Modern aircraft structures demand fasteners that can handle extreme shear and tensile loads, resist vibration-induced loosening, and be installed with consistent, controlled preload. Standard solid-shank rivets serve well in many applications, but there are structural joints — wing spars, fuselage frames, engine mounts, and control surface attach points — where engineers reach for something stronger and more reliable. Two fastener families dominate these high-load applications: Hi-Lok fasteners and lockbolt fasteners. Both are two-piece systems that combine a pin (or bolt) with a collar, and both achieve their clamping force through a mechanically defined, repeatable preload. For the AMT airframe technician, understanding how each system works, how to install and inspect it correctly, and how to remove it without damaging surrounding structure is not just a test requirement — it is a core competency for anyone working on transport-category or high-performance aircraft.
Hi-Lok Fastener System
The Hi-Lok fastener was developed specifically to provide a consistent clamp-up load without requiring a torque wrench on every installation. It consists of two components: a threaded pin and a threaded collar. The pin resembles a standard bolt but has a hex recess (or wrenching feature) in its head end, and the collar is a thin, nut-like ring with a built-in wrenching hex that is designed to shear off at a precise torque value.
How the Hi-Lok Works
During installation, the pin is inserted through the pre-drilled and countersunk (or straight) hole from the accessible side. A hex key (Allen wrench) is used to hold the pin stationary by engaging the recess in the head. The collar is then threaded onto the exposed end. As the collar is tightened, it draws the joint together and develops clamp-up load. When the torque reaches the engineered breakaway value — controlled by the machined shear groove and wrenching hex geometry designed into the collar — the outer wrenching hex shears cleanly off, leaving the inner, smooth portion of the collar locked in place. This breakaway feature is the key advantage: the installer cannot over-torque or under-torque the fastener, because the collar itself controls the final load.
Hi-Lok pins are manufactured in several alloy systems, including steel, titanium, and aluminum, selected based on the strength and weight requirements of the structure. Steel Hi-Loks are used in high-strength steel structure, while titanium Hi-Loks are preferred in areas requiring a high strength-to-weight ratio. Aluminum Hi-Loks exist for lower-load applications in aluminum structure. Specific finish and color-coding conventions vary by manufacturer and applicable NAS/MS specification rather than a single universal scheme. The collar material must be compatible with the pin material to prevent galvanic corrosion; for example, aluminum collars are used on aluminum pins, and titanium or stainless collars are used on titanium pins in areas where weight savings are critical.
Hi-Lok Identification and Part Numbering
Hi-Lok part numbers encode the fastener's diameter, grip length, and head style. The grip length is critical: it must match the total thickness of the materials being joined so that the threaded portion of the pin extends the correct distance beyond the material for collar engagement. Too short a grip and the collar will bottom out before developing full clamp-up; too long a grip and the collar threads may not reach the shear groove properly. Always consult the aircraft's structural repair manual (SRM) or engineering drawing to determine the correct grip length for a given stack-up.
Lockbolt Fastener System
Lockbolt fasteners — produced under trade names such as Huck or Composi-Lok — work on a different principle. Rather than a threaded engagement, lockbolts use a swaged collar that is plastically deformed into annular grooves on the pin's shank. Installation requires a pneumatic or hydraulic pulling tool that simultaneously pulls the pin's tail stub while a swage anvil drives the collar material into the grooves. When the pin tail fractures at a controlled break groove, the installation is complete and the collar is permanently locked.
Types of Lockbolts
There are three basic lockbolt configurations. Pull-type lockbolts are the most common in airframe work. The installation tool grabs the pull stem (the pintail) protruding from the collar side and pulls it, swaging the collar and then breaking off the stem flush or below flush. Stump-type lockbolts have a short pin extension for swaging rather than the long pull stem used on pull-type fasteners; they are used in locations where clearance is restricted and a pull tool cannot operate, with a separate swage tool driven over the collar. Blind lockbolts — such as the Huck BOM — are used where only one side of the joint is accessible, similar in concept to blind rivets but capable of significantly higher structural loads.
Lockbolt Pin and Collar Materials
Lockbolt pins are typically made from alloy steel or titanium, and the collars from aluminum, titanium, or steel, depending on the application and required strength. The collar material is softer than the pin material by design, allowing it to flow into the pin grooves without cracking. The number of annular grooves and the collar wall thickness are engineered to produce a precise clamp-up load and shear strength when properly swaged. Because swaging is permanent, the collar — and often the pin — must be drilled out for removal, which is a more involved process than removing a threaded fastener.
