Machine Screws General Data

This page summarizes common machine screw requirements and conventions from ASME B18.6.3 (inch series), including material guidance, Unified thread practice, thread length rules, and length tolerance.

Materials and minimum strength

Machine screws are commonly manufactured from carbon steel and (unless otherwise specified) have a minimum tensile strength of 60,000 psi.

They may also be produced from higher-strength steels, stainless steel, brass, monel, aluminum alloys, or other materials as agreed between the manufacturer and purchaser.

Threads and thread class

Unless otherwise noted, machine screw threads are Unified, Class 2A, in the UNC and UNF series (or UNRC/UNRF at the manufacturer’s option).
For very small sizes, see: Thread dimensions for No.0000, No.000 and No.00 .
For threads with additive finish, the maximum diameters of Class 2A may be exceeded by the amount of allowance.

Full thread length guidance

Sizes No.5 and smaller

For nominal lengths ≤ 3× diameter: full thread extends to within one pitch of the bearing surface (or closer if practicable).
For nominal lengths > 3× diameter up to and including 1 1/8 in.: full thread extends to within two pitches of the bearing surface (or closer if practicable).
For longer nominal lengths (unless otherwise specified): minimum full thread length is 1.0 in.

Sizes No.6 and larger

For nominal lengths ≤ 3× diameter: full thread extends to within one pitch of the bearing surface (or closer if practicable).
For nominal lengths > 3× diameter up to and including 2 in.: full thread extends to within two pitches of the bearing surface (or closer if practicable).
For longer nominal lengths (unless otherwise specified): minimum full thread length is 1.50 in.

Body diameter and end finish

The body diameter shall not be less than the Class 2A minimum pitch diameter and not greater than the basic major diameter of the thread.
Unless otherwise specified, machine screws shall have plain sheared ends.

Tolerance on length

The following table summarizes length tolerance (negative tolerance shown) by nominal screw size range.

Nominal screw length	0000 through 00	0 through 12	1/4 through 3/4
Up to 1/2 in., incl.	−0.01	−0.02	−0.03
Over 1/2 to 1 in., incl.	−0.02	−0.03	−0.03
Over 1 to 2 in., incl.	—	−0.06	−0.06
Over 2 in.	—	−0.09	−0.09

How machine screws are designated

Machine screws are typically designated in this sequence: nominal size (number or fraction/decimal), threads per inch, nominal length, product name (head type + drive), optional point, material, and optional protective finish.

Example: 1/4−20 × 1 1/4 Slotted Truss Head Machine Screw, Steel, Zinc Plated

Reference

ASME B18.6.3 – 2003, Machine Screws and Machine Screw Nuts