ENGINEERING REFERENCE
Tolerance Fit Chart — ISO 286 & ANSI B4.1
Complete ISO 286 & ANSI B4.1 tolerance fit chart with clearance, transition & interference fit tables. Includes CNC machining cost guide and material-specific press fit guidance.
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ISO 286 & ANSI B4.1
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Clearance / Transition / Interference
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CNC Cost & Tolerance Guide
Engineering Fits
Three fit types at a glance
ClearanceFree sliding, rotating
TransitionLocating, light tap
InterferencePress fit, permanent
Cost GuideIT Grade vs. Machining Cost
Reference Chart
ISO 286 & ANSI B4.1
FIT FUNDAMENTALS
What Are Engineering Fits?
An engineering fit defines the dimensional relationship between two mating parts — typically a shaft and a hole of the same nominal size. Fit selection determines whether parts slide freely, locate precisely, or lock together permanently.
Every mechanical assembly — from a precision bearing housing to a dowel pin in an automotive gearbox — depends on getting the fit right. Specify a shaft too large for its mating bore, and you’ll need a hydraulic press to assemble it. Specify it too loose, and you’ll get vibration, misalignment, and premature wear. The tolerance fit chart is the engineer’s fundamental reference for making these decisions correctly.
Clearance Fits
Shaft diameter is always smaller than the hole — guaranteed space between parts. Allows free rotation or sliding without binding. Common in rotating shafts, pistons, and sliding guide rails.
Transition Fits
May produce either small clearance or small interference depending on actual manufactured dimensions. Provides accurate location without heavy assembly force — a light tap with a mallet is often sufficient.
Interference Fits
Shaft is always larger than the hole — assembly requires force from a press, thermal expansion, or cooling. Once assembled, friction holds parts together, often eliminating need for fasteners or adhesives.
ISO 286 & ANSI B4.1
Tolerance Fit Chart — Complete Reference
Clearance, transition, and interference fits across both ISO 286 (metric) and ANSI B4.1 (inch) standards. Includes limit deviation tables and practical application guidance.
Note: All values per ISO 286-2:2010 and ANSI B4.1-1967 (R2024). Negative result values indicate interference. Always consult the current edition of the governing standard for final design and inspection values.
ISO 286 Preferred Fits — Hole-Basis (H7 Base)
Most commonly specified ISO fits for CNC-machined parts at three representative diameters. All values use the H7 hole as the basis. Clearance values are positive; interference values are negative.
| ISO Fit | Fit Type | Ø10 mm Result | Ø25 mm Result | Ø50 mm Result | Typical Application |
|---|---|---|---|---|---|
| H7/g6 | Close Sliding | 0.005–0.029 | 0.007–0.041 | 0.009–0.050 | Precision guiding, hand assembly, minimal play |
| H7/f7 | Free Running | 0.013–0.046 | 0.016–0.055 | 0.025–0.089 | Rotating shafts, moderate speeds, general machinery |
| H7/h6 | Precision Locating | 0.000–0.021 | 0.000–0.034 | 0.000–0.046 | Location with negligible play, hand-push assembly |
| H7/js6 | Light Transition | −0.006 to +0.015 | −0.008 to +0.020 | −0.010 to +0.025 | Dowel pins, accurate location, light tap assembly |
| H7/k6 | True Transition | −0.001 to +0.019 | −0.002 to +0.027 | −0.002 to +0.034 | Gear hubs, couplings, snug fit with light force |
| H7/m6 | Tight Transition | −0.007 to +0.012 | −0.008 to +0.021 | −0.009 to +0.030 | High-accuracy location, possible light interference |
| H7/n6 | Transition/Interference | −0.010 to +0.008 | −0.014 to +0.012 | −0.016 to +0.018 | Permanent location, may require press |
| H7/p6 | Light Press | −0.013 to +0.006 | −0.020 to +0.004 | −0.027 to +0.006 | Bearings, bushings, arbor press assembly |
| H7/r6 | Medium Press | −0.017 to −0.002 | −0.027 to −0.007 | −0.035 to −0.010 | Gears on shafts, medium-torque connections |
| H7/s6 | Heavy Press | −0.021 to −0.006 | −0.033 to −0.012 | −0.045 to −0.018 | High-torque assemblies, permanent joints |
| H7/u6 | Force/Shrink | −0.025 to −0.009 | −0.040 to −0.017 | −0.058 to −0.027 | Railway wheels, heavy shrink fits, permanent |
ISO 286-2 Limit Deviation Tables (H7 Hole + Common Shafts)
All values in micrometres (µm). Multiply by 0.001 to convert to millimetres. To compute fit: hole upper deviation − shaft lower deviation = max clearance; hole lower deviation − shaft upper deviation = min clearance (or max interference if negative).
