How to Measure NAS Bolts

National Aeronautical Standards (NAS) bolts are exclusive to aircraft construction. Due to federal regulations, measuring these types of bolts require an understanding of how the bolt is utilized. The smooth portion of the bolt, the area below the hex head leading down to the threads is the grip. The grip equals the thickness of material being held together. The diameter of the bolt must conform to strict engineering standards for shear strength in holding the materials. Grip length is measured in 1/6-inch increments. The bolt diameter is measured in 1/16-inch increments.

NAS bolts are used for airplane construction.

Set the dial calipers to zero. Close the jaws on the calipers. Loosen the adjustment screw on the dial face. Rotate the dial so it reads exactly zero.
Measure the overall length of the NAS bolt from under the bolt head to the end of the threads. This is the overall under head length.
Lay the open jaws of the caliper against the threads. Measure the length of the threads. This is the NAS bolt thread length.

Subtract the overall head length from the thread length. The final calculation is the grip.
Confirm the grip by measuring it with the calipers. When finding the total grip length be sure to add the thickness of washers used under the bolt head.