What is ANSI B18.2.1?

Often times, ANSI (American National Standards Institute) or ASME (American Society of Mechanical Engineers)  B18.2.1 will show up on a drawing, some certification documents, and even our website . This number simply refers to dimensional standards for a variety of common bolts, such as hex bolts, heavy hex head bolts, and square head bolts. Most ASTM specifications do not address dimensional tolerances of fasteners.  Instead, most ASTM bolt specifications refer to an ASME standard for this information.  Bolt manufacturers must comply with ASME standards for dimensions such as head height, width across the flats, body diameter, bolt length tolerances, thread lengths, and dimensional tolerances in general.  ASME B18.2.1 does not have any bearing on the strength or the chemistry of the bolt, which is typically called out by an ASTM or SAE grade designation.



    Is B18.2.1 a standard head and thread for UNC socket head cap screws? I’ve got a job that specifies ASTM S354 Grade BD material and ASME B18.2.1 “form”. That’s a new one for me.

    @Steve- ASME B18.2.1 covers the standard dimensions for many headed bolts like hex, heavy hex, square, etc. It does not cover socket heads, because socket head cap screws are covered by ASME B18.3. ASTM A354 BD bolts are more commonly made to the B18.2.1 hex bolt standards, whereas socket head cap screws are typically made per ASTM A574.

    @Thomas- Most of the dimensions that we use to generate our internal cut sheets are pulled directly from that ASME standard, so from a dimensional standpoint, you could call it that. However, there are many grades of fastener that could all have those same dimensions, so if you were using only that standard for your cut sheet, it would be missing the grade and mechanical information.

    Dear Mike
    I need to consult you if this standard also applies to stud bolts and rods.
    Thanks you.

    Franco R.

    @Franco- ASME B18.2.1 applies to hex bolts and other headed products. Unheaded studs and rods are covered by ASME B18.31.2 and B18.31.3 respectively.

    I m Mohamed Mzid and I work as mechanical Tech in Tunisia with shell and we have S/D this year must be to keep some bolt as contingency because we have Hydro test of 60 vessels all spool removed and some spectacle blind inserted, all bolting as ANSI B16 2.1 and hot dipped Galvanized and the spic of stud ASTM A320-L7M and Nuts A194-7M
    do you have a chart for knowing the long of stud if I have the size and the rate of flange ?
    and if I have the spectacle blind or spacer must be to add with initial length

    @Faiz- Since we do not manufacture metric fasteners, we do not have a copy of the latest metric standard.


    Would you know the yield strength of a ASME B.18.2.1 bolt of the following size: 1.5 X 6 UNC ?

    Thank you,
    Carlos Muller
    General Dynamics Mission Systems

    @Carlos- ASME B18.2.1 only covers the dimensions of the bolt, not the steel type or mechanical properties. If you know the grade, we can help determine the minimum yield strength.

    Hi Mike,

    I hope you can help me. We need to attached a inside metal railing (200lbs) to a wooden beam (14×5) – we have 10 attachment points. The railing contractor suggested using 3/8 x 2 1/2 wooden lag bolts but mentioned that he wanted to make sure we don’t rip of the heads using an impact wrench. Also what time of head and coating would you recommend.

    Albrecht Gass

    @Albrecht Gass- I would be careful using an impact wrench when installing 3/8″ lag bolts, as the heads would likely shear off with a powerful wrench like that. As for head style and coating, a hex head will be the most common head style of lag bolt. Outdoor applications normally require a hot dip galvanized coating, whereas for indoor applications, plain finish or electro zinc coated is probably sufficient.

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