Hi-Lok fasteners explained for aerospace assembly and repair
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What Hi-Lok fasteners are
Hi-Lok fasteners, often searched as hi lok fasteners, are two-part aerospace structural fasteners made up of a threaded pin and a compatible self-locking collar. Their value is not just that they hold parts together. The collar is designed so its wrenching portion breaks off at a controlled installation torque, helping create repeatable preload when the specified pin, collar, hole condition, grip length, and tool are used together.
Public product literature from major aerospace fastener manufacturers describes the system as combining useful characteristics of a bolt-and-nut assembly and a pin-and-collar fastener. In actual aircraft work, however, a Hi-Lok should be treated as a controlled engineering part, not as a generic replacement for any aircraft bolt, rivet, or blind fastener.
For broader context on related hardware categories, see the Breezcats fasteners section. This article focuses on how the Hi-Lok concept works, where it is commonly specified, and what buyers, maintenance planners, and technical readers should verify before making assumptions from appearance alone.
How the pin and collar system works
A Hi-Lok assembly has two primary components. The pin passes through the prepared joint, and the collar threads onto the pin from the opposite side. During installation, the tool holds the pin stationary, usually through a recess in the pin end, while the collar is rotated. As torque increases, the collar tightens and clamps the joint. At the designed torque level, the wrenching portion of the collar shears off, leaving the installed collar body in place.
This break-off feature is the most visible part of the system, but it is only one part of the design. The intended preload depends on the correct collar, the correct pin series, the applicable dash number, the grip range, the hole condition, and the installation method. A collar that looks similar may not have the same material, locking feature, thread form, strength, or break-off behavior. For that reason, aviation maintenance documents and aerospace drawings normally identify these fasteners by exact part numbers or standards rather than by appearance.
Public product literature from Howmet Aerospace and LISI Aerospace drawing information describes common Hi-Lok design elements such as controlled break-off collars, pin-and-collar assembly drawings, head markings, grip dimensions, and system strength considerations. These sources are useful for understanding the concept, but they do not replace the controlling aircraft structural repair manual, engineering drawing, service bulletin, or approved maintenance procedure.
Where Hi-Lok fasteners are used
Hi-Lok fasteners are most closely associated with aerospace structures because aircraft joints often require high strength, close tolerance, low weight, repeatability, and traceable documentation. They may be used in tension or shear applications when the engineering data specifies the correct type. Flush head versions are relevant where an aerodynamic exterior surface is involved. Protruding head versions may be used in internal or non-flush locations when allowed by the drawing.
The term Hi-Lok does not describe a single universal part. It covers families of pins, collars, sizes, materials, head styles, coatings, and strength levels. Public drawing pages for Hi-Lok pin series show examples of variables such as nominal diameter, grip, head geometry, thread details, and collar compatibility. The lower strength of the pin or collar can govern the assembled system strength, so the correct match matters.
In production, the system can support repeatable installation across many fasteners. In maintenance or repair, its usefulness depends on approved instructions. A technician or purchaser should not assume that a Hi-Lok can replace a solid rivet, a blind fastener, a lockbolt, or a conventional bolt unless the relevant engineering authority allows it. Aircraft structures are load paths, not general metalwork.
How Hi-Lok compares with other aerospace fasteners
Hi-Lok fasteners are often described as sitting between familiar fastener categories. That comparison is helpful, but it has limits. The table below gives a practical comparison for technical readers and procurement teams.
| Fastener type | Typical advantage | Key limitation | Important selection note |
|---|---|---|---|
| Solid rivet | Light, proven, permanent joint with strong fatigue history in aircraft sheet metal | Usually requires access for bucking and skilled installation | Do not replace with another fastener type unless approved data allows it |
| Standard bolt and nut | Familiar threaded assembly and possible removability | Preload depends on torque procedure, lubrication, locking method, and inspection controls | Torque values and locking features must match the engineering requirement |
| Swaged collar or lockbolt | High clamp capability and permanent mechanical locking | Requires specific pullers or swaging tools, and removal can be more involved | Pin, collar, and tool compatibility control the result |
| Blind fastener | Can be installed from one side where backside access is limited | Not automatically equivalent to a solid rivet or structural pin-collar fastener | Use only where the aircraft or engineering data specifies it |
| Hi-Lok fastener | Controlled collar break-off can support repeatable preload with the specified parts | Requires correct pin, collar, grip, hole preparation, and access to both sides | Part number, dash number, and collar compatibility are more important than appearance |
Selection details that matter
The practical question is rarely whether a fastener is simply a Hi-Lok. The important question is which exact Hi-Lok assembly is specified. A well-written purchasing or maintenance request should include enough information to prevent a visually similar but nonconforming substitution.
- Pin series and dash number. The pin callout identifies geometry, diameter, grip, material, finish, head style, and other controlled details. A small change in the dash number can be meaningful.
- Collar series. The collar is not an accessory selected after the fact. It is part of the engineered assembly and may control preload, locking behavior, and installed height.
- Grip length. Grip must match the actual joint stack and the permitted grip range. If it is too short or too long, clamp-up, thread engagement, and installed condition can be affected.
- Load direction. Some applications are primarily shear, some are tension, and many structural joints see combined loads. Head style and strength selection must follow the drawing.
- Hole preparation. Aerospace fasteners often depend on hole diameter, roundness, finish, countersink condition, edge distance, and alignment. A correct fastener installed in a poor hole is still a risk.
