Grip length is the length of the unthreaded portion of a bolt's shank. That's an important consideration when dealing with critical applications that involve a lot of vibration, namely, aircraft and racing. As a rule, no more than one thread should be within the bolt hole because vibrations can cause the threads to ream out the hole. When it comes to such applications, it's essential to use aircraft bolts, which are varied enough to meet almost any specifications including the perfect grip length for the job. The part numbers for aircraft bolts are preceded by NAS, MS or AN.
Find the last digits of the NAS- or MS-part number for the given bolt in question. The digits will follow either an A, L or a dash. Divide the number by 16 to find the grip length in inches. For example, the part number NAS464-5L20 has 20 for the last digits, so dividing by 16 indicates a grip length of 1 1/4 inches. For AN bolts, refer to a chart to find the grip length.
Choose a bolt with a grip length equal to or slightly greater than the thickness of the material that you will be bolting. Use a washer to make up for the amount the grip length is greater than the thickness.
Thread a nut on the bolt and tighten it with a torque wrench. Refer to the torque specifications for the given bolt diameter. Make sure the nut isn't bottomed out on the last thread and that the end of the bolt is showing two full threads when the nut is tightened.
Non-aircraft bolts have grip lengths that vary according to the manufacturer. For any given length and diameter, each manufacturer generally offers only one grip length. So, if you're concerned about grip length, use aircraft bolts.