| Nominal Ø (mm) | H7 Hole (µm) |
|---|---|
| 1–3 | +10 / 0 |
| 3–6 | +12 / 0 |
| 6–10 | +15 / 0 |
| 10–18 | +18 / 0 |
| 18–30 | +21 / 0 |
| 30–50 | +25 / 0 |
| 50–80 | +30 / 0 |
| 80–120 | +35 / 0 |
| 120–180 | +40 / 0 |
| 180–250 | +46 / 0 |
| Nominal Ø (mm) | g6 | f7 | h6 | k6 | m6 | n6 | p6 | r6 | s6 |
|---|---|---|---|---|---|---|---|---|---|
| 1–3 | −2 / −8 | −6 / −16 | 0 / −6 | +6 / 0 | +8 / +2 | +10 / +4 | +12 / +6 | +16 / +10 | +20 / +14 |
| 3–6 | −4 / −12 | −10 / −22 | 0 / −8 | +9 / +1 | +12 / +4 | +16 / +8 | +20 / +12 | +23 / +15 | +27 / +19 |
| 6–10 | −5 / −14 | −13 / −28 | 0 / −9 | +10 / +1 | +15 / +6 | +19 / +10 | +24 / +15 | +28 / +19 | +32 / +23 |
| 10–18 | −6 / −17 | −16 / −34 | 0 / −11 | +12 / +1 | +18 / +7 | +23 / +12 | +29 / +18 | +34 / +23 | +39 / +28 |
| 18–30 | −7 / −20 | −20 / −41 | 0 / −13 | +15 / +2 | +21 / +8 | +28 / +15 | +35 / +22 | +41 / +28 | +48 / +35 |
| 30–50 | −9 / −25 | −25 / −50 | 0 / −16 | +18 / +2 | +25 / +9 | +33 / +17 | +42 / +26 | +50 / +34 | +59 / +43 |
| 50–80 | −10 / −29 | −30 / −60 | 0 / −19 | +21 / +2 | +30 / +11 | +39 / +20 | +51 / +32 | +60 / +41 | +72 / +53 |
| 80–120 | −12 / −34 | −36 / −71 | 0 / −22 | +25 / +3 | +35 / +13 | +45 / +23 | +59 / +37 | +73 / +51 | +93 / +71 |
| 120–180 | −14 / −39 | −43 / −83 | 0 / −25 | +28 / +3 | +40 / +15 | +52 / +27 | +68 / +43 | +88 / +63 | +117 / +92 |
| 180–250 | −15 / −44 | −50 / −96 | 0 / −29 | +33 / +4 | +46 / +17 | +60 / +31 | +79 / +50 | +106 / +77 | +151 / +122 |
ANSI B4.1 Fit Classes (Inch)
For inch-based drawings, ANSI B4.1 organizes fits into five class groups: RC (running & sliding), LC (locational clearance), LT (locational transition), LN (locational interference), and FN (force & shrink).
