Ask what a bolt does in an airframe and the honest answer is: it depends on the joint. Some joints load the bolt across its axis, with the joined members trying to slide past one another. Others load it along its axis, trying to pull the joint apart. Aerospace fastener standards split along the same line. Shear bolts and tension bolts are shaped, heat treated, and installed differently because they carry load differently, and choosing between them is not a matter of preference. It is a matter of matching the fastener to the load path the joint was designed around.
Key takeaways
- ✓Airframe joints are designed around a load path: shear joints load the bolt across its axis, tension joints load it along its axis.
- ✓Shear bolts put a smooth, close-tolerance grip in the shear plane; the NAS6203-NAS6220 hex head family is a classic example of the style.
- ✓Tension bolts typically carry 12-point heads and are installed at high preload so the clamped joint, not the fastener, absorbs the load cycles.
- ✓Grip length must match the thickness of the joined stack so that threads stay out of bearing.
- ✓The two most common mistakes, threads in the shear plane and the wrong grip length, are easy to make and hard to see after assembly.
Two Loading Modes in Airframe Joints
Structural joints put fasteners to work in two basic ways. In a shear joint, the load runs perpendicular to the bolt axis. Skin splices, lap joints, and lug-and-clevis connections all behave this way: the joined members try to slide past each other, and the bolt shank bears against the walls of the holes. In a tension joint, the load runs along the bolt axis. The members try to separate, and the bolt holds them clamped together. Fittings that redirect concentrated loads, such as some engine mount and wing attach fittings, are typically designed this way.
Real joints often see a mix of both, and the stress engineer accounts for that when the fastener is called out. That is the point for anyone buying or installing hardware: the drawing callout already encodes the loading assumption. The buyer's job is not to reclassify the joint but to supply exactly the fastener the callout defines, in the right material, diameter, and grip.
Shear Bolts: A Smooth Grip in the Shear Plane
A shear bolt is built so that its smooth, unthreaded shank, the grip, sits in the shear plane where the joined members meet. The shank is typically held to a close tolerance so it fills the hole and bears evenly, and close-tolerance shanks pair with reamed holes to limit joint movement. Head design follows the job: because a shear bolt is not asked to develop high axial preload, many families use compact hex heads with relatively thin profiles, saving weight where full tension capability is not required.
The classic example of the style is the NAS6203-NAS6220 close-tolerance hex head family, where the trailing digits typically track the nominal diameter in sixteenths of an inch and a dash number selects the grip. If callout structure like that reads as alphabet soup, our guide to reading NAS part numbers unpacks it with a worked example. The habit to build is simple: every character in the callout selects something real, and grip is the one buyers most often get wrong.
Tension Bolts: Preload Is the Point
A tension bolt is designed to be stretched. Installed at high preload, it clamps the joint faces together so firmly that normal service loads never separate them; the load cycles pass through the clamped members while the bolt tension stays nearly constant. That is what protects the bolt from fatigue, and it is why preload, developed through controlled torque or other tensioning methods, is the heart of a tension installation.
The hardware looks different too. Tension bolts commonly use 12-point external drive heads, which take high installation torque in tight spaces and provide a robust wrenching surface for the preloads involved. They are typically paired with tension-rated nuts and washers, and they are commonly made from higher strength materials than general purpose hardware. Torque values depend on thread condition, plating, and lubrication, so the applicable installation document, not habit, sets the number.
Why Grip Length Matters
Grip is the unthreaded length of the shank, and on a shear bolt it must span the full thickness of the joined stack. Get it right and the smooth shank bears in every hole. Get it too short and threads end up in bearing: the effective shank area drops, contact concentrates on the thread crests, and the thread roots act as stress risers exactly where the load is highest. Get it too long and the nut bottoms out on the thread runout before it ever clamps the joint.
That is why grip selection is measured, not guessed. Assemblers determine the stack thickness, often with a grip gauge in the actual hole, and select the dash number to suit. Small adjustments with washers are typically permitted only within the limits the applicable specification or drawing allows. When a stack lands between available grips, the answer comes from the engineering documents, not from improvisation on the shop floor.
Material Notes
The common shear and tension bolt families come in a familiar set of materials, though the details belong to each standard rather than to any general rule. Cadmium-plated alloy steel is the traditional workhorse, and many close-tolerance shear bolt families are typically heat treated to strength levels around 160 ksi. CRES grades such as A286 appear where corrosion resistance or elevated temperature capability matters. Titanium alloys, such as Ti-6Al-4V, trade higher cost for significant weight savings and are common in weight-critical structure. Finish and lubrication vary by variant and affect installation torque, so confirm material and finish codes from the standard and your drawing rather than assuming them from the family name.
Common Mistakes
- Threads in the shear plane. Usually the result of a grip one size too short, or a substitution made for availability. The joint may assemble and torque normally, which is what makes the error dangerous: nothing looks wrong from the outside.
- Wrong grip, long. A grip that outruns the stack leaves the nut clamping on thread runout, or demands more washers than the specification permits.
- Swapping bolt types without approval. A tension bolt in a shear application, or the reverse, changes head geometry, strength, and installed behavior. Substitutions belong to engineering, not to the parts crib.
- Misreading the callout. Diameter and grip both live in the part number, and transposing a dash number orders a different bolt. Verify the full callout at the quote, on the PO, and again at receiving.
Sourcing Shear and Tension Bolts
Start from the complete callout on your drawing, including material and finish codes, and quote against exactly that string. Instaparts' catalog covers more than 266,000 aerospace parts; browse the bolts category or start from our aerospace bolts overview and get a fast online quote. Every Instaparts shipment includes a certificate of conformance with full AS9120 traceability, so the paperwork behind the bolt is as sound as the bolt itself.