A N A M E R I C A N N A T I O N A L S T A N D A R D
ASME B18.2.1-2010
[Revision of ASME B18.2.1-1996 (R2005)]
Square, Hex, Heavy
Hex, and Askew Head
Bolts and Hex, Heavy
Hex, Hex Flange,
Lobed Head, and Lag
Screws (Inch Series)
INTENTIONALLY LEFT BLANK
ASME B18.2.1-2010
[Revision of ASME B18.2.1-1996 (R2005)]
Square, Hex, Heavy
Hex, and Askew Head
Bolts and Hex, Heavy
Hex, Hex Flange,
Lobed Head, and Lag
Screws (Inch Series)
A N A M E R I C A N N AT I O N A L S TA N D A R D
Three Park Avenue • New York, NY • 10016 USA
Date of Issuance: October 13, 2010
This Standard will be revised when the Society approves the issuance of a new edition. There will
be no addenda issued to this edition.
ASME issues written replies to inquiries concerning interpretations of technical aspects of this
Standard. Interpretations are published on the ASME Web site under the Committee Pages at
http://cstools.asme.org as they are issued.
ASME is the registered trademark of The American Society of Mechanical Engineers.
This code or standard was developed under procedures accredited as meeting the criteria for American National
Standards. The Standards Committee that approved the code or standard was balanced to assure that individuals from
competent and concerned interests have had an opportunity to participate. The proposed code or standard was made
available for public review and comment that provides an opportunity for additional public input from industry, academia,
regulatory agencies, and the public-at-large.
ASME does not “approve,” “rate,” or “endorse” any item, construction, proprietary device, or activity.
ASME does not take any position with respect to the validity of any patent rights asserted in connection with any
items mentioned in this document, and does not undertake to insure anyone utilizing a standard against liability for
infringement of any applicable letters patent, nor assumes any such liability. Users of a code or standard are expressly
advised that determination of the validity of any such patent rights, and the risk of infringement of such rights, is
entirely their own responsibility.
Participation by federal agency representative(s) or person(s) affiliated with industry is not to be interpreted as
government or industry endorsement of this code or standard.
ASME accepts responsibility for only those interpretations of this document issued in accordance with the established
ASME procedures and policies, which precludes the issuance of interpretations by individuals.
No part of this document may be reproduced in any form,
in an electronic retrieval system or otherwise,
without the prior written permission of the publisher.
The American Society of Mechanical Engineers
Three Park Avenue, New York, NY 10016-5990
Copyright © 2010 by
THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
All rights reserved
Printed in U.S.A.
CONTENTS
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Committee Roster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Correspondence With the B18 Committee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 General Data for Both Bolts and Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3 Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 Lag Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figures
1 Underhead Fillet for Long Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 Underhead Fillet for Short Screws Threaded Full Length . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 LG, Maximum and LB, Minimum for Short Screws Threaded Full Length . . . . . . . . . . . 15
Tables
1 Dimensions of Square Head Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Dimensions of Hex Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Dimensions of Heavy Hex Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 Dimensions of Askew Head Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5 Length Tolerances for Bolts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6 Dimensions of Hex Cap Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7 Dimensions of Heavy Hex Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8 Dimensions of Hex Flange Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9 Dimensions of Lobe Head Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10 Dimensions of Underhead Fillets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11 LG, Maximum and LB, Minimum Limitations for Short Screws Threaded
Full Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12 Maximum Grip Gaging Lengths, LG, and Minimum Body Lengths, LB,
for Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13 Length Tolerances for Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14 Dimensions of Square Lag Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
15 Dimensions of Hex Lag Screws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
16 Dimensions of Lag Screw Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Mandatory Appendix
I Gage and Gaging Practices for External Lobed Head Drive System . . . . . . . . . . . . . . . . 23
Nonmandatory Appendices
A Formulas for Bolt and Screw Head Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
B Weight in Pounds of 100 Steel Hex Cap Screws for Given Diameter/Length
Combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
C Countersunk Center Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
iii
FOREWORD
American National Standards Committee B18 for the standardization of bolts, screws, nuts,
rivets, and similar fasteners was organized in March 1922, as Sectional Committee B18, under
the aegis of the American Engineering Standards Committee (later the American Standards
Association, then the United States of America Standards Institute, and, as of October 6, 1969,
the American National Standards Institute, Inc.), with the Society of Automotive Engineers and
The American Society of Mechanical Engineers as joint sponsors. Subcommittee 2 was subse-
quently established and chargedwith the responsibility for technical content of standards covering
wrench head bolts and nuts.
Subcommittee 2, after appraisal of the requirements of industry, developed a proposed standard
series of bolt head and nut dimensions. This proposal was finally approved and designated a
tentative American Standard in February 1927.
