How to Specify Fits and Tolerances: ISO 286 Hole Basis and Shaft Basis System

ISO 286 provides a complete, standardized system for specifying clearance, transition, and interference fits that eliminates ambiguity between design and manufacturing — correctly using hole-basis and shaft-basis systems with IT grades and fundamental deviations is a core skill for any mechanical designer.

Whenever two mating parts must fit together — a shaft in a bearing bore, a pin in a hole, a key in a keyway — the designer must specify tolerances that guarantee the correct type of fit: running clearance, snug push fit, or press fit. ISO 286-1 (the system of fits and tolerances for cylindrical features) provides a standardized vocabulary of 20 fundamental deviations, 18 IT grades, and hole-basis and shaft-basis conventions that make fit specification predictable, communicable, and achievable with standard tooling. This article explains the ISO 286 system from first principles through practical fit selection.

The Three Types of Fits

Before examining the ISO 286 notation, the three functional categories of fit must be understood:

• Clearance fit: The shaft is always smaller than the hole for all permissible size combinations. There is always a positive clearance (gap) between the mating surfaces. Used for rotating or sliding fits where relative motion is required.
• Transition fit: Depending on the actual sizes of the hole and shaft, the fit may result in clearance or interference. The tolerance zones for hole and shaft overlap. Used when alignment is important but some flexibility in assembly force is acceptable — the part may push together with finger pressure or may require a light mallet.
• Interference fit: The shaft is always larger than the hole for all permissible size combinations. The shaft must be pressed, heated, or cooled to assemble; the resulting joint transmits torque and axial load without fasteners. Used for permanent or semi-permanent assemblies.

IT Grades: International Tolerance Grades

ISO 286 defines 20 standard tolerance grades, designated IT01, IT0, IT1 through IT18, representing increasingly loose tolerances:

• IT01–IT4: Gauge and precision measuring instruments, master gauges
• IT5–IT7: Precision engineering fits — bearings, precision spindles, precision instruments
• IT7–IT11: General engineering fits — most industrial machinery, shafts and holes in typical equipment
• IT12–IT18: Raw material tolerances, sheet metal, rough castings

The IT grade tolerance value increases with both the nominal size and the grade number. For example, for a nominal diameter in the 50–80 mm range:

• IT5: 13 μm
• IT6: 19 μm
• IT7: 30 μm
• IT8: 46 μm
• IT9: 74 μm
• IT11: 190 μm

Each step up in IT grade approximately increases the tolerance by a factor of 1.6. This geometric progression means that tolerances for all nominal sizes scale proportionally — the system is size-independent in its structure.

Fundamental Deviations: Defining the Tolerance Position

The IT grade defines the width of the tolerance zone, but not where the zone is positioned relative to the nominal size. The fundamental deviation (also called the fundamental offset or allowance) defines where the tolerance zone starts.

For holes (internal features), fundamental deviations are designated by UPPERCASE letters A through ZC:

• A, B: Large positive deviations (hole much larger than nominal — used for very loose clearance fits)
• C, D, E, F, G: Moderate positive deviations (clearance fits)
• H: Zero deviation — the fundamental deviation is at the nominal size. The tolerance zone extends entirely above (positive) the nominal. H is the standard hole for hole-basis fits.
• JS: Symmetric bilateral deviation (zone centered on nominal — half above, half below)
• K, M, N: Small negative to zero deviations (transition fits)
• P, R, S, T, U, V, X, Y, Z: Increasing negative deviations (interference fits, lower and lower limit of hole)

For shafts (external features), fundamental deviations are designated by lowercase letters a through zc, with mirror-image logic:

• a, b: Large negative deviations (shaft much smaller than nominal — very loose clearance)
• c, d, e, f, g: Moderate negative deviations (clearance fits)
• h: Zero upper deviation — shaft is at or below nominal. Standard shaft for shaft-basis fits.
• js: Symmetric bilateral
• k, m, n: Small positive deviations (transition fits)
• p, r, s, t, u: Increasing positive deviations (interference fits)

Hole-Basis vs Shaft-Basis System

Hole-basis system (preferred): The hole is H (fundamental deviation = 0, tolerance zone above nominal) and the shaft deviation is varied to achieve the desired fit. This is preferred in industry because:

• Holes are made with standard drills and reamers — changing the hole size requires changing the tool
• Shafts are made by turning and can be adjusted to almost any diameter continuously
• It is more economical to vary the shaft than to maintain a variety of precision bore tools
• Standard H7 bore reamer and H6 bore reamer cover most precision engineering needs

Shaft-basis system: The shaft is h (fundamental deviation = 0, tolerance zone below nominal) and the hole deviation is varied. Used when the shaft is a standard commercial diameter — a standard 20mm ground shaft bar, a standard key, or a standard fastener — and the mating hole must be varied to achieve the fit. Less common than hole-basis.

