A Guide to the ISO Fits and Tolerances System — Clearance, Transition & Interference Fits, Hole-Basis vs. Shaft-Basis Selection
A practical guide to the ISO system of fits and tolerances for mating shaft and hole components — clearance, transition, and interference fit categories, how the hole-basis and shaft-basis systems work, and how to read and specify the standard tolerance grade and fit designation on a forged or machined component drawing. Shivam Forge, Rajkot, India. Call +91-9265772827.
Every mating shaft and hole pair — a bearing seated in a housing bore, a pin in a bracket, a gear hub on a shaft — needs its two dimensions specified not as a single nominal number but as a controlled range, because two parts machined to precisely the same nominal diameter will not necessarily assemble or perform the way the designer intended. The ISO system of limits and fits exists to solve this problem in a standardized, internationally recognized way: rather than each designer inventing a bespoke tolerance scheme, the ISO system defines a structured set of tolerance grades (controlling how tight or loose the allowable size variation is) and fundamental deviations (controlling where that variation sits relative to the nominal size), combined into a fit designation that fully and unambiguously specifies how a given shaft and hole pair will behave when assembled — whether they'll always have clearance and rotate or slide freely, always require force to assemble and stay permanently locked together, or sit in a transitional zone that could go either way depending on where within tolerance each individual part actually lands. Understanding this system matters directly for anyone specifying or purchasing forged shaft or bore-containing components, since the difference between a clearance fit and an interference fit isn't a minor manufacturing detail — it's the difference between a bearing that rotates freely and one that's permanently pressed in place, and getting the fit designation wrong, or failing to understand what a supplier's drawing note actually specifies, can produce a component that's dimensionally within tolerance and still functionally wrong for its intended assembly.
A fit where the hole is always sized larger than the shaft across the full tolerance range of both parts, guaranteeing a gap between them at assembly — used wherever the shaft needs to rotate or slide freely within the hole, such as a plain bearing journal or a sliding pin, with the specific tolerance grade selected controlling how tight or loose that guaranteed clearance actually is.
A fit where the shaft is always sized larger than the hole across the full tolerance range of both parts, requiring the shaft to be pressed or shrink-fitted into the hole and creating a permanent, non-rotating, non-sliding connection through the resulting elastic clamping force — used for permanently mounted components like a press-fitted bearing inner race or a gear hub intended never to move relative to its shaft.
A fit where the tolerance ranges of the shaft and hole overlap, meaning any individual assembled pair could end up with either a small clearance or a small interference depending on exactly where each part's actual size falls within its allowed tolerance band — used where accurate location and easy assembly matter more than either guaranteed free rotation or guaranteed permanent locking, such as locating a gear or pulley that will also be secured by a key or fastener.
The fit category isn't a separate choice from the tolerance grades themselves — it emerges directly from how the selected hole tolerance range and shaft tolerance range relate to each other on the nominal dimension, meaning specifying a fit designation like H7/g6 or H7/p6 is what actually determines, by the ISO system's defined tables, whether the resulting fit is clearance, transition, or interference.
The hole-basis system fixes the hole's fundamental deviation at H (meaning its lower limit equals the nominal size, with tolerance added only in the positive direction) and achieves different fit types by varying the shaft's tolerance designation — the dominant convention in practice, largely because internal bores are generally more difficult and costly to machine to a custom size than external shaft diameters, so standardizing the hole simplifies tooling for the more constrained feature.
The shaft-basis system instead fixes the shaft's fundamental deviation at h (its upper limit equals the nominal size, with tolerance added only in the negative direction) and varies the hole's tolerance designation to achieve different fit types — used less commonly than the hole-basis system, typically in applications like line shafting where a single continuous shaft mates with multiple different hole components along its length, making a fixed shaft size more practical than a fixed hole size.
The number in a fit designation (the 7 in H7, or the 6 in g6) refers to an ISO standard tolerance (IT) grade, which sets how tight or loose the allowable size variation is regardless of where that variation sits — lower IT grade numbers indicate tighter tolerances, appropriate for precision fits like bearing seats, while higher numbers indicate looser, more economical tolerances appropriate for less critical dimensions.
The letter in a fit designation (the H, g, or p) refers to the fundamental deviation, which sets where the tolerance zone sits relative to the nominal size — uppercase letters denote hole tolerance zones and lowercase letters denote shaft tolerance zones by ISO convention, with the specific letter (a through zc) indicating the tolerance zone's position from large negative deviation through large positive deviation, directly determining whether a given shaft letter paired with a given hole letter produces clearance, transition, or interference.
Every mating shaft and hole assembly depends on a controlled dimensional relationship between the two parts, not merely on each part individually falling somewhere close to a nominal size — and the ISO system of limits and fits exists specifically to make that relationship explicit, standardized, and internationally interpretable rather than left to ad hoc interpretation on a drawing-by-drawing basis. At the core of the system are two independent controls applied to any toleranced feature: an ISO tolerance (IT) grade, which sets how wide the allowable size variation band is regardless of where it sits, and a fundamental deviation, which sets exactly where that band sits relative to the feature's nominal size. Combining an IT grade with a fundamental deviation letter produces a complete tolerance specification for a single feature — an H7 hole, for instance, or a g6 shaft — and pairing a hole specification with a shaft specification produces a complete fit designation, such as H7/g6, that fully defines how the two parts will behave when assembled together.
