Socket head cap screw dimensions including 6-32, 10-32, 1/4-20, 3/8-16, 7/16-20, 1/2-13,
etc., screw sizes are shown in the following chart according to ASME B18.3-2003 standard.
Socket head cap screw is a
type of cap screw with cylindrical head, smooth and knurled cylindrical sides, a
flat bearing surface and hexagon or spline sockets for drive.
Some important notes from the ASME B18.3:
- The diameter range of socket head cap screws is from 0.06 inch to 4 inches according to
- Thread shall conform to Unified external threads with radius root: Class 3A UNRC and UNRF
for screw sizes No. 0 through 1 inch inclusive, Class 2A UNRC and UNRF for sizes over 1 inch through 1
1⁄2 inches inclusive, and Class 2A UNRC for larger sizes.
- Alloy steel cap screws shall conform to ASTM A547 and stainless steel cap screws shall conform to ASTM F837.
Note1: Screws with nominal lengths less than the smallest length
given in the dropdown list are full form (complete) threads. In this case, the
threads extend to within two pitches of the head for sizes 0 (0.060 in) through
5/8 in inclusive; and shall extend as close to the head as is practice for sizes
larger than 5/8 in.
Note2: For determination of min body length (LB), maximum
grip gaging length (LG) and min thread length (LT) of screws with
nominal lengths greater than the largest length given in the dropdown list, see
ASME B18.3 standard.
Note3: Screws having nominal lengths falling between those for which given in the Nominal Length Dropdown List shall have
LG and LB dimensions conforming with those of the next shorter nominal length for the respective screw size.
For example: for a
in. size screw, 1.88 in. long, LG =
0.50 in. and LB=
Note1: Body refers to the unthreaded cylindrical portion of the
shank for those screws not threaded to head.
Note2: Head may be plain or knurled at the option of the
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 Socket Head Cap Screws, ASME B18.3, 8 - 32 x 2.00, 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.
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 Gaging :
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.