Limits and Fits for a Shaft in a Bore

A fit is how tightly a shaft sits in a bore. Two tolerance zones set it: one for the hole and one for the shaft. Metric drawings give a pair of classes, such as H7/g6 (ISO 286). Inch drawings give a standard class, such as LC5 (ASME B4.1). This page turns either one into the actual limits of size, the clearance or interference they produce, and a picture of the two zones.

Gear drawings use these fits in several places: the gear bore, bearing seats, pilots, and splined or keyed hubs. Pick the nominal size, pick a class, and read the limits. Click a row in the table of other fits to compare.

Limits, read first.
  • Plain cylindrical fits only. It gives the size limits of a round shaft and a round bore at the reference temperature. It does not size keyways, splines, tapered fits or threads.
  • Metric (ISO 286-1) covers sizes above 0 mm up to 500 mm. Any hole letter A to ZC and any shaft letter a to zc work, in grades IT01 to IT18. Sizes above 500 mm are not included.
  • Inch (ASME B4.1) covers the standard fits only. These are classes RC1–9, LC1–11, LT1–6, LN1–3 and FN1–5. All are hole basis, in sizes to 200 in. Classes LT and LN1 are tabulated only up to 19.69 in, and FN3 only from 0.95 in.
  • The two systems are not interchangeable. The same letters give similar but not identical limits, because the size steps and rounding differ. Use the system your drawing standard calls for.
  • No allowance for service. The page gives size limits only. It does not calculate temperature effects, surface finish, press force or shrink temperature. It also leaves out thin-hub stress, loss of fit under load and holding torque. Check hub stress and torque capacity for any interference fit.
  • The values are transcribed from the standards. Check any value you put on a drawing against the current edition of ISO 286 or ASME B4.1. The checks built into this page catch typing errors, not an edition change.
Fit Lookup
Hole & Shaft Limits, in mm or inches
Standard and Size
ISO 286 is used with metric drawings. ASME B4.1 is the US inch standard.
Changing the units converts the size you have entered. Results show both units.
The size the hole and the shaft share, before any deviation.
Fit
The Fit
Limits of Size
Deviations are measured from the nominal size. Positive is larger. The dashed line is the nominal size. If the hole zone is entirely above the shaft zone, the fit is always loose. If the shaft zone is entirely above the hole zone, it is always tight.
Other Fits at This Size
Click a row to load that fit.
Show detailed calculation steps
How It Works

Both systems fix the hole and the shaft by a zone: where it sits relative to the nominal size, and how wide it is. The width is the tolerance grade. It grows with the size and with the grade number. A bigger grade number is looser and cheaper. The position is the fundamental deviation, the limit closest to the nominal size, set by a letter.

\[ \text{hole: } D_{\max} = D + ES,\; D_{\min} = D + EI \] \[ \text{shaft: } d_{\max} = D + es,\; d_{\min} = D + ei \] \[ C_{\min} = EI - es,\qquad C_{\max} = ES - ei \]

D is the nominal size. ES and EI are the upper and lower deviations of the hole. es and ei are the upper and lower deviations of the shaft. C is the clearance.

A positive Cmin means the fit is always loose (clearance). A negative Cmax means the shaft is always larger than the hole (interference). If Cmin is negative and Cmax is positive the parts may assemble either way, which is a transition fit.

ISO 286: the letter and the grade

The class H7 is the letter H with grade 7. A capital letter is a hole and a small letter is a shaft. H puts the lower limit of the hole on the nominal size (EI = 0). h puts the upper limit of the shaft there (es = 0). So H7/g6 is a hole-basis fit, and G7/h6 would be shaft-basis. Letters a to g leave the shaft smaller than nominal, which gives clearance. Letters j to n sit about the nominal size, which gives a transition fit. Letters p to zc make the shaft larger, which gives interference. Hole letters mirror them, with a small correction for the finer grades of K to ZC.

ASME B4.1: classes instead of letters

The inch standard lists ready-made classes. Each is a hole grade and a shaft grade with a letter, and the table gives the resulting limits for each size range. Running and sliding fits (RC) and locational clearance fits (LC) always clear. Locational transition fits (LT) may go either way. Locational interference (LN) and force and shrink fits (FN) always interfere.

Reference Guide
Reference
Fit

Common Fits and What They Are For

ISO classCharacterASME near equivalent
H11/c11Loose runningRC8, RC9
H9/d9Free runningRC7
H8/f7Close runningRC4
H7/g6Sliding, accurate locationLC5
H7/h6Locational clearanceLC2
H7/k6Locational transitionLT3
H7/n6Locational transition, tighterLT5
H7/p6Locational interferenceLN2
H7/s6Medium driveFN2
H7/u6Force fitFN4

The pairs have the same letters and grades, so they are close in character. They are not numerically identical, because the size ranges and rounding differ. The limits for ISO H7/g6 and ASME LC5 are a few tenths of a thousandth apart. Use the table to translate intent, not to substitute one for the other on a drawing.