- Material and coating. Alloy choice and surface treatment affect strength, corrosion behavior, temperature capability, and compatibility with surrounding materials.
- Tool access. The system needs access for the pin and collar, plus enough clearance for the installation tool. Offset and specialty tools may be required in restricted locations.
- Documentation. Certification, traceability, and conformity records are central in aerospace supply chains. A low-cost substitute without documentation can be unusable.
These details are why procurement teams should avoid vague requests such as Hi-Lok equivalent unless acceptable alternatives are explicitly defined by engineering or purchasing specifications. In aerospace, equivalence is demonstrated through standards and approvals, not through visual similarity.
Standards, documentation, and source control
Hi-Lok fasteners exist within a standards-heavy aerospace environment. The Aerospace Industries Association maintains the National Aerospace Standards library, and its public materials describe NAS parts as including high-strength precision fasteners. AIA also states that the National Aerospace Standards Committee develops and maintains aerospace part standards and that AIA itself does not supply parts or maintain supplier lists for NAS or MS parts.
Defense and aerospace documentation systems add another layer. The DLA ASSIST public record for Airframe Fastener Usage Guidelines lists the document as active, with a DID date of May 10, 2023 and a next review due date of May 8, 2028. Its stated purpose is to support selection and application of mechanical fasteners for air vehicles and aircraft stores, balancing performance, reliability, standardization, cost, logistics, and maintenance. That kind of document illustrates the broader point: fasteners are not isolated commodities in aircraft work. They are selected within a controlled design and maintenance system.
The FAA advisory material commonly referenced for acceptable aircraft inspection and repair practices is also important background, but it should be read correctly. General advisory material does not override a type design, structural repair manual, aircraft maintenance manual, service bulletin, or approved engineering disposition. For certified aircraft, the controlling data determines what fastener may be installed, how the hole is prepared, and what inspection is required after installation. See also: Buying Guides.
Common mistakes in installation and purchasing
Many Hi-Lok problems begin before the tool touches the aircraft. The first mistake is treating the break-off collar as proof that the installation is automatically correct. Collar break-off indicates that a designed torque event occurred at the collar. It does not prove that the correct parts were selected, the grip was right, the hole was acceptable, or the surrounding structure was undamaged.
A second mistake is mixing pin and collar types because they appear to thread together. Thread engagement alone is not enough. The collar must be compatible with the pin and the application. A mismatch can change preload, installed height, locking performance, strength, or removability. Color coding, head markings, and shop familiarity can help experienced personnel identify parts, but the controlled document remains the authority.
A third mistake is ignoring removal risk. Removing a structural fastener can enlarge a hole, damage a countersink, disturb coatings, or create the need for an oversize repair. Oversize fasteners are not casual replacements; they must be specified and installed under approved instructions. In maintenance planning, removal method and follow-up inspection should be part of the job plan, not an afterthought.
A fourth mistake is assuming all aerospace fastener documentation is current because a catalog page exists online. Catalogs, drawings, standards, and manufacturer references can be revised. When the part is used for regulated aerospace work, buyers and technicians should verify the current revision level through approved channels before purchase or installation.
Why Hi-Lok still matters in aerospace supply chains
The continued relevance of Hi-Lok fasteners comes from a combination of engineering and production value. Controlled preload can help reduce variation in structural assembly. The two-piece system can be compact enough for locations where other fasteners are harder to install. The range of head styles, sizes, and material combinations makes the system adaptable across many applications when the design permits it.
For distributors and maintenance organizations, that adaptability also creates a long tail of similar-looking parts. A shelf label that simply says Hi-Lok is not enough. The useful information is the complete callout, including pin series, dash number, collar series, material, finish, grip, standard, revision context, and certification requirements. As aerospace supply chains continue to emphasize traceability and conformity, documentation can be as important as the physical fastener.
For engineers and technical buyers, the safest way to think about Hi-Lok is as a controlled fastening system rather than a single item. The pin, collar, prepared hole, tooling, and approval basis work together. When those elements align, the system offers repeatable structural fastening performance. When one element is guessed, substituted, or undocumented, the advantage can disappear.
Frequently asked questions
Are Hi-Lok fasteners the same as bolts?
No. A Hi-Lok uses a threaded pin and a self-locking collar with a controlled break-off feature. It shares some characteristics with a bolt-and-nut assembly, but it is selected, installed, and documented as a specific aerospace fastening system.
Can a Hi-Lok replace a rivet or blind fastener?
Only if the approved engineering data allows that substitution. Solid rivets, blind fasteners, lockbolts, bolts, and Hi-Lok assemblies can serve different structural purposes. The controlling drawing, repair manual, or approved engineering instruction must define the acceptable replacement.
Why does the collar break off during installation?
The wrenching portion of the collar is designed to shear at a controlled torque level. This helps create a repeatable preload when the correct pin, collar, grip, hole preparation, and tool are used together.
How should grip length be chosen?
Grip length should be chosen from the applicable drawing, part standard, or repair instruction after confirming the actual material stack. It should not be guessed from appearance or replaced with the nearest available length without approval.
Is Hi-Lok a generic term?
Public product literature identifies Hi-Lok as a trademark associated with Hi-Shear Corporation. In everyday shop language, people may use the term loosely, but purchasing and installation should rely on exact part numbers and approved documentation.
