Drilled head bolt guide: types, uses, and how to choose the right one

What is a drilled head bolt?

A drilled head bolt is a threaded fastener with one or more small holes bored through its head, specifically engineered to receive safety wire that mechanically prevents loosening under sustained vibration or dynamic loading. Unlike standard hex head bolts or socket head cap screws, the drilling adds a passive, wire-based locking mechanism that cannot be defeated by heat, chemical degradation, or fatigue cycling — which is exactly why it is the mandatory fastener of choice in aerospace, motorsports, and heavy industrial environments.

The concept is elegantly simple. Once the bolt is torqued to spec, a strand of stainless safety wire threads through the hole and connects to an adjacent bolt or anchor point. The wire is twisted so that any tendency of the bolt to rotate in the loosening direction immediately tensions the wire — creating a self-arresting feedback loop. According to recent research, vibration-induced fastener loosening accounts for roughly 20–30% of mechanical field failures across aviation and heavy equipment sectors, making this a problem worth solving properly.

Where did the drilled head bolt come from?

The design traces back to early military aviation, where the consequences of an in-flight fastener failure were catastrophic. The U.S. military eventually codified safety wiring requirements under MS20995 and related specifications, and the practice migrated into commercial aviation, Formula-class racing, and industrial turbomachinery. Today the drilled head bolt sits within a broader family of bolt fastener types and standards that spans everything from structural bolts to castellated bolt designs using cotter pins.

Single-hole vs. cross drilled bolt: what's the difference?

A single-hole variant has one bore running through the head, usually perpendicular to the bolt axis. A cross drilled bolt — sometimes called a dual-port design — has two holes at 90° to each other, allowing wire to exit in multiple directions. Cross-drilled heads are preferred on flange bolt clusters where space constraints force irregular wire routing. Single-hole designs are simpler to manufacture and adequate for most standard bolted joint applications where wire routing is straightforward.

Types of drilled head bolts explained

Not all drilled head bolts are interchangeable. Head geometry, drive type, material, and hole configuration each affect installation torque, wire routing capability, and compatibility with surrounding hardware. Here is a practical breakdown of the main variants a U.S. buyer will encounter.

Hex head drilled bolt (most common)

The standard hex head bolt with a drilled head is the most widely stocked form. Its six-sided profile accepts standard wrenches and socket drives, making installation fast. You'll find this configuration specified as AN hex bolts in aerospace applications and as Grade 5 or Grade 8 hardware in motorsports. The lateral drilling is typically centered in the hex flat for maximum residual head strength — per AN/MS standards, the drilling reduces rated tensile strength by less than 5%, a negligible tradeoff for the locking reliability gained.

Socket head cap screw (drilled)

A drilled socket head cap screw is the go-to choice when radial clearance is tight. The cylindrical head offers a larger contact area and higher drive torque than a comparable hex design, which matters on high-strength metric bolt assemblies in engine bay applications. In practice, actual testing on cylinder head bolt installations confirms that socket head variants tolerate repeated torque-angle sequences better than hex variants in confined aluminum bores — a useful detail if you're working on cylinder head bolt replacements for 4G64 or similar engines.

Flange bolt with drilled head

A drilled flange bolt integrates a wide bearing flange beneath the head, spreading clamping force across a larger surface area — critical on aluminum castings where point loading causes fretting damage over heat cycles. Engine head bolt applications on turbocharged platforms commonly use this geometry. The flange does constrain wire routing angles, so cross-drilled variants are often specified here.

Castellated bolt and cotter pin bolt (related designs)

Worth distinguishing: a castellated bolt uses a notched nut and a cotter pin bolt uses a split pin through the shank, not the head. These are categorically different from drilled head bolts — they lock via nut-side rather than head-side wire tension. Mixing them up in a spec document is a surprisingly common error that shows up in procurement disputes.

Torque specifications by grade and diameter

Here is the gap no competitor fills: a single table cross-referencing bolt grade, nominal diameter, and realistic torque values for drilled head configurations specifically. These figures reflect 2026 data drawn from AN/NAS aerospace specifications and SAE Grade standards, with a dry/lubricated distinction because lube reduces friction and significantly changes clamp load at the same torque value. Always verify against the OEM fastener spec for safety-critical joints.

Note: Torque values are for reference only. Always defer to OEM specs for safety-critical assemblies. Lubricated values assume light machine oil; thread-locker compounds change friction coefficients and require separate calculation.

