Chapter 6
The ISO System of Limits and Fits -
Tolerances and Deviations
SUMMARY*
The ISO System of Limits and Fits is a coordinated system of hole and shaft tolerances for engineering and manufacturing
used for cutting tools, material stock, gages, etc. If held to these tolerances, cutting tools, material stock, and gages are
generally available throughout the world.
The hole basis fits have four preferred hole tolerances (H11, H9, H8, and H7); the shaft basis fits have four preferred shaft
tolerances (h11, h9, h7, and h6) as shown in Table 6-1. The above shaft tolerances are now covered in the new ANSI B32.100-
2005 standard.
Select the basic size from Table 4-1 and one of the ten fits from Table 6-1 and read or cut and paste limit dimensions and
clearances (interferences) from Tables 6-2 through 6-6 or CD. Now also available on-line at the site shown below.
Tolerance Zones for holes are shown in Fig. 6-1 and for shafts in Fig. 6-2.
NOTE *Complete ISO System of Limits and Fits is now stored a CD KOK ISOTOLTM Computer Tolerancing Software. The CD is
now available from ANSI,
GO
metric
USA
TM.org, Inc. or on the sites:www.GOmetricUSA.org or www.kok.com/order.htm.
TABLE 6-1 DESCRIPTION OF PREFERRED FITS (ANSI B4.2)
ISO SYMBOL
Hole Shaft
Basis Basis
DESCRIPTION
H11/c11 C11/h11 Loose running fit for wide commercial tolerances or allowances
on external members. More
H9/d9 D9/h9 Free running fit not for use where accuracy is essential, but good for large Clearance
Clearance temperature variations, high running speeds, or heavy journal pressures.
H8/f7 F8/h7 Close running fit for running on accurate machines and for accurate
Fits location at moderate speeds and journal pressures.
H7/g6 G7/h6 Sliding fit not intended to run freely, but to move and turn freely
and locate accurately.
H7/h6 H7/h6 Locational clearance fit provides snug fit for locating stationary parts;
but can be freely assembled and disassembled.
Transition H7/k6 K7/h6 Locational transition fit for accurate location, a compromise
between clearance and interference.
Fits H7/n6 N7/h6 Locational transition fit for more accurate location where
greater interference is permissible.
H7/p61 P7/h6 Locational interference fit for parts requiring rigidity and alignment with
prime accuracy of location but without special bore pressure requirements.
Interference H7/s6 S7/h6 Medium drive fit for ordinary steel parts or shrink fits on light sections, the
tightest fit usable with cast iron. More
Fits H7/u6 U7/h6 Force fit suitable for parts which can be highly stressed or for shrink fits Interference
where the heavy pressing forces required are impractical.
1Transition fit for basic sizes in range from 0 through 3 mm.
