrication, wear, temperature, and required guidance.
Accurately Located Removable Component
H7/h6 may provide a close-clearance relationship with zero minimum clearance, while H7/js6 can create a transition relationship. Confirm whether any interference is acceptable before selecting between them.
Press-Mounted Hub or Gear
H7/m6 or H7/p6 may be evaluated as starting points, but the required interference must be calculated from transmitted load, component geometry, material strength, wall thickness, surface finish, and assembly force. Keys, splines, or other torque-transfer features may still be required.
Rolling-Element Bearing Seats
Do not apply one generic ISO fit to every bearing seat. Follow the bearing manufacturer’s recommendations based on which ring rotates relative to the load, load magnitude, internal clearance, operating temperature, and shaft or housing material.
Thermal or Heavy Interference Assembly
Do not select a shrink fit solely from a generic recommendation chart. Calculate the required interference and resulting component stresses, then verify the assembly temperatures, material limits, geometry, and available manufacturing capability.
A practical workflow for confident fit selection and why it pays off.
10. Is there a practical ISO fit selection process?
Absolutely. While tolerance selection involves technical knowledge and judgement, a structured approach helps prevent overthinking and avoid costly mistakes.
Here’s a step-by-step framework to guide your fit selection:
Fit Selection Workflow
Use the following ISO fit selection workflow to move from functional requirements to production-ready dimensional limits.
- Define the function. Must the components slide, rotate, locate, transmit torque, or remain permanently assembled?
- Define the required relationship. Must clearance always be present, is either clearance or interference acceptable, or must interference be guaranteed?
- Establish the nominal size. ISO limit deviations vary with nominal-size range, even when the same fit designation is used.
- Review operating conditions. Consider load, speed, temperature, lubrication, vibration, wear, corrosion, and expected service life.
- Review the material and surface condition. Include thermal expansion, stiffness, wall thickness, coatings, plating, and surface finish.
- Define the assembly and maintenance method. Confirm whether hand assembly, pressing, heating, cooling, disassembly, or replacement is required.
- Confirm manufacturing capability. Verify that the selected process can repeatedly achieve the hole and shaft limits.
- Define inspection and acceptance. Select appropriate measurement equipment, measurement locations, sampling, and conformity rules.
- Check the complete assembly. Review tolerance stack-up and validate critical fits through engineering calculation, prototype assembly, or application-specific testing.
Example Use Case
Sliding guide pin and bushing: If the pin must assemble easily and slide with controlled play, H7/g6 may be evaluated as a starting point. Speed, lubrication, wear, alignment, and allowable clearance must still be confirmed.
Press-mounted gear hub: If the hub must resist relative movement and press assembly is acceptable, H7/p6 may be evaluated as a starting point. The required interference, contact pressure, hub stress, torque capacity, and assembly force must be calculated before release.
Engineering note: ISO fit classes do not define every production and inspection requirement by themselves. Confirm the nominal-size range, current ISO 286 table values, material condition, surface finish, operating temperature, measurement method, assembly requirements, and project-specific acceptance criteria before releasing a drawing for production.
Need H7/g6, H7/h6, Press-Fit, or Precision-Mated Parts Manufactured?
Davion Manufacturing supports precision CNC machining and CNC turning services involving controlled hole-and-shaft fits, bearing seats, bushings, sleeves, pins, mating diameters, and other tolerance-critical features. Upload your CAD file and technical drawing for an engineering-led manufacturability and quotation review.
Send the technical information currently available. Missing manufacturing details can be clarified during RFQ review.