That behavior falls into one of three categories depending entirely on how the two parts' tolerance ranges relate to each other. A clearance fit results when the hole's tolerance range sits entirely above the shaft's, guaranteeing some gap between the two parts under every possible combination of actual sizes within tolerance — the appropriate category for any application where the shaft needs to rotate or slide relative to the hole in normal operation, such as a plain bearing journal, a sliding guide pin, or a shaft passing through a clearance hole for a fastener. An interference fit results when the relationship reverses entirely — the shaft's tolerance range sits entirely above the hole's — guaranteeing the shaft is always oversized relative to the hole, requiring the two parts to be pressed together (or, for larger interference amounts, assembled using thermal expansion via heating the hole or cooling the shaft) and producing a permanent connection held by elastic clamping force rather than any mechanical locking feature. A transition fit sits between these two extremes: the tolerance ranges of the shaft and hole genuinely overlap, meaning depending on where each individual part's actual size lands within its own allowed range, an assembled pair might end up with a very small clearance or a very small interference — appropriate for applications prioritizing accurate, repeatable location during assembly, frequently paired with a key or dowel providing the actual functional retention or torque transfer.
In practice, the overwhelming majority of fit specifications use the hole-basis system, in which the hole's fundamental deviation is fixed at H (meaning the hole's minimum size equals its nominal size, with all tolerance added in the positive direction) and different fit outcomes are achieved entirely by varying the shaft's fundamental deviation letter and IT grade against that fixed hole. This convention exists for a practical manufacturing reason rather than an arbitrary standards decision: internal bores are generally more difficult and expensive to machine to a precise, non-standard size than external shaft diameters are, since boring or reaming to a specific size typically requires dedicated or adjustable tooling, whereas external diameters can be turned or ground to essentially any specific size with comparable ease. Standardizing the hole tolerance and matching shaft sizes to it — H7/g6 for a standard clearance running fit, H7/k6 for a light transition location fit, H7/p6 for a light interference fit, among many standard combinations the ISO tables define — lets a shop use a consistent set of reaming or boring tooling across many different fit applications, adjusting only the more easily controlled shaft diameter to achieve the desired functional relationship. The shaft-basis system, fixing the shaft's deviation at h instead, sees comparatively limited use, generally reserved for applications like continuous line shafting where a single shaft size must mate with multiple different hole components along its length.
For engineers and purchasers specifying forged shaft blanks, bearing housings, or other bore-containing components where a correctly executed fit designation genuinely matters to assembly and function, Shivam Forge machines mating features to the specified ISO tolerance grade and fundamental deviation, verified through dimensional inspection appropriate to the fit's precision requirement. Contact our engineering team at +91-9265772827 or sales@shivamforge.com with your drawing and fit designation, or your functional requirement if the fit specification hasn't yet been finalized, for a manufacturability review and quotation.
A clearance fit guarantees the hole is always larger than the shaft across both parts' full tolerance range, leaving a gap that allows free rotation or sliding. An interference fit guarantees the opposite — the shaft is always larger than the hole — requiring the shaft to be pressed or shrink-fitted in, creating a permanent connection held together by elastic clamping force rather than free relative motion.
H7 specifies the hole's tolerance: fundamental deviation H (lower limit at nominal size) with IT grade 7 (a moderately tight tolerance). g6 specifies the shaft's tolerance: fundamental deviation g (a small negative deviation below nominal) with IT grade 6. Together, H7/g6 is a standard, widely used clearance fit — commonly specified for rotating shafts in plain bearings where reliable running clearance is needed without excessive looseness.
The hole-basis system fixes the hole's tolerance and varies the shaft's tolerance to achieve different fit types, and it dominates in practice mainly because internal bores are generally harder and more expensive to machine to a custom, non-standard size than external shaft diameters are. Standardizing the hole to a fixed tolerance and machining shafts to match, using standard tooling like reamers sized to the fixed hole tolerance, is typically more economical than the reverse.
It depends on the intended function: clearance fits suit components that need to rotate or slide relative to each other in service, interference fits suit components meant to be permanently joined without relative motion, and transition fits suit components needing accurate, repeatable location during assembly without either guaranteed free motion or guaranteed permanent locking, often where a key, pin, or fastener will provide the actual torque or axial retention. Reviewing your specific component's functional requirement against these three categories is the right starting point before selecting a specific IT grade and deviation letter.
Yes. Provide your drawing with the specified fit designation (or the functional requirement if the fit hasn't yet been finalized) and our engineering team will confirm manufacturability, machining allowance, and quotation for your specific shaft or bore component.
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