Posted on September 11, 2026 Kyle Salem Advanced Imaging Equipment
Each assembly uses a precision pin and threaded Hi-Lok collar to secure aircraft skins, frames, ribs, spars, fittings, and other structural joints. Similar systems should not be assumed to be Hi-Lok products or interchangeable with them.
Correct selection involves more than matching diameter. Grip length, thread, head style, material, finish, and application must agree with approved engineering data. Parts that physically fit are not necessarily an authorized combination.
Hi-Lok fasteners are aerospace structural fasteners consisting of a threaded pin and a break-off collar. During installation, the pin is held against rotation while the collar is driven until its wrenching section separates at the specified torque-off point. The remaining portion stays threaded on the pin.
Hi-Lok assemblies generally remain installed until maintenance or repair requires removal. They are removable, but are not intended for routine disassembly. The applicable drawing, repair manual, or engineering instruction determines whether an assembly is permitted.
The pin transfers shear and tensile loads. It normally has a manufactured head, smooth grip, threaded end, and tail recess that prevents rotation during tightening.
Pin configurations differ by diameter, grip length, thread, head style, material, strength rating, finish, and approved collar pairing. Two pins with similar dimensions may therefore have different structural applications.
The collar threads onto the pin and compresses the joint. A calibrated groove separates its wrenching section from the installed portion.
As Howmet’s Hi-Lok fastening-system overview explains, the wrenching portion shears away at its designed torque level. This creates a recognizable endpoint, but torque-off alone does not prove a particular clamp load or acceptable joint condition.
Results also depend on part selection, grip, coating, hole quality, material stack, and procedure. A collar should never be chosen only because it starts on the threads.
Hi-Lok numbers must be interpreted through the drawing for the particular series. Consider HL18-6-6 as an example. HL identifies the Hi-Lok family, while 18 identifies the pin series. The HL18 series specifies a protruding shear-head pin made from alloy steel. In this designation, the first dash number, -6, represents a nominal diameter of 6/32 inch, or 3/16 inch. The second -6 represents a maximum grip of 6/16 inch, or 3/8 inch.
Head style, material, finish, strength, and thread are defined by the HL18 drawing; they cannot all be decoded from the suffix. Other series may follow different requirements, and collars use their own part numbers. Verify both complete designations against the current manufacturer drawing and approved aircraft data rather than treating this example as universal.
Governing technical data provides exact procedures and acceptance criteria. A general sequence is:
Torque-off means the collar reached its break-off point. Correct pairing, suitable grip, and acceptable hole and structural conditions still require inspection.
Grip must suit the combined material thickness. An incorrect choice can affect seating, protrusion, and load transfer. Use the drawing, manufacturer data, repair manual, or engineering instructions not simply whether a pin enters the hole.
Hi-Lok installation tools engage both sides: one holds the pin’s recess while another drives the collar. Common equipment includes:
Tool choice depends on fastener size, collar, access, and procedure. Howmet’s installation-tool guidance addresses standard and restricted-access work. Readers can also see how aircraft tooling supports installation and maintenance.
Poor tool engagement can damage the collar or recess, rotate the pin, or mark the structure. Confirm compatibility before work begins.
A solid rivet forms a shop head through deformation. A Hi-Lok pin accepts a threaded break-off collar. An aerospace bolt generally uses a nut, while a lockbolt has a swaged collar.
Understanding how common aviation fastener types differ clarifies the roles of bolts, rivets, pins, and threaded hardware. They are not automatically interchangeable; joint design and approved data determine the correct choice.
Inspect head and collar seating, protrusion, gaps, finish, grip, hole condition, and pin rotation. Incorrect protrusion, visible gaps, structural damage, or poor seating may require further evaluation.
Removal may use a collar splitter, cutter, drill, or another specified tool. Protect the hole, parent material, coatings, and nearby systems. Inspect afterward for enlargement, corrosion, cracking, or other damage.
Do not describe a requirement only as “Hi-Lok.” Confirm complete pin and collar numbers, manufacturer, family, diameter, thread, grip, head style, materials, finish, effectivity, and documentation.
When an identifier is known, Hi-Lok fastener part-number information may help distinguish pins, collars, and similar records. Manufacturer or NSN details can add context, but the applicable IPC, structural drawing, repair manual, or engineering approval remains the authority for installation eligibility.
For a quotation, provide the part number, component designation, mating part when known, quantity, condition, certification needs, and deadline. A listing does not establish aircraft eligibility or substitution approval.
They are threaded pin-and-collar fasteners. Their break-off collar and installation method distinguish them from conventional bolts and solid rivets.
No. Separation identifies the designed torque-off point. It does not independently verify clamp load or compensate for incorrect parts, grip, coatings, hole condition, or installation.
Collars are generally treated as one-time installation components. Follow the applicable maintenance instructions rather than reusing a collar because it appears undamaged.
Use the approved material-stack thickness and specified grip data. Diameter, head, material, finish, collar compatibility, and structural requirements must also match.
No. Diameter alone is insufficient. Confirm the thread, material, finish, product family, and approved pin-and-collar pairing in authoritative technical data.
Use the specified removal method and tool. Protect the hole and surrounding material, then inspect both before selecting a replacement.
A Hi-Lok fastener combines a structural pin with a threaded break-off collar. Reliable installation depends on the approved pairing, grip, diameter, material, finish, hole preparation, tooling, and inspection. The collar’s torque-off feature provides an installation endpoint, not automatic proof of a specific clamp load or acceptable joint. Always follow the applicable aircraft documentation when identifying, installing, removing, or replacing Hi-Lok fasteners.
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