Low head socket cap screw dimensions are shown in the following chart according to ASME B18.3-2003 standard.

Low head socket cap screw is a type of cap screw which is recommended for lighter fastening operations. It's not suggested for use in critical high strength applications where socket head cap screws should normally used. These fasteners are designed to be used in applications where head height clearance is a problem. Due to their reduced height, and smaller socket size, they normally cannot be properly preloaded as a standard socket cap screw and should not be subjected to high dynamic loads.

Some important notes from the ASME B18.3:

- The diameter range of low head hex cap screws is from 0.1120 inch to 0.6250 inch according to standard.

- Thread shall conform to Unified external threads with radius root: Class 3A UNRC and UNRF for screw sizes No. 0 through 1 inch inclusive.

- These screws are not ordinarily made in spline socket.

- These screws are not ordinarily made in Stainless Steel.

- For exact screw length, consult with the supplier.


Nominal diameter (inches)


Dimensions of Low Head Hexagon Socket Cap Screws
Parameter Symbol Value Unit
Nominal Size --- --- ---
Body Diameter [D]1 Dmax --- in
Dmin ---
Head Diameter [A]2 Amax ---
Amin ---
Head Height [H] Hmax ---
Hmin ---
Max. Chamfer or Radius 3 Cmax ---
Nom. Hexagon Socket Size J --- ---
Min. Key Engagement Tmin ---
Max. Chamfer or Radius 4 Kmax ---
Fillet Juncture Diameter
at Bearing Surface [F]
Fmax --- in
Fmin ---
Max. Fillet Length FL_max ---
Min. Juncture Radius Rmin ---
Before Threading Diameter
(In threaded to the head screws)
Nmin ---

Note1: Body refers to the unthreaded cylindrical portion of the shank for those screws not threaded to head.
Note2: Head shall be plain, not knurled.
Note3: The intersection of the top of the head and the side of the head may be chamfered or radiused within the limits of C, at the manufacturer's option.
Note4: The edge between the bearing surface and the side of the head may be broken, but the radius or chamfer shall not exceed Kmax.
Note:5 See the "Definitions" section for the details of the fillet.


Designation: Product Name, Designation of the Standard, Nominal Size - Thread Pitch x Nominal Length, Material, Protective Finish (if required) Ex: Hexagon Low Head Socket Head Cap Screws, ASME B18.3, 8 - 32 x 0.75, Alloy Steel, Zinc Plated

Fillet: For all lengths of screws, the form of the underhead fillet shall be optional, provided that it is smooth and continuous concave curve fairing into the bearing surface of the head, and the screw shank is within the envelope established by the limits for fillet extension, length, and juncture radius given in the results table.

Socket Head Cap Screw Underhead Fillets

Long Screw (Full Body Screw): Screw which is not threaded for full length.

Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UN form screw threads: The UN thread is intended for general purpose fastening operations.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

Short Screw (Thread to Head Screw): A screw which is threaded for full length.

Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.

Thread Gages and Gauging : Specifications and dimensions for the gages used on Unified inch screw thread is defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread plug gages, snap gages, thread ring gages and plain diameter gages for internal and external threads are covered in same standard.


Link Usage
Dimensions of Hex Keys (Allen Keys) and Bits Calculator covers inch series Hex Keys (Allen Keys) and Bits according to ASME B18.3-2003 standard.
Unified Inch Screw Thread Sizes & Tolerances Calculator Unified inch screw threads and tolerances calculator was developed to calculate basic major, minor and pitch diameters of the external (bolt) and internal (nut) inch thread according to ASME B1.1-2003 standard. In addition to basic size calculations, limits of sizes calculations of different tolerance classes can be done according to same standard.

  • ASME B18.3-2003, Socket Cap, Shoulder, Set Screws, Hex, and Spline Keys