Hex-head bolts

In M8-1.0 × 20, 8 mm is nominal diameter, 1.0 mm is pitch and 20 mm is length. On a hex-head fastener, length starts beneath the head. In 1/4-20 × 2, the second number is 20 threads per inch. Check thread extent and head clearance separately, and distinguish partially threaded bolts from screws threaded nearer the head. Fastener Mart, Understanding Metric Fasteners, Bolt Depot, length section.

Identity and terminology

The external hex accepts a wrench or socket. A cap screw with an internal hex uses a different drive. Bolt Depot, How to Identify Nuts, Bolts, and Screws.

ISO 4014:2022 is Fasteners — Hexagon head bolts — Product grades A and B, edition 5, published June 2022. Its public abstract covers steel and stainless steel, metric coarse-pitch threads M1.6–M64, and product grades A and B. Those are the document's boundaries, not a definition of every hex-head fastener. ISO 4014:2022 catalog and abstract.

ISO 4017:2022 names hexagon head screws. Its public scope has the same material, coarse-pitch size range and product-grade boundaries. Its foreword identifies the sixth edition as a technical revision replacing ISO 4017:2014. A fine-pitch fastener or different material cannot be assigned to these coarse-pitch editions merely because its head is hexagonal. ISO 4017:2022, foreword and clause 1.

Fully and partially threaded forms

Bossard lists BN 21200 as a fully threaded hex-head screw associated with ISO 4017 and withdrawn DIN 933. Its alternatives include BN 21201, a partially threaded hex-head bolt associated with ISO 4014 and DIN 931. This establishes the catalog's full/partial correspondence; its references omit edition years and do not establish universal DIN/ISO interchangeability. Bossard BN 21200, description and alternative products.

Partial threading leaves a smooth body portion between the head and threaded end. “Fully threaded” describes the product form; it does not promise full-profile threads through the head junction or end chamfer. The inspected Bossard BN 58 drawing illustrates these distinct regions on a screw labeled fully threaded. Bossard BN 58, article 1052403, page 1, distributed by TME.

Geometric synthesis: equal nominal diameter and under-head length do not establish equal threaded reach. A smooth body and threaded portion occupy different axial regions. Whether an assembly needs threads, smooth shank or clearance at a particular location depends on its joint geometry; neither form is generically approved as a replacement for the other.

Dimensions describe different features

Hex-head fastener length runs from under the head to the end, excluding head height. Bolt Depot distinguishes this from countersunk-head length measurement. Bolt Depot, length section.

The BN 58 drawing distinguishes the following features. Its table gives M4 for article 1052403, but does not explicitly print units or dimensional tolerances on the inspected page; numerical table values are therefore not reproduced here as an acceptance specification. Bossard BN 58, page 1 drawing and table.

Pitch and threaded length answer additional questions. Pitch is the axial distance between adjacent threads; the threaded portion's axial extent is different from the overall under-head length. Explanatory synthesis: head clearance, thread spacing and threaded reach are separate geometric conditions. No one diameter/length pair resolves all three.

Metric and inch descriptions

Fastener Mart explains M8-1.0 × 20 as an 8 mm nominal diameter, 1.0 mm pitch and 20 mm length. This is a notation example, not an ISO 4014/4017 coarse-pitch product example. The supplier describes omitted pitch as a coarse-thread convention, while including pitch in its own listings to avoid ambiguity. For hex heads it measures length under the head. Fastener Mart, Understanding Metric Fasteners.

An inch description uses a different second quantity: Albany County Fasteners explains 1/4″-20 × 2″ as a 1/4 inch diameter, 20 threads per inch and 2 inch length. The 20 is a count per inch, not a 20 mm pitch or thread length. Albany County Fasteners, Thread Pitch Vs. Threads Per Inch (TPI).

For inch dimensional scope, ASME lists B18.2.1-2012 (R2021) as covering nine inch-series bolt and screw product types, including hex forms. Its description places heavy hex structural bolts in B18.2.6 instead. These families cannot be collapsed into one generic head-dimension table. The public listing was inspected; its full dimensional tables were not. ASME B18.2.1 listing and description.

Material, property class and surface finish

Bossard BN 21200 separately specifies steel, property class 8.8, and a zinc-flake surface designation under ISO 10683. This illustrates distinct material, mechanical-property and coating attributes; none is supplied by an M-size alone. Its catalog also says ISO wrench sizes for M10 and M12 are available on request, illustrating why paired DIN/ISO labels do not settle every head dimension. Bossard BN 21200, attributes and variants.

Product grades A/B are separate from mechanical property classes: ISO 4017's public references distinguish ISO 4759-1 on tolerances from ISO 898-1 on carbon/alloy-steel mechanical properties and ISO 3506-1 on stainless-steel grades/classes. This page does not reproduce their requirements or assign strength values. ISO 4017:2022, clause 2.

Applicability and evidence limits

Assembly-specific requirements remain outside this general reference: dimensional tolerances and thread fit, engagement, required material/class and coating, mating hardware, loads, environment, installation procedure and substitution approval. The descriptions establish no safe preload, tightening torque, fatigue capacity, corrosion life or customer acceptance.

Reviewed 2026-10-30 PDT. ISO 4014's publication date is June 2022; the ISO 4017 text identifies edition 6 (2022), and ASME identifies its 2012 edition reaffirmed in 2021. These source dates differ from this review date. The inspected supplier pages and BN 58 PDF show no explicit content revision date. The date-shaped Albany URL and generated “Printed” date are not verified publication/update dates.

Coverage comprises public ISO abstract/OBP text, ASME's public description, supplier-authored HTML and the visually inspected one-page Bossard PDF hosted by TME. Full paywalled standards and their dimensional tables were not inspected. Several direct raw HTML requests returned HTTP 403 where the research service returned readable text; that access difference does not establish a new source revision. No stock, installed customer hardware or assembly approval was verified.

Related reference: Hardware and component references.