Reading the Tolerance Designation

A complete ISO 286 fit is written as: nominal diameter / hole designation / shaft designation

Example: 30 H7/g6 means:

• 30 mm nominal diameter
• H7: Hole with H fundamental deviation and IT7 grade tolerance
• g6: Shaft with g fundamental deviation and IT6 grade tolerance

On a drawing, the hole and shaft are each individually specified:

• Hole drawing: ∅30 H7 (with the tolerance limits explicitly shown: ∅30.000 +0.021/+0.000 for a 30mm H7 hole)
• Shaft drawing: ∅30 g6 (∅30.000 -0.007/-0.020 for a 30mm g6 shaft)

The min and max clearance can be calculated: min clearance = lower limit of hole – upper limit of shaft = 30.000 – 29.993 = 0.007 mm; max clearance = upper limit of hole – lower limit of shaft = 30.021 – 29.980 = 0.041 mm. This is a running/sliding clearance fit.

Practical Fit Selection Guide

Fit Designation	Type	Clearance/Interference Range (30mm)	Assembly Method	Typical Application
H7/h6	Clearance (close running)	+0 to +0.041 mm clearance	Slide by hand	Locating fits, precision sliding parts
H7/g6	Clearance (running)	+0.007 to +0.041 mm	Easy hand assembly	Sliding fits, spigots, general machine parts
H7/f7	Clearance (easy running)	+0.020 to +0.062 mm	Hand assembly, free rotation	Plain bearings, lightly loaded shafts
H7/e8	Clearance (loose running)	+0.040 to +0.107 mm	Very easy hand assembly	Rough guides, clearance holes
H7/k6	Transition	-0.018 to +0.021 mm	Light press or mallet	Gears, pulleys, locating bushings
H7/n6	Transition (push)	-0.028 to +0.011 mm	Press or heated assembly	Coupling hubs, tight locating fits
H7/p6	Interference (light press)	-0.042 to -0.008 mm	Arbor press (room temp)	Bushings, sleeves, lightly loaded press fits
H7/s6	Interference (medium press)	-0.067 to -0.033 mm	Thermal or hydraulic press	Bearing races, coupling pins
H7/u6	Interference (heavy press)	-0.101 to -0.067 mm	High-force press or shrink fit	Permanent joints, heavy torque transmission

Calculating Tolerance Limits from ISO 286

For any combination of nominal diameter, deviation letter, and IT grade, the tolerance limits can be looked up in ISO 286-1 tables or calculated:

• Find the IT grade tolerance value (ei or ES) for the nominal diameter range from the standard tables
• Find the fundamental deviation (lower deviation for shafts ei, upper deviation for holes ES or lower deviation EI) for the deviation letter and diameter range
• Upper/lower limits = nominal + fundamental deviation ± IT tolerance value

Most CAD software (SolidWorks, CATIA, NX) has ISO 286 tables built in — entering the hole or shaft designation automatically populates the tolerance limits. Engineering handbooks (Machinery’s Handbook, Shigley’s) and online ISO 286 calculators are also readily available.

Specifying Fits on Engineering Drawings

On assembly drawings: the fit designation is stated adjacent to the relevant dimension: ∅30 H7/g6. This gives the reader the complete fit information in one line.

On individual part drawings: each part shows its own tolerance with the limit values (more useful for inspection) alongside the ISO designation. For the shaft: ∅30 g6 (∅29.993/∅29.980) — the limit values are shown in parentheses or as the primary tolerance with the ISO designation as a note.

For bearing fits specifically: bearing manufacturers publish specific recommendations for inner ring (shaft) and outer ring (housing) fits based on load magnitude, rotation (inner ring or outer ring rotating), and operating temperature. Always consult the bearing manufacturer’s application engineering data for bearing fit selection — do not use general ISO 286 fit tables for precision bearing applications.

Conclusion

ISO 286 transforms fit specification from an art into a science. The combination of IT grade (defining tolerance width) and fundamental deviation (defining tolerance position) provides a complete vocabulary for any fit type — running clearance, transition, or interference. The hole-basis system with H7 holes and varied shaft deviations covers the vast majority of industrial engineering applications and allows standard tooling to be used for holes while shaft diameters are adjusted to achieve the target fit. Mastering the H7/g6 running fit, H7/k6 transition fit, and H7/p6 light interference fit — and knowing how to calculate or look up the exact tolerance limits — is indispensable for any mechanical design engineer working with mating cylindrical components.