Gear

Where Fits Come Up on a Gear Drawing

  • Gear bore on a shaft. A keyed gear is commonly given a transition or light clearance bore such as H7 on a k6 or h6 shaft. A keyless press fit uses an interference class and needs a hub stress and torque check.
  • Bearing seats. Shaft seats are often k5, m5 or j5. Housing seats are often H7 or K7. The bearing maker sets them by load and by which ring rotates. Follow the bearing catalogue.
  • Gear on a spline or a pilot. Piloting diameters use fit classes. The spline teeth have their own tolerance system.
  • Running of the bore. A fit class controls size, not concentricity. Add a runout or position tolerance when the gear accuracy grade needs it.
These are typical practices, not rules. The right fit depends on the load, speed, material and how the part is assembled and serviced.
?

Choosing Between Clearance, Transition and Interference

  • Clearance if the parts must slide, turn, or be assembled and removed by hand.
  • Transition for accurate location with a key or a set screw doing the driving, where a light tap assembles it.
  • Interference if the fit itself is to hold the part against load, with or without a key.

Tighter grades cost more to make, and the extra cost rises quickly: going from IT8 to IT6 on both parts is a large step. Use the loosest class that does the job, and use hole-basis fits because standard reamers, broaches and gauges make the hole.

Std

Source Standards

  • ISO 286-1:2010, Geometrical product specifications (GPS): ISO code system for tolerances on linear sizes, Part 1: basis of tolerances, deviations and fits. Tables 1, 4 and 5 are the source of the grade and deviation values.
  • ISO 286-2 gives the tables of limit deviations for the classes. The values here are derived from Part 1 by the standard's general rule, so Part 2 is a useful independent check.
  • ASME B4.1-1967 (R1994), Preferred limits and fits for cylindrical parts. Tables 5 to 9 are the source of the fit classes.
  • ASME B4.2 is the metric counterpart that uses the ISO classes in a preferred selection.
Check the current edition of each standard before using a value on a drawing.
JS

Using the Tables on Another Page

The metric rules and tables are in js/iso286-math.js. The inch tables are in js/asme-b41-data.js and js/asme-b41-math.js. They work in the browser and in Node and know nothing about this page.

var f = Iso286.fit('H7', 'g6', 25);   // size in mm
f.minClearance     // 7   (micrometres)
f.maxClearance     // 41
f.holeMin, f.holeMax, f.shaftMin, f.shaftMax   // mm

var a = AsmeB41.fit('LC5', 2.0);      // size in inches
a.minClearance     // 0.4  (thousandths of an inch)
a.shaftUpper, a.shaftLower

The page script, js/shaft-bore-fits.js, reads the form and draws the results.

Worked Examples

Inch: a 2.000 in shaft in a 2.000 in bore, class LC5 (H7/g6). The size is in the range over 1.97 up to 3.15 in. The table gives clearance 0.4 to 2.3 thousandths and hole limits 0 to +1.2.

  • Hole 2.0000 to 2.0012 in
  • Shaft upper = −0.4 thou = 1.9996 in; lower = 1.2 − 2.3 = −1.1 thou = 1.9989 in

Metric: a 25 mm shaft in a 25 mm bore, H7/g6. The size is in the range above 18 mm up to 30 mm. IT7 is 21 µm and IT6 is 13 µm. H7 gives the hole 0 to +21 µm. The fundamental deviation for g is −7 µm, so the shaft is −7 to −20 µm.

  • Hole 25.000 to 25.021 mm (0.98425 to 0.98508 in)
  • Shaft 24.980 to 24.993 mm
  • Clearance minimum 0 − (−7) = 7 µm, maximum 21 − (−20) = 41 µm. Always loose, so it slides.
Notes
  • Size ranges are “over X up to and including Y”. A size exactly on a boundary belongs to the smaller range.
  • The ISO js and JS classes are shown as ±IT/2. Sometimes the grade is odd and IT/2 is not a whole number of micrometres. The standard allows rounding there. The page shows the exact half.
  • The inch table values are the printed values of the standard, which are rounded to a half of a ten-thousandth in places. A few cells differ from a strict grade calculation by the standard’s own rounding.
  • The mm and inch sizes shown are the same limits in two units. The conversion is 1 in = 25.4 mm exactly.

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