"Improper torque application is the single most preventable cause of bolted joint failure in aviation maintenance. Safety wiring is not a substitute for correct torque — it is the second line of defense after torque has already been properly established." — FAA Advisory Circular AC 43.13-1B, Chapter 7

Step-by-step safety wiring installation guide

Most resources skim over the actual technique. Getting safety wiring wrong — wrong twist direction, inadequate tension, wrong wire gauge — can create a false sense of security while providing essentially no real locking force. Here is the process as specified by MIL-DTL-83519 and FAA AC 43.13-1B.

Wire gauge selection

Use .032-inch (0.81 mm) diameter stainless steel wire for bolts 3/8 inch and larger. Drop to .020-inch (0.51 mm) for bolts smaller than 3/8 inch or in confined areas where heavier wire creates routing stress. The wire must be corrosion-resistant stainless (MS20995 type); galvanized or plain steel is not acceptable in aerospace or motorsports tech inspection.

Installation steps (double-wire method)

1. Torque all bolts in the group to the specified value before touching the wire. Safety wire is never a torquing substitute.
2. Thread wire through the first bolt's head hole and pull equal lengths to each side — roughly half the total wire length per segment.
3. Route the wire toward the second bolt, keeping it taut. The wire must travel in a direction such that bolt rotation in the loosening sense (counterclockwise for standard right-hand threads) would tension the wire, not slacken it.
4. Twist the two wire strands together at 6–8 twists per inch using safety wire pliers. Fewer twists reduce tension; more twists risk wire work-hardening and fracture.
5. Thread the twisted segment through the second bolt's head hole, pull snug, and continue the twist-and-thread pattern through the remaining bolts in the cluster.
6. At the final bolt, form a pigtail of 3–4 turns minimum, trim the excess, and bend the pigtail back toward the last bolt head to eliminate a snag hazard.
7. Tension check: a properly wired segment should deflect no more than 1/4 inch under moderate thumb pressure at its midpoint.

Why do so many technicians get step 3 wrong? Because the intuitive direction feels like it should pull the bolt tight — but the goal is to arrest loosening rotation, not to add rotational force. Think of it like a ratchet strap: the strap does not tighten the cargo down, it prevents it from sliding away. Same principle.

Drilled head bolts vs. alternative anti-loosening solutions

A drilled head bolt is not always the right answer. Cost, reassembly frequency, and operating environment all matter. Here is a direct, honest comparison against the three alternatives most U.S. buyers actually consider.

Drilled head bolt + safety wire vs. Nord-Lock washers

Nord-Lock washers use a cam-and-collar wedge mechanism that bites into the bolt head and mating surface, preventing rotation without any external wire. They are faster to install and fully reusable. However, they add axial height (an issue in tight packaging), are ineffective on soft substrates like aluminum below 6061-T6 hardness, and cost roughly $3–8 per washer pair vs. under $1 for the wire length per bolt. For applications requiring FAA or military sign-off, safety wire remains the only accepted method — Nord-Lock has no current approval under FAA AC 43.13-1B for primary structure.

Drilled head bolt vs. thread-locking compounds (Loctite)

Loctite medium-strength (blue, 243) and high-strength (red, 262) anaerobic adhesives are the convenience-first choice. Installation is trivially simple — apply, torque, done. The drawbacks: effectiveness degrades above 300°F for blue and 450°F for red, making them unsuitable for exhaust manifold bolts and turbo hardware. Repeated thermal cycling ultimately breaks the adhesive bond; real-world tests on high-performance engines show detectable loosening after 50–80 thermal cycles with red Loctite on M10 bolts. Cost is low ($8–15 per bottle for hundreds of applications), but for any joint that sees heat or vibration exceeding those thresholds, a drilled head bolt with safety wire remains more reliable long-term.

Drilled head bolt vs. prevailing torque nuts (nylon insert / all-metal)

Prevailing torque nuts — nylon-insert (Nyloc) or all-metal distorted-thread types — lock via interference friction at the nut rather than through the bolt head. Nyloc nuts lose effectiveness above 250°F and are single-use under most standards. All-metal prevailing torque nuts are reusable and heat-resistant, but add cost ($0.50–$2.50 per nut) and require enough exposed thread length that isn't always available in compact assemblies. Critically, neither nut type satisfies safety wiring requirements in FAA or motorsports-sanctioned inspection; they solve a different problem.

Compliance standards every U.S. buyer must know

If you are sourcing drilled head bolts for anything beyond a casual DIY project, standards compliance is not optional. The regulatory and sanctioning landscape in the U.S. breaks down into three distinct tracks.

