- Product Grades and Tolerances for Fasteners as per IS / ISO
Indian Standard IS 1367 is on technical supply conditions for threaded steel fasteners. There are many parts in this standard. Part number 2 is on product grades and tolerances. The equivalent ISO standard is ISO – 4759 – part 1. Information on product grades and tolerances is given in this article. For full detail / more information, please refer latest version of the standard.
The standard specifies tolerance for bolts, screws, studs and nuts. It specifies tolerance level and tolerances of form and position. Tolerance levels for many features of fasteners are given in the standard. For example, tolerance level for thread (one of the features) is given below.
FeatureTolerance for Product GradesABCInternal thread (nuts) 6H 6H 7H External threads (bolts, screws and nut end of studs) 6g 6g 8g
Tolerances of form and position are used to specify tolerance for the following.
- Concentricity, symmetry and run-out
- Parallelism and
Note:Tolerance of form and position are important for use of fasteners in critical applications. To ensure that you get quality product, it is recommended that you buy fasteners manufactured by reputed company.
The product grades covered in this standard refer to the quality of the product and to the size of the tolerances where Grade A is the most precise and Grade C, the least precise. The above designation of product grades as A, B and C is a change from designation of product grades employed in 1967 version. The following table gives the equivalent product grades in the old and revised versions.
Product grade in the 1967 editionEquivalent grade in the present standardPrecision (P) Grade A Semi-precision (S) Grade B Black (B) Grade C
It can see that tolerance 6g for external threads for product grades A and B makes them more precise than product grade C having tolerance 8g. As per IS 1363 (Part 1) – 1984, no finish is required for hexagon head bolts of product grade C except for the thread. A small die seam across the bearing surface is permissible.
In general for industrial use, product grades A and B are specified as wide / high tolerances are given for product grade C.