| Class | Type | Description | Result at 1.0″ |
|---|---|---|---|
| RC1 | Clearance | Close sliding — precise guiding, no perceptible play | 0.0003–0.0010″ |
| RC2 | Clearance | Sliding — small clearance, precision location | 0.0004–0.0014″ |
| RC3 | Clearance | Precision running — low speeds, light bearing pressures | 0.0006–0.0020″ |
| RC4 | Clearance | Close running — moderate speeds and pressures | 0.0006–0.0026″ |
| RC5 | Clearance | Medium running — higher speeds, heavier pressures | 0.0014–0.0039″ |
| RC6 | Clearance | Medium running — general machinery | 0.0020–0.0055″ |
| RC7 | Clearance | Free running — large temp variations, high speeds | 0.0026–0.0071″ |
| RC8 | Clearance | Loose running — dirty, corrosive, or cold environments | 0.0045–0.0115″ |
| RC9 | Clearance | Very loose running — extreme conditions, wide tolerances | 0.0070–0.0180″ |
| LC2 | Locational Clearance | Guaranteed clearance — hand assembly | 0.0000–0.0012″ |
| LT1 | Locational Transition | Slight clearance or interference — light tap required | −0.0002 to +0.0008″ |
| LN1 | Locational Interference | Guaranteed interference — precise permanent location | 0.0001–0.0006″ |
| FN1 | Force/Shrink | Light drive fit — arbor press | 0.0001–0.0009″ |
| FN2 | Force/Shrink | Medium drive fit — hydraulic press | 0.0003–0.0013″ |
| FN3 | Force/Shrink | Heavy drive fit — press with thermal expansion | 0.0004–0.0019″ |
| FN4 | Force/Shrink | Force fit — shrink or high-force press | 0.0013–0.0023″ |
| FN5 | Force/Shrink | Shrink fit (high force) — heating of outer part required | 0.0018–0.0030″ |
| ANSI Class | Approximate ISO Equivalent | Notes |
|---|---|---|
| RC1 | H5/g4 or H6/g5 | Close sliding |
| RC4 | H8/f7 or H7/f7 | Close running |
| RC7 | H9/d9 or H9/e9 | Free running |
| LC2 | H6/h5 | Locational clearance |
| LT1 | H7/js6 | Transition — slight interference or clearance |
| FN1 | H7/p6 | Light press |
| FN2 | H7/r6 or H7/s6 | Medium press |
| FN4 | H7/u6 | Heavy force fit |
Standards Referenced
Data compiled from ISO 286-1:2010, ISO 286-2:2010, ANSI B4.1-1967 (R2024), and JIS B 0401. Always verify against the standard specified on your drawing.
Hole-Basis vs. Shaft-Basis
Hole-basis (H-series) is the industry standard for CNC machining — it is easier to adjust a shaft diameter than to ream a hole to a non-standard size. Shaft-basis (h-series) is used when multiple fits are required along a single shaft.
PRACTICAL SELECTION
How to Choose the Right Fit
Three fits cover roughly 80% of CNC machining applications. Use this framework to narrow your selection systematically.
01
Define Assembly Requirements
Is the joint meant to move (rotation/sliding), locate precisely, or transmit torque without fasteners? Determine if disassembly is required for maintenance or service.
02
Select Fit Category
Clearance for moving parts. Transition for accurate location with light assembly force. Interference for permanent joints or torque transmission without keys.
03
Apply the 80% Rule
Start with H7/g6 (sliding), H7/k6 (locating), or H7/p6 (light press). These three fits cover the majority of general engineering applications.
04
Consider Material Pairing
Steel shaft in aluminum housing requires 30–50% less interference than steel-on-steel. Check the material considerations table before finalizing.
05
Account for Temperature
If the assembly operates across a wide temperature range, calculate interference at both min and max temperatures. Aluminum expands roughly twice as fast as steel.
Example Workflow: Bearing Housing Assembly
A 25 mm steel shaft must be assembled into an aluminum housing with a bearing mounted on the shaft. The bearing inner ring requires a locating fit (H7/k6) on the shaft. The bearing outer ring needs a clearance fit (H7/h6) in the housing for easy assembly and service. The housing itself is pressed into the final assembly with a light press fit (H7/p6).
H7/k6Bearing inner ring — locating
H7/h6Bearing outer ring — clearance
H7/p6Housing — light press
Material Considerations for Press Fits
Press fit interference cannot be specified in isolation — the material pairing determines how much interference the assembly can tolerate before the weaker material yields. This is one of the most overlooked aspects of fit selection.
| Material Pairing | Recommended Interference (µm per mm Ø) | Key Consideration |
|---|---|---|
| Steel shaft → Steel housing | 0.8–2.0 µm/mm | Highest strength; predictable elastic deformation; standard press fit calculations apply |
| Steel shaft → Aluminum housing | 0.4–1.2 µm/mm | Reduce interference 30–50% vs. steel/steel; aluminum’s lower elastic modulus means it deforms more |
| Aluminum shaft → Aluminum housing | 0.2–0.8 µm/mm | Lowest interference; risk of galling during assembly; use lubrication |
| Steel shaft → Plastic housing (POM, Nylon) | 0.08–0.4 µm/mm | Creep and stress relaxation over time; interference will decrease; consider mechanical retention |
| Steel shaft → Brass/Bronze housing | 0.4–1.0 µm/mm | Good bearing properties; lower interference than steel/steel; suitable for bushings |
| Stainless steel → Stainless steel | 0.6–1.5 µm/mm | Risk of galling without lubrication; use anti-seize compound during press assembly |
Example calculation: A 25 mm steel shaft pressed into an aluminum housing should use approximately 10–30 µm of total interference. The same shaft in a steel housing can safely use 20–50 µm. Applying the steel/steel interference value to an aluminum housing risks cracking or permanently deforming the aluminum.
