ASTM A325 structural bolts are virtually identical to standard heavy hex bolts in regards to configuration. One main difference however is the thread length. Because of the application, ASTM A325 bolts have a shorter than normal thread length. For example, a 1” x 4” A325 structural bolt has a 1-3/4” thread length whereas a 1” x 4” A307 hex bolt has a 2-1/4” thread length.
The A325 specification contains specific language limiting these bolts to these thread lengths. Section 1.5 of the specification reads as follows:
1.5 This specification is applicable to heavy hex structural bolts only. For bolts of other configurations and thread lengths with similar mechanical properties, see Specification A449.
There is however a supplemental requirement under the ASTM A325 specification (S1), which discusses the scenario where an A325 can be fully threaded. The difference, however is that this supplemental requirement depends on the length of the bolt, whereas a standard A325 thread length is diameter dependent only. The supplemental requirement “S1”reads, “Bolts with nominal lengths equal to or shorter than four times the nominal bolt diameter shall be threaded full length.” This means that if the structural bolt in question exceeds four times the diameter in length, it cannot be fully threaded. One thing to note is that fully threaded A325s are readily available in certain diameters and have “A325T” stamped on the head.
So how does the current supplemental requirement “S1” work with the newly implemented ASTM F3125? ASTM F3125 has its own supplemental requirement if a nonstandard thread length is needed. This supplemental requirement is designated by “S2” and needs to be specified if the thread length differs from the standard thread length in any regard, including fully threaded. This not only means that a bolt can be threaded half way up the shank or fully threaded, but it also means that the bolt can technically be fully threaded with the bolt length exceeding four times the diameter. Any F3125 Grade A325 bolt with a longer than standard thread length requires a head stamp “A325S”.