Answer: 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, “A325 T”.
| Nominal Bolt Size, in. | 1/2 | 5/8 | 3/4 | 7/8 | 1 | 1-1/8 | 1-1/4 | 1-3/8 | 1-1/2 |
|---|---|---|---|---|---|---|---|---|---|
| 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.
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