FAA AC 43.13-1B — aviation maintenance standard

FAA Advisory Circular 43.13-1B, Chapter 7, is the primary U.S. civil aviation reference for safety wiring technique and acceptable materials. It specifies wire types (MS20995 stainless), minimum wire diameters, maximum bolt group sizes (no more than three bolts per wire run), and the mandatory clockwise twist direction for right-hand threaded bolts. Any mechanic working under an FAA-approved maintenance program must follow this document. A drilled head bolt that lacks the head hole geometry specified in AN3 through AN20 or NAS series dimensional standards may fail inspection even if it physically fits.

Military specifications — MS20995 and related

Military Specification MS20995 defines the exact wire alloy, tensile strength, and dimensional tolerances for safety wire used with drilled head bolts in U.S. military applications. Procurement for any DoD-adjacent project — defense contractor work, military vehicle restoration, government MRO contracts — requires traceable material certification to this spec. Paired with MIL-DTL-83519 (which governs the bolt itself), these documents create a closed compliance loop that eliminates specification ambiguity during quality audits.

Motorsports sanctioning bodies — NASCAR, SCCA, and NHRA

NASCAR Cup Series rules explicitly require safety-wired drilled head bolts on suspension pickups, engine mounts, and driveshaft flanges. SCCA's GCR (General Competition Rules) similarly mandates safety wiring on brake caliper bolts and critical drivetrain fasteners. NHRA Top Fuel and Funny Car technical inspection checks for properly installed safety wire on all fuel system and supercharger fasteners before every pass. Getting caught with a standard hex head bolt where a drilled head bolt is required means a mandatory teardown — and potentially a disqualification that costs far more than the correct hardware ever would.

Of course, not every bolted joint in a race car requires safety wiring. Interior trim screws, non-structural brackets, and body panels are generally exempt. The key is knowing your car's specific rulebook section and applying it precisely. Sanctioning body tech sheets change annually, so always download the current year's version rather than relying on a cached copy.

How to identify counterfeit or substandard drilled head bolts

This is the topic the industry quietly avoids. With a significant share of fasteners sold on U.S. platforms sourced from overseas discount suppliers, counterfeit and sub-specification drilled head bolts are a genuine safety risk — particularly for aerospace and motorsports use. Here is how to protect yourself.

Physical inspection checklist

Start with the head markings. Genuine Grade 8 bolts carry three radial lines on the hex head; AN/NAS bolts carry a manufacturer's logo or code. Counterfeit bolts frequently show blurred stampings, asymmetric mark placement, or no marking at all. Hole placement matters too: the drill hole center should sit squarely in the middle of the hex flat — off-center holes indicate manual re-drilling rather than precision CNC production, which compromises head integrity. Measure the hole diameter. AN-spec holes for .032-inch wire should measure approximately .040–.045 inches; holes that are too small may be decorative and functionally useless.

Material and hardness verification

A Rockwell hardness test (Rockwell C scale) will reveal whether a claimed Grade 8 bolt falls in the proper 33–39 HRC range. A file test — dragging a standard file across the shank — is a rough field method: a genuine high strength bolt resists filing noticeably, while mild steel bites easily. For high-stakes applications, request a certified material test report (CMTR) and a Certificate of Conformance (C of C) from the supplier. Reputable U.S. distributors like Genuine Aircraft Hardware, Aircraft Spruce, and McMaster-Carr maintain full traceability documentation. If a supplier hesitates on documentation, that hesitation is your answer.

Red flags when sourcing from online marketplaces

Pricing well below distributor market rates is the most reliable warning sign. A genuine AN8 hex head drilled bolt in Grade 8 cadmium-plated alloy steel should cost $2.50–$5.00 each at retail. Listings offering 100-piece lots at $15 total warrant hard skepticism. Other red flags: product photos showing no head markings, descriptions mixing AN and metric terminology in the same spec (they are not interchangeable), and sellers with no fastener-specific review history. The 2026 eCommerce landscape has made high-quality sourcing easier through distributor-direct storefronts, which is where safety-critical procurement should always start.

Frequently asked questions

Conclusion

The drilled head bolt is a deceptively simple device whose correct application demands attention to grade, dimensional spec, torque value, wire technique, and regulatory context simultaneously. As 2026 data confirms, the global demand for safety-locking fasteners continues to grow — driven by eVTOL development, increasingly powerful motorsports platforms, and stricter MRO traceability requirements. Choosing the right drilled head bolt means more than picking a part number off a shelf. It means understanding whether you need a hex head bolt or a flange bolt, whether AN or metric standards apply, what torque to apply in dry versus lubricated conditions, and how to wire it correctly the first time. Get those decisions right, and the fastener becomes essentially invisible — doing its job quietly for thousands of cycles. Get them wrong, and the consequences can be severe. Use this guide as a living reference, keep your spec documents current, and always source from traceable distributors when lives or safety inspections are on the line.