Is there a standard thread length for bolts?

Depending on the type of bolt you are using the thread length can vary greatly. There is not an inclusive thread length standard that covers all fasteners. Depending on the applications, manufacturer, and a number of other factors the amount thread may change. It is important to clearly communicate at time of purchase your desired thread lengths on headed bolts in addition to bent anchor bolts and threaded rods. Below is a brief outline of common fasteners and the thread lengths generally found in the marketplace.

Hex / Heavy Hex Bolts

ASME B18.2.1 which covers hex, heavy hex, and square head bolts, states bolts 6″ in length and under have a thread length that is twice the diameter plus 1/4″. Bolts that are longer than 6″ have a thread length of twice the diameter plus 1/2″. For example, a 1/2 x 6 hex bolt will have 1-1/4″ of thread, whereas a 1/2 x 6-1/2 hex bolt will have 1-1/2″ thread. In our experience, most “off the shelf bolts” that are longer than 12″ have 6″ of thread.

Structural Bolts

Structural bolts like ASTM A325 and A490 specify shorter thread lengths since these bolts are used for steel-to-steel connections and the amount of grip length is critical thread lengths are much shorter. A325 heavy hex bolts with the S1 supplement are to be fully threaded. Those with the S1.2 supplement must be marked, “A325T”.

Nominal Bolt Size (in.) 12 58 34 78 1 118 114 138 112
Thread Length (in.) 1.00 1.25 1.38 1.50 1.75 2.00 2.00 2.25 2.25

Carriage Bolts

ASME B18.5 it states that the thread lengths for carriage bolts 6″ in length and under have a thread length that is twice the diameter plus a 1/4″ and bolts longer than 6″ have a thread length of twice the diameter plus 1/2″. However, the import carriage bolts we stock are fully threaded for bolts 6” long and under and 6” of thread for bolts longer than 6”.

Lag Bolts

The minimum thread length shall be equal to one-half the nominal screw length plus 1/2″ or 6”, whichever is less.

Anchor Bolts

For any type of bent bolt or straight rod there are no thread standards. The thread length of anchor bolts change significantly depending on the application, projection from the top of concrete, and a number of other factors. Most structural drawings will outline the exact dimensions of the anchor bolts, including thread length. It is important to remember that anchor rods often specify different thread lengths on each end.

Written December 1, 2008 by
Dane McKinnonDane McKinnon

Phone: 503.219.6991
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8 comments

  1. 1″-8 SHCS thread engagement. I know the old rule of thumb is 1 1/2 times dia. If this is true then why are the nuts only .831 thick? My real question is what is the minimum length of threads a tapped hole must have before it fails in steel (P-20) with using a 1″-8 SHCS

    1. @Michael Croff – All things being equal, they shouldn’t be. Bolts made with a finished head pattern should have the same head dimensions regardless of length, and the same is true for bolts made with heavy hex pattern heads. If you have two different bolts that were made to different grades, for example, if the 1-1/8 x 5 bolt was made to ASTM A325 and the 1-1/8 x 12 bolt was made per SAE J429 grade 5, then they would have different dimensions, but that is due to the specification differences, not the length.

  2. I have a customer with a Cat motor grader punched for 3/4″ bolt holes. We have a cutting edge punched for 5/8″ holes in stock. We would like to sell him our product that is in stock, but the bolt holes are different sizes. Is there anything, like a sleeve or transtition piece, that could assist us with this issue? Thanks

  3. @Matt Viecelli:

    A sleeve can be fabricated using A513 type 5 tubing, but it will not be as strong. The smaller bolts can not support loads as well as larger ones for the same grade bolt.

    If you use 3/4″ OD, 0.65″ wall DOM and do some lathe work (cut to length, ream to size), you could make spacers, but it can not be guaranteed that the resulting assembly will be strong enough.

    The costs of the proper engineering may be unreasonable compared to having the right sized holes machined in the cutting edge unless the cutting edge is VERY common. Adding the cutting edge to inventory may be a better choice than adding adapters and special bolts to inventory.

  4. Existe alguna norma ó estandar que me indique cual es el largo adecuado (ó minimo) para tornilleria en estructura de acero??
    Cuantos hilos del tornillo A-325 deben sobresalir de la tuerca (Nuts) una vez aplicado el torque.
    Gracias.

    1. @Guillermo – Una buena regla general es que dos hilos deben sobresalir del extremo de la tuerca. Sin embargo puede haber excepciones sobre una base de caso por caso, pero esta llamada tiene que ser hecha por un ingeniero.

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