ASME B18.3 Low Head Hexagon Socket Cap Screws
ASME B18.3 Dimensions of Low Head Hexagon Socket Cap Screws
| Nominal Size (Basic Screw Diameter) |
Body Diameter, D [Note (2)] |
Head Diameter, A [Note (3)] |
Head Height, H |
Maximum Chamfer or Radius, C |
Nominal Hexagon Socket Size, |
Minimum Key Engagement, |
Maximum Chamfer or Radius, |
|||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| [Note (1)] | Max | Min | Max | Min | Max | Min | [Note (4)] | J | T | K | ||
| 4 | (0.1120) | 0.1120 | 0.1075 | 0.183 | 0.176 | 0.059 | 0.053 | 0.009 | ... | 0.050 | 0.038 | 0.008 |
| 5 | (0.1250) | 0.1250 | 0.1202 | 0.205 | 0.198 | 0.065 | 0.059 | 0.012 | 1/16 | 0.062 | 0.044 | 0.008 |
| 6 | (0.1380) | 0.1380 | 0.1329 | 0.226 | 0.218 | 0.072 | 0.066 | 0.013 | 1/16 | 0.062 | 0.050 | 0.008 |
| 8 | (0.1640) | 0.1640 | 0.1585 | 0.270 | 0.262 | 0.085 | 0.079 | 0.014 | 5/64 | 0.078 | 0.060 | 0.008 |
| 10 | (0.1900) | 0.1900 | 0.1840 | 0.312 | 0.303 | 0.098 | 0.092 | 0.018 | 3/32 | 0.094 | 0.072 | 0.008 |
| 1/4 | (0.2500) | 0.2500 | 0.2435 | 0.375 | 0.365 | 0.127 | 0.121 | 0.025 | 1/8 | 0.125 | 0.094 | 0.010 |
| 5/16 | (0.3125) | 0.3125 | 0.3053 | 0.437 | 0.431 | 0.158 | 0.152 | 0.033 | 5/32 | 0.156 | 0.110 | 0.010 |
| 3/8 | (0.3750) | 0.3750 | 0.3678 | 0.562 | 0.556 | 0.192 | 0.182 | 0.040 | 3/16 | 0.188 | 0.115 | 0.010 |
| 7/16 | (0.4375) | 0.4375 | 0.4294 | 0.625 | 0.618 | 0.223 | 0.213 | 0.047 | 7/32 | 0.219 | 0.135 | 0.015 |
| 1/2 | (0.5000) | 0.5000 | 0.4919 | 0.750 | 0.743 | 0.254 | 0.244 | 0.055 | 1/4 | 0.250 | 0.151 | 0.015 |
| 5/8 | (0.6250) | 0.6250 | 0.6163 | 0.875 | 0.867 | 0.316 | 0.306 | 0.070 | 5/16 | 0.312 | 0.250 | 0.015 |
GENERAL NOTES:
(a) This product, although a high-strength precision fastener, is recommended for lighter fastening
applications. It is not suggested for use in critical high-strength applications where socket head cap
screws should normally be used. They are designed to be used in applications where head height clearance
is a problem. Because of their reduced head 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.
(b) For additional requirements, refer to Notes (5) through (12), and also section 2, General Data, of
this Standard.
NOTES:
(1) Nominal Size. Where specifying nominal size in decimals, zeros preceding decimal and in the fourth
decimal place shall be omitted.
(2) Body. The term body refers to the unthreaded cylindrical portion of the shank for those screws not
threaded to the head.
(3) Head Diameter. Heads shall be plain, not knurled.
(4) Head Chamfer. The top of the head shall be flat. The intersection of the top of the head and side of
the head may be chamfered or radiused within the limits of C, at the manufacturer’s option.
(5) Bearing Surface. The plane of the bearing surface shall be perpendicular to the axis of the shank,
with a maximun deviation of 1 deg, obtained by holding the screw on the body or major thread diameter
within 1 diameter of the bearing surface of the head, but beyond the maximum length of the fillet, FL, and
inspecting on an optical comparator, or comparable inspection equipment, rotating the shank 360 deg.
(6) Edge of Head. The edge between the bearing surface and the side of the head may be broken (rounded or
chamfered), but the radius or chamfer measured along the bearing surface shall not exceed the values
listed for K.
(7) Runout
(a) The runout of the head with the axis of the shank shall be within 2% of the maximum basic screw
diameter dimension, D, or 0.006 in., whichever is greater.
Runout for (a) above is defined as the full indicator movement (FIM) obtained by holding the screw on the
body or major screw thread diameter within 1 diameter of the bearing surface of the head, but beyond the
maximum length of the fillet, FL, rotating 360 deg and indicating on the outside diameter of the
head.
(b) The runout of the socket with the axis of the shank of the screw shall be within 3% of the maximum
screw diameter, D, or 0.005 in., whichever is greater for sizes through 1⁄2 in. diameter and 6% for sizes
above 1⁄2 in. diameter.
Runout for (b) above is defined as the full indicator movement (FIM) obtained by holding the screw on the
body or major screw thread diameter within 1 diameter of the bearing surface of the head, but beyond the
maximum length of the fillet, FL, rotating 360 deg, indicating on each of the six hexagon flats.
(c) The conformance of screws to shank straightness or camber limitations set forth as De in Table 6 shall
be checked by the use of the procedures and typical gage illustrated in ASME B18.2.9.
(8) Fillet. For all lengths of screws, the form of the underhead fillet shall be optional, as depicted in the illustration above Tables 2 and 3, provided it is a 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 specified in Tables 2 and 3.
(9) Length. The length of the screw shall be measured parallel to the axis of the screw from the plane of the bearing surface under the head to the plane of the flat of the point. The portion of the screw contained within dimension L is commonly called the shank. The basic length dimension on the product shall be the nominal length expressed as a two-placed decimal.
(10) All other characteristics are the same as Notes (3) through (6), (8), (13), (14), and (18) through
(21) of Table 1 for socket head cap screws.
[EXCEPTIONS: Since these are not full-size socket screw products, the
mechanical requirements of ASTM A574
will not apply.]
(11) The minimum tensile requirements for Alloy Steel Low Head Socket Cap Screws are the same as for
Alloy
Steel Socket Button Cap Screws, for the same length and diameter, as per ASTM F835.
(12) Designation. To promote uniformity and understanding in communications relating to products
conforming to this Standard, it is recommended that Hexagon Low Head Socket Cap Screws be designated in
accordance with the following data, preferably in the sequence shown:
(a) product name
(b) designation of the standard
(c) nominal size (number, fractional or decimal equivalent)
(d) thread pitch
(e) nominal length (fractional or decimal equivalent)
(f) material
(g) protective finish, if required
EXAMPLES:
Hexagon Low Head Socket Cap Screws, ASME 818.3, 8-32 3⁄4 , Alloy Steel
Hexagon Low Head Socket Cap Screws, ASME B18.3, 0.164-32 0.750, Alloy Steel, Zinc Plated. (For plated
products, see para.
1.7, Introductory Notes.)
For the recommended B18 part identifying numbering system (PIN), see ASME B18.24.