Installation Requirements and Hole Preparation
Both Hi-Lok and lockbolt systems are highly sensitive to hole quality. The hole must be drilled to the correct diameter (typically a close-tolerance or interference-fit hole, depending on the application), properly deburred, and, for flush-head fasteners, countersunk to the correct angle and depth. Hi-Lok and lockbolt pins used in critical structural locations are often installed with a controlled interference fit, the exact value of which is specified in the applicable SRM or engineering drawing rather than a single universal figure; a properly specified interference fit eliminates fretting and improves fatigue life significantly. Installing an interference-fit fastener requires careful reaming to final diameter, application of a thin wet coat of sealant or primer on the shank for corrosion protection, and driving the pin squarely into the hole to avoid cocking.
Clamp-up adequacy is verified differently for each system. For Hi-Loks, the clean shear of the collar hex is the go/no-go indicator. If the hex does not shear during installation, the fastener must be removed and the cause investigated — often a hole that is too large, allowing the pin to pull through without building proper load, or an incorrect collar. For lockbolts, a properly swaged collar will have material fully filling the pin grooves with a smooth, even appearance; witness marks, splits, or incomplete swage are rejectable conditions.
Inspection Criteria
Installed Hi-Lok and lockbolt fasteners are inspected visually and, in some cases, with a tap test. Rejection criteria include: cracks in the collar or pin head, inspected using the magnification or NDI method specified in the applicable maintenance manual or SRM; rotation of the pin head (indicating loss of clamp-up or thread failure); corrosion under the head or at the collar interface; missing or partially sheared collar hex on a Hi-Lok (indicating improper installation); and incomplete swage on a lockbolt collar. Any fastener that can be rotated by hand or that shows head movement under moderate finger pressure must be removed and replaced. Hi-Lok pins that have been improperly installed and show a collar that did not shear must not be re-torqued — the collar must be removed and a new collar installed because the wrenching hex is a one-time element.
Removal Procedures
Removing a Hi-Lok fastener is relatively straightforward if care is taken. A collar removal tool (a specially shaped chisel or split blade) is used to cut through the collar without damaging the pin or surrounding structure. The collar is split and peeled away, and the pin is then pushed or pulled out. The hole is inspected before a replacement fastener is installed. Lockbolt removal requires drilling out the collar using a drill bit sized slightly smaller than the pin shank, then driving the pin clear. Some technicians use an end mill or collar splitter to avoid enlarging the hole. In all cases, avoid drilling into the structure itself, as an oversized hole may require a bushing or larger-diameter replacement fastener per the SRM.
Why It Matters
Hi-Lok and lockbolt fasteners are found throughout primary structure on commercial, military, and high-performance general aviation aircraft. A single improperly installed fastener in a spar cap or fuselage skin splice can create a stress concentration that initiates fatigue cracking — a condition that may not become visible until after significant damage has propagated. The controlled preload of these systems is not an engineering nicety; it is a calculated contributor to the joint's fatigue life and load-sharing behavior. Technicians who understand why the collar breakaway torque matters, or why an interference fit is specified, will install these fasteners correctly even under shop-floor time pressure.
Key Numbers and Rules
- Hi-Lok collar breakaway: the shear of the outer hex is the sole indicator that correct installation torque has been achieved — no separate torque wrench is needed or appropriate.
- Interference fit tolerance: value is specified per the SRM or engineering drawing and varies by application; there is no single universal figure for all Hi-Lok and lockbolt installations.
- Grip length selection: the grip must match the material stack-up thickness so that threads engage correctly and collar seats against the material surface.
- Collar compatibility: collar material must be compatible with pin material to prevent galvanic corrosion (e.g., aluminum collar on aluminum pin, titanium or stainless on titanium pin).
- Rejection triggers: pin rotation, collar cracks, incomplete swage, corrosion under head, or a Hi-Lok collar that did not shear during installation.
- Removal rule: never re-torque a Hi-Lok collar that failed to shear — remove and replace the collar only, then reinspect the hole before reinstallation.
Common Test Traps
- Confusing the shear-off hex with a defect: the missing outer hex on an installed Hi-Lok collar is correct — it means the fastener was properly torqued. A collar that still has its hex intact is the problem condition.
- Assuming lockbolts are removable like bolts: lockbolts use a swaged (permanent) collar, not a threaded nut; they must be drilled out, not unscrewed.
- Wrong grip length: selecting grip length based on one layer of material rather than the total stack-up is a classic mistake that produces either an under-loaded or an improperly seated collar.
- Galvanic corrosion from mismatched materials: using a steel collar on a titanium pin or an aluminum collar in a steel joint violates compatibility requirements and will cause accelerated corrosion.
- Drilling out a lockbolt with an oversized bit: using a drill that matches the collar OD rather than a smaller pilot drill risks enlarging the structural hole, which may require engineering disposition before the repair can proceed.