When a fully threaded bolt is specified, how much of an unthreaded shank is allowed?
Fully threaded bolts are often referred to as “tap bolts”. The unthreaded portion of a bolt is often referred to as the “grip”. ASME B18.2.1 is the dimensional standard that covers hex, heavy hex, and square head bolts as well as lag screws. This specification states for fully threaded bolts and lag screws that are up to and including 1” in diameter, the unthreaded length under the head should not exceed 2.5 times the thread pitch. For bolts that are larger than 1” in diameter, the unthreaded portion should not exceed 3.5 times the thread pitch. ASME B18.5 which covers round head bolts (including carriage bolts and countersunk bolts) and ASME B18.2.6 which covers structural bolts follow the same formula for determining the minimum grip length for a fully threaded bolt.
For example, a fully threaded 1” diameter Unified National Coarse bolt with 8 threads per inch would have an unthreaded shank with a length that does not exceed .35” (2.5 ÷ 8). A fully threaded 1½” diameter Unified National Coarse bolt with 6 threads per inch would have an unthreaded shank with a length that does not exceed .58” (3.5 ÷ 6).
Fully threaded bolts can be found readily available in select grades and sizes.
- F3125 Grade A325T fully threaded structural bolts – These bolts are defined under Supplementary requirement S1 in the ASTM F3125 specification. An A325T cannot exceed four diameters in length. These bolts are mass-produced and readily available in the marketplace. Fully threaded A325 structural bolts longer than four diameters in length can be custom manufactured and would be marked with “A325S” rather than “A325T”.
- A307 Grade A tap bolts – Mass-produced and readily available in the marketplace. Used primarily for construction applications.
- SAE J429 Grade 5 tap bolts – Mass-produced and readily available in the marketplace. Used primarily for automotive, equipment, and OEM applications.