IS 1367 Part 5 : 2002Technical Supply Conditions for Threaded Steel Fasteners - Part 5: Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel - Nuts with Specified Proof Load Values

IS 1367 Part 5:2002 specifies the mechanical properties of nuts (with specified proof-load values) made of carbon and alloy steel — the strength side of the *nut* in a bolted assembly. It is the nut counterpart to IS 1367 Part 3 (bolt mechanical properties) and ensures the nut is strength-matched to the bolt so the joint fails (if at all) by bolt yielding, not nut stripping.

It sits in the fastener stack:

• IS 1367 Part 3 — mechanical properties of bolts/screws (the matched-bolt side) · IS 1363 Part 1 / IS 1364 Part 1 — bolt/nut *dimensions*
• IS 800 — steel design (bolted connections) · IS 2062 — structural steel

A bolted connection is only as strong as its weakest component, and a mismatched nut is a classic hidden weak link. IS 1367 Part 5 controls the nut so it isn't:

• Proof load — the axial load the nut must sustain without stripping or permanent deformation; the nut property class is matched to the bolt property class (so a class-8 nut for a class-8.8 bolt, etc.) so the nut develops the bolt's full strength
• A lower-class or substandard nut strips the threads before the bolt reaches its capacity — the connection fails at well below design load, often suddenly
• Marking/identification so the nut class is verifiable

The engineering point: the bolt property class (IS 1367 Part 3) is meaningless if the nut is weaker — specifying a high-strength bolt and accepting an unmarked/low-grade nut is a common, dangerous mismatch. The connection strength assumed in the IS 800 design requires the matched nut with its proof load, not just the right bolt.

Scenario: a structural bolted connection using property-class bolts.

Step 1 — bolt class from design: IS 800 → required bolt size and property class (e.g. 8.8) per IS 1367 Part 3.

Step 2 — specify the matched nut: the corresponding IS 1367 Part 5 nut class (matched to the bolt class, with its specified proof load) — *explicitly*, not 'a nut to suit'.

Step 3 — dimensions: bolt/nut geometry per IS 1363/IS 1364; ensure thread fit.

Step 4 — verify markings: check the nut property-class marking on delivery — unmarked/lower-class nuts strip before the bolt yields.

Step 5 — assemble correctly: matched bolt+nut(+washer), correct tightening for the connection type.

A matched nut lets the connection develop the bolt's designed capacity; a weak nut makes the joint fail by thread stripping far below the IS 800 design load — invisibly, until it lets go.

1. Specifying the bolt class but not the nut class. The nut must be strength-matched (IS 1367 Part 5 proof load) or it strips before the bolt yields.

2. Accepting unmarked / lower-grade nuts. A substandard nut is the connection's hidden weak link — verify the class marking.

3. Assuming 'a nut is a nut'. Nut proof load scales with class; an under-class nut defeats a high-strength bolt.

4. Thread/fit mismatch. Wrong thread or fit causes stripping/galling independent of class.

5. Ignoring it in the IS 800 connection. The design capacity assumes a matched assembly — bolt class alone doesn't deliver it.

• IS 1367 Part 3 — mechanical properties of bolts/screws (the matched-bolt side)
• IS 1363 Part 1 — grade-C hex bolt dimensions · IS 1364 Part 1 — grade A/B bolt/nut dimensions
• IS 800 — steel design (bolted connections) · IS 2062 — structural steel
• [IS 6623] IS 6623 — high-strength friction-grip nuts (slip-critical work)

IS 1367 Part 5 is reaffirmed and enforces a principle every connection engineer must internalise: a bolted joint is only as strong as the weakest of bolt, nut and threads, and the nut is the most commonly mismatched. Specifying a high-strength bolt (IS 1367 Part 3) and then accepting an unmarked or lower-class nut produces a connection that strips and fails well below its design capacity, often suddenly — and it is invisible on a drawing that only names the bolt. The discipline is simple and non-negotiable: specify the matched nut class with its proof load explicitly, verify the class markings on both bolt and nut at delivery, ensure thread fit, and treat the bolted connection as a designed *assembly* per IS 800. The strong-bolt/weak-nut mismatch is one of the most preventable connection failures there is.