A first revision of the document was designated as an American Standard in March 1933 and
was followed by a second revision that was granted approval as an American Standard in
January 1941.
Following reorganization of the B18 Committee in 1947, Subcommittee 2 was asked to expand
the standard on head proportions into a complete product standard. A proposal covering square
and hexagon head bolts and nuts, hexagon head cap screws, and automotive hexagon head bolts
was prepared and submitted to the B18 Committee in April 1950. While this draft was under
consideration, the B18 Committee received a proposal from the British Standards Institution for
unification of dimensions on products incorporating Unified screw threads. The Committee
welcomed the opportunity of discussing the proposals and an American-British-Canadian
Conference was held in New York, June 1 and 2, 1950.
It was agreed in the conference that the essentials of unification could be accomplished by
selection of mutually satisfactory across-the-flats dimensions, since this would permit the use of
the same wrenches and because other features would rarely affect interchangeability. After due
consideration, suitable existing across-the-flats dimensionswere selected for the hexagon products
affected.
In its meeting of October 13, 1950, Subcommittee 2 agreed to incorporate in the proposed
standard the conference recommendations on 1⁄4-in. hexagon head bolts, 5⁄8-in. hexagon head cap
screws and automotive hexagon head bolts, 5⁄16-in. and 3⁄8-in. regular hexagon and square nuts,
and 7⁄16-in. light and regular hexagon and square nuts. At a subsequent meeting of Subcommittee
2, further changes were adopted in order to combine the light and regular series of nuts and to
combine the automotive hexagon head bolt, hexagon head cap screw, and regular hexagon head
close tolerance bolt.
In view of the progress made in the United States and the urgency of standardization for
mutual defense, the British Standards Institution sponsored a second conference in London in
April 1951, to complete the unification of certain hexagon bolts and nuts.
At a meeting on June 8, 1951, Subcommittee 2 reaffirmed its acceptance of the unified dimen-
sions, which corresponded with those in the March 1951 draft, but attempted to select better
nomenclature for the unified products. A final draft incorporating the nomenclature “Finished
Hexagon Bolts and Nuts” and containing numerous editorial changes was submitted for letter
ballot in September 1951. Following approval by the B18 Committee and the sponsors, the
proposal was presented to the American Standards Association for approval and designation as
an American Standard. This was granted on March 24, 1952.
Recognizing the Standardwas in need of additional refinements, Subcommittee 2 began immedi-
ately to revise it, removing inconsistencies with respect to fillets, improving the length tolerances
on heavy hexagon bolts, and incorporating numerous other corrections and clarifications. The
most noteworthy editorial change was a decision to combine the coverage for hexagon cap screws
and square head set screws from the B18.2 Standard with the coverage for slotted head cap
screws and slotted headless set screws from the B18.6 Standard and publish them in a separate
iv
document. The requirements for the unified hexagon cap screws and finished hexagon bolts
being identical in the overlapping sizes, these data would now be available in two publications.
Following approvals by the B18Committee and sponsor organizations, the proposalwas submitted
to the American Standards Association and declared an American Standard on February 2, 1955.
A revision of this document comprised of numerous editorial corrections, and inclusion of an
appendix for grade markings was duly approved and designated an American Standard on
April 18, 1960.
At a meeting in February 1960, Subcommittee 2 approved a recommendation to reduce the head
heights for heavy, heavy semifinished, and heavy finished hexagon bolts that was subsequently
approved by letter ballot of the B18 Committee on August 16, 1960. A proposed Standard for
heavy hexagon structural bolts submitted and accepted by Subcommittee 2 at its October 17,
1960 meeting was approved by letter ballot of the B18 Committee on May 9, 1961. To meet the
urgent needs of the steel construction industry, it was considered necessary to publish the Standard
for the structural bolts immediately. Consequently, Appendix IV to ASA B18.2-1960 containing
coverage for the revised heavy hexagon bolts and the new heavy hexagon structural bolts was
released in 1962. In October of 1961, Subcommittee 2 appointed a subgroup to review all product
standards for square and hexagon bolts, screws, and nuts and to recommend simplifications that
would be compatible with technical, production, and distribution advances that had occurred
over the prior several years. The subgroup presented its recommendations at a meeting of
Subcommittee 2 in October of 1962. It was agreed that the internally and externally threaded
products should be published in separate documents as suggested, and draft proposals for each
were completed.
