Grip Length on Aerospace Bolts and Screws: How to Pick the Right Dash Number for Your Joint Stack

September 16, 2026

The dash number on a bolt sets its grip length, which decides whether the smooth shank or the threads carry the bearing load in a flight joint. Measuring the full joint stack and matching it to the right AN dash number keeps the shank bearing against the hole and the joint secure through years of flight cycles.

Key Takeaways

The dash number sets a bolt’s grip, the unthreaded shank length, and it should match the joint stack thickness or come very close, per FAA AC 43.13-1B.
A correct grip keeps the shank bearing smooth against the hole walls, since threads have a smaller cross section and give a weaker bearing surface.
On AN bolts, the first number gives diameter in sixteenths of an inch, and the dash number’s last digit counts eighths of an inch, with any earlier digits counting whole inches. NAS and MS parts use their own coding.
Measure the full stack with a calliper, including primer, sealant and shims, and share that thickness with your supplier so they can confirm the grip before production.

Two bolts laid side by side may look alike, but only by checking the dash number at the end of the part number can you see the difference; this grip determines the unthreaded section of the shank that passes through the joined parts. In a flight joint, grip length decides whether the smooth shank or the threads carry the bearing load inside the hole. Choose it well, and the joint holds its clamp and shape through years of flight cycles. That is why buyers of aerospace bolts and screws check the dash number first.

Fortunately, the process is relatively straightforward and repetitive. All one needs to do is determine the joint stack and understand the dash numbering system to select aerospace bolts and screws in no time at all. A few minutes with a calliper and a standards chart usually settles the question.

What Grip Length Means on Aerospace Bolts

Grip is the plain, unthreaded part of the shank. The joint stack is the total thickness of every layer the bolt passes through, including sheet, fitting and any spacers.

The goal is straightforward. The FAA’s AC 43.13-1B guidance says grip length should match the material thickness, or come very close. That way the smooth shank bears against the hole walls, and the threads sit outside the joint where the nut can do its job.

Why does this matter so much? Threads have a smaller cross section and sharper contours than the shank, so the shank gives a steadier bearing surface.

How to Read the Dash Number on AN Bolts

AN hex bolts use a clear coding pattern. Let’s break it down with a common part, AN4-7:

The number after “AN” gives the diameter in sixteenths of an inch, so AN4 means 4/16, or 1/4 inch.
The dash number gives the length. The last digit counts eighths of an inch, and any digits before it count whole inches.
AN4-7 is 7/8 inch long. AN4-12 is 1 and 2/8 inches, which works out to 1 1/4 inches.

The dash number sets overall length, and each size has a matching grip figure in the standard’s table. NAS and MS parts follow their own coding, so check the relevant sheet before you assume the same rule applies.

Measuring Your Joint Stack the Right Way

A careful measurement saves a second order later. I have seen a planner catch a stack error simply by measuring after sealant went on rather than before.

Next steps at the bench:

Measure the full stack with a calliper at the hole location.
Include primer, sealant or shims that sit between the layers.
Pick the dash number whose grip matches that thickness or sits just above it.
Check thread engagement with the nut installed.

AC 43.13-1B allows washers to take up small differences when the grip runs slightly long. That flexibility helps, though it should stay a small adjustment rather than a habit.

Ordering Aerospace Screws and Bolts With Confidence

Your drawing should state the full part number, including the dash number. When you send an enquiry, share the stack thickness too so that the supplier can confirm the grip before production.

For special joints, a custom length to the drawing often gives the neatest fit. Is your current stack measured, recorded and matched to the dash number on your parts list?