COST & CAPABILITY
What Tolerance Fits Cost to Machine
Tolerance costs money — but not linearly. Each step up in IT grade requires slower cutting speeds, more frequent tool changes, additional setup, and more inspection.
At Baetro, we quote parts across the full tolerance spectrum daily. The cost relationship is consistent: moving from IT8 to IT7 typically increases cost 30–50%, while IT5 and tighter can be 3–6 times the baseline cost per part.
IT10–IT8
Standard Machining
Baseline cost (1.0×). Standard feeds and speeds, single setup, basic inspection. Most CNC-machined parts function perfectly at IT7–IT8. Specify tight tolerances only where functionally required.
IT7–IT6
Precision Machining
1.5–3.5× cost. Reduced cutting speeds, thermal stabilization, in-process measurement, CMM inspection. Achievable with precision turning, boring, and high-end CNC with temperature control.
IT5 & Tighter
Grinding & Honing
3.5–6.0× cost. Specialist equipment, very slow material removal, 100% inspection, higher scrap rate. Cylindrical grinding, honing, or lapping required for IT5 and tighter.
Manufacturing Process Capability by IT Grade
At Baetro, our standard precision CNC machining holds tolerances to ±0.001″ (0.025 mm) — comfortably within IT7 for most diameter ranges. For critical bearing fits and press-fit bores requiring IT6, our CNC turning and grinding capabilities deliver tighter tolerances with full CMM verification and inspection reports included with every shipment.
IT7–IT8Standard CNC milling/turning
IT6Precision turning, grinding
±0.001″Standard tolerance capability
Tolerance Fit FAQs
Frequently Asked Questions
A clearance fit guarantees a gap between shaft and hole — the shaft is always smaller, allowing free movement. An interference fit guarantees the shaft is larger than the hole — assembly requires force and the joint is typically permanent. A transition fit falls between the two: it may produce a small clearance or small interference depending on the actual manufactured dimensions.
The notation 25H7/g6 breaks down as: 25 = 25 mm basic size; H = hole deviation letter (hole’s lower limit is zero); 7 = hole IT grade; g = shaft deviation letter (shaft is undersized); 6 = shaft IT grade. For 25 mm: H7 hole = 0/+0.021 mm, g6 shaft = −0.007/−0.020 mm. Result: 0.007–0.041 mm clearance.
Three fits cover roughly 80% of CNC machining applications: H7/g6 (close sliding — hand assembly, minimal play), H7/k6 (transition/locating — light tap assembly, accurate position), and H7/p6 (light press — arbor press assembly, permanent or semi-permanent joint).
Different material pairings tolerate different amounts of interference. A steel shaft in a steel housing can safely use 20–50 µm of interference at 25 mm diameter. The same steel shaft in an aluminum housing should use only 10–30 µm — roughly 30–50% less — because aluminum’s lower elastic modulus means it deforms more. Temperature changes also affect interference: an aluminum housing expands roughly twice as fast as a steel shaft.
Yes, and nonlinearly. Moving from IT8 to IT7 typically increases cost 30–50%. Going from IT7 to IT6 can double the cost. IT5 and tighter can be 3–6 times the baseline cost per part. The key to cost-effective design is specifying tight tolerances only on the features that functionally require them.
There is no official one-to-one mapping — ISO 286 and ANSI B4.1 use different calculation methods. For practical design reference: ANSI RC4 roughly corresponds to ISO H8/f7 or H7/f7; ANSI FN2 roughly corresponds to ISO H7/r6 or H7/s6. Always verify against the standard specified on your drawing.
Standard CNC milling and turning reliably hold ±0.025 mm to ±0.05 mm (IT7–IT8 range). With attention to tool wear, thermal stabilization, and in-process measurement, ±0.01 mm (IT6) is achievable on critical features. Tolerances tighter than ±0.005 mm (IT5) typically require grinding, honing, or lapping — separate processes beyond standard CNC machining.
Get Parts Machined to Your Exact Tolerance
The right fit on a drawing means nothing if it’s not held in the machine. Upload your CAD file for an instant quote — our engineers review every fit selection and provide free DFM feedback before machining begins. ISO 9001 certified, 30+ CNC machines, parts ship in 3–7 days.