The proposed revision for square and hex bolts and screws incorporated the following subgroup
recommendations: consolidation of hexagon head cap screws and finished hexagon bolts into a
single product, consolidation of heavy semifinished hexagon bolts and heavy finished hexagon
bolts into a single product, elimination of regular semifinished hexagon bolts, a new length
tolerancing pattern for all bolts and screws, documentation of a positive identification procedure
for determining whether an externally threaded product should properly be designated a bolt
or a screw, and an abbreviated and purified set of product nomenclature reflecting application
of the identification procedure. Letter ballot of this proposal to the B18 Committee in March,
1964 resulted in several comments that were resolved to the satisfaction of the committee in June
of 1964. Following acceptance by the sponsor organizations, the revision was submitted to the
American Standards Association and was designated American Standard ASA B18.2.1 on
September 8, 1965.
Subcommittee 2 continued to further develop refinements initiated by the simplification sub-
group and revisions reflecting changes in manufacturing practices and consumer requirements.
This work culminated in Subcommittee acceptance of a 1970 proposal incorporating, in addition
to numerous editorial changes, revisions in the following significant areas: addition of coverage
for askew head bolts and hex head lag screws, addition of straightness requirements to applicable
products, addition of minimum fillet to square and hex bolts and lag screws, application of UNR
threads and new concepts for controlling thread length on products having Unified threads, and
clarification of grade markings, thread runout gages, and formulas for dimensions. Also included
were refinements to hex cap screw and heavy hex screw requirements consisting of the addition
of wrenching height and revision of underhead fillets, washer face thicknesses, and controls on
angularity of bearing face. The proposed revision, after approval by letter ballot of the B18
Committee in March 1970, was subsequently approved by the sponsors and submitted to the
American National Standards Institute for designation as an American National Standard. This
was granted on January 18, 1972.
Numerous user complaints on interference of the elliptical fillet added in the 1972 revision
resulted in the appointment of a subcommittee to study the problem. They recommended reverting
back to the max./min. radius fillet specified in the 1965 version with the elliptical fillet retained
for use when specified by the user. Further refinements in the definition of the fillet for short
length screws were added to “Hex Cap and Heavy Hex Screws.” Geometric tolerancing was
updated to conform to American National Standard Y14.5, Dimensioning and Tolerancing. The
transition length of the hex cap screw was changed to equal 5 coarse (UNC) threads. Few, if any,
users accepted the 1972 values that were designed to reduce tooling by providing the same body
v
length for adjacent lengths. On screws, separate straightness requirements have been deleted,
and the combination thread runout and straightness gage described in Appendix I is specified.
Straightness as a variable based on length has been applied to bolts with gaging described in
Appendix 11. Acceptability of screw threads based on gaging systems established by American
National Standard B1.3-1979 has been added to each type of screw or bolt, except lag screws.
This proposal was approved by letter ballot of the Subcommittee and B18 in January 1980.
Following acceptance by the secretariat organizations, the revision was referred to the American
National Standards Institute and granted recognition as an American National Standard on
June 24, 1981.
In 1991, it was recognized that B18.2.1 required extensive revision to better meet the needs of
conformance with Public Law 101-592. Included in these considerations were improved definition
of a full body versus a reduced body and those dimensions that should be certified to ensure
product fit, form, and function. Other dimensions given for each product would only be examined
in the event of a dispute. Also, the term “finished hex bolt,” which is today’s cap screw, should
be dropped. Additionally, a weight table has been included to assist users.
Furthermore, it was felt that the heavy hex structural bolt, heavy hex nut, hardened steel
washers, and compressible washer-type direct tension indicators should be included in a new
standard for fasteners intended for use in structural applications. For this reason, the heavy hex
structural bolt was removed from this Standard. The new table for maximum grip gaging lengths
and minimum body lengths for cap screws and heavy hex screws was included for the first time
in the 1996 edition to assist users and is similar to the pattern used for metric bolts and screws.
The Subcommittee 2 agreed to undertake the revision of B18.2.1 during the first quarter of
2008. The Standard was updated to incorporate the new format and additional sections as refined
in ASME B18.12.1. The notes that had followed every table were reorganized into the body of
the Standard to eliminate the redundancy created by repeating the same table notes under
numerous tables. This revision adds flange head and lobed head screws and extends the size
range of heavy hex head cap screws from 3 in. to 6 in. in diameter. The thread details for lag
screws were redefined to align with the way all other spaced threads are defined. Designated
inspection characteristics were eliminated from each product type, and a general section on
quality assurance was created stating that all products must meet the requirements in the Standard
according to ASME B18.18.2. The title of the Standard was revised to indicate that the flange
head and lobed head screws have been added to the Standard.
Suggestions for improvement of this Standard will be welcomed. They should be sent to The
American Society of Mechanical Engineers, Secretary, B18 Main Committee, 3 Park Avenue,
New York, NY 10016-5990.
This revision was approved as an American National Standard on July 8, 2010.
vi
ASME B18 COMMITTEE
Standardization of Bolts, Nuts, Rivets, Screws,
Washers, and Similar Fasteners
(The following is the roster of