How close to the underside of the head do A325T threads need to be?

A325-T-heavy-hex-boltUnder the new F3125 and original A325 specification, fully threaded structural bolts are allowable if the length of the bolt does not exceed 4 times the diameter (for more information please refer to our article: What is an A325T bolt?). We recently had a customer ask about how close the threads need to be to the underside of the head. This issue is addressed directly in the S1 supplemental requirement of both specifications. Both specifications contain the exact same language:

The distance from the under-head bearing surface to the first complete (full form) thread, as measured with a GO thread ring gauge, assembled by hand as far as the thread will permit, shall not exceed the length of 2-1/2 threads for bolt sizes 1 in. and smaller, and 3-1/2 threads for bolt sizes larger than 1 in.


For a 1”-8 bolt:  1” of thread length contains 8 threads per inch = 1 ÷ 8 = .125” per thread
.125” per thread x 2.5 threads = .3125” or 5/16” permissible unthreaded shank

For 1½”-6 bolt:  1” of thread length contains 6 threads per inch = 1 ÷ 6 = .167” per thread
.167” per thread x 3.5 threads = .585” or ≈37/64” permissible unthreaded shank

If you have any questions about A325 bolts or need any other help, please contact one of our helpful team members today.


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