Drilled vs Undrilled Bolts: Castle Nuts, Cotter Pins, and Safety Wire

Every structural bolt installation on a certificated or experimental aircraft requires a positive locking method — something that prevents the nut from backing off under vibration and thermal cycling. The design drawing or repair manual tells you which method: castle nut with cotter pin, safety wire through a drilled head, or a self-locking nut. These are not interchangeable. The method is part of the design intent, and selecting the wrong hardware is an engineering deviation, not a substitution. This guide covers the three primary locking methods, the hardware required for each, and the installation technique that actually makes them work.

The Decision Is Made by the Design

When a drawing calls for a drilled shank bolt with castle nut and cotter pin, it does so because the designer wanted positive mechanical retention that is inspectable on every maintenance cycle. When it calls for a self-locking nut, the designer accepted the prevailing torque of the insert as adequate locking for that joint under its expected loads and vibration environment. Substituting one for the other changes the design — it requires engineering review, not just a trip to the parts bin.

Know the part number suffix before ordering. For AN bolts: the "H" suffix designates a drilled shank bolt (AN4H-7 = 1/4-28 UNF, 7/16" grip, drilled shank). No suffix, no "H" = undrilled shank (AN4-7 = same dimensions, no hole). Drilled head bolts have two small holes through the hex head faces for safety wire — these are identified in the part number or description, and are visually obvious when you're holding the hardware.

Locking Methods Reference

The fiber insert in AN365/MS20365 self-locking nuts is temperature-limited — do not use fiber-insert locknuts in exhaust-adjacent applications above approximately 250°F. Use all-metal AN363 or MS20363 locknuts where temperatures exceed the fiber insert limit, or use drilled-bolt castle nut installations where positive mechanical retention is preferred regardless of temperature.

Castle Nuts and Cotter Pins — The Torque-Then-Advance Rule

AC 43.13-1B Section 7-139 is the governing reference. The rule is clear and absolute: torque to the low end of the specified range first, then advance (never back off) to the nearest cotter pin slot that aligns with the bolt hole.

• If no cotter pin slot aligns within the specified torque range, the problem is grip length. The bolt is either too long (excess thread protruding past the nut, slots don't align until past max torque) or the washer stack is wrong. Replace the hardware with the correct grip length bolt — do not torque beyond the maximum or back off below the minimum.
• AN310 castle nut size selection: AN310-4 mates with 1/4-28 (AN4 bolt). AN310-5 mates with 5/16-24 (AN5). AN310-6 mates with 3/8-24 (AN6). AN310-8 mates with 1/2-20 (AN8). The castle nut number suffix is the AN bolt size it matches.
• Cotter pin size: MS24665 series. Use the largest diameter cotter pin that fits the hole without forcing — oversized pins stress the hole and make removal difficult; undersized pins can back out. Standard aircraft practice for AN4 (1/4-28) bolt: MS24665-42 (3/32" diameter).
• Cotter pin installation: Insert through slot and hole. Bend one leg over the end of the bolt, flat against the threaded end. Bend the other leg down against a flat of the nut. Do not bend the legs more than 90 degrees from the installed position — repeated bending to 180 degrees fatigues the pin material and creates a fracture risk. Cut legs to length if they extend more than 1/2" beyond the bend — protruding cotter pin legs are a laceration hazard and can foul adjacent mechanisms.

Safety Wire Technique — Drilled Head Bolts

Safety wire through drilled bolt heads provides positive mechanical retention in a direction that requires the bolt to loosen before the wire can go slack. The wire is installed in tension such that any tendency of the bolt to loosen tightens the wire further. When wire breaks or goes slack, the bolt can't be assumed to be at torque — that's the sign-and-symptom maintenance crews look for on walk-around inspection.

• Wire specification: MS20995C stainless safety wire, 0.032" diameter for most aircraft fasteners up to 5/8" bolt size. Use 0.041" diameter for large fittings above 5/8" and high-vibration zones where the heavier wire resists fatigue-induced breakage better.
• Wire direction: The fundamental rule — the wire must be installed such that if the bolt starts to loosen (rotate counterclockwise looking at the head), the wire goes into tension. Visualize the rotation direction of loosening, then run the wire so that rotation pulls against the wire anchor.
• Double-twist (pliers) method: Thread the wire through the first bolt hole. Run to the second bolt, thread through its hole. Grasp both wires with safety wire pliers and twist at 6–8 twists per inch throughout the run. Cut the wire end leaving approximately 3/8" to 1/2" of pigtail. Bend the pigtail back away from personnel handling paths — a straight pigtail end is a puncture hazard.
• Single-wire method: Used where double wire won't fit. Single wire threads through the hole, wraps around the bolt head, and anchors to structure or an adjacent fitting. Less preferred than double-wire for vibration resistance.
• Maximum bolts per wire run: AC 43.13-1B limits a single safety wire run to three fasteners maximum. Longer runs mean the wire may go slack at intermediate bolts before the end bolt is adequately tensioned.
• Pigtail inspection: The pigtail end should be folded back at 90 degrees, not left pointing outward. On annual inspection, check that wire is tight with no broken strands, no cuts from adjacent structure, and no green corrosion indicating moisture intrusion.

When Drilled Head vs Drilled Shank

The two drilled configurations serve different installation geometries and different retention philosophies:

• Drilled shank bolt (H suffix): The castle nut is the primary locking element. The bolt does not rotate during installation — torque is applied to the nut. The drilled shank is accessed from the nut end after the castle nut is installed. Used when nut accessibility is straightforward and positive mechanical retention (inspectable cotter pin) is desired.
• Drilled head bolt: Safety wire through the head is the primary retention. Used where nut access is limited (blind nuts, platenuts, recessed locations) or where the application environment (engine baffles, exhaust attach, high-vibration control linkage hard points) makes castle nut installation impractical. The bolt head is accessible for safety wire inspection even when the nut end is buried.
• Both drilled: Some high-vibration primary structural applications specify bolts with both drilled heads and drilled shanks — the castle nut provides primary retention and the safety wire through the head provides secondary retention. This redundant locking is called out explicitly in maintenance manuals for critical installations.
• Never install an undrilled bolt in a location that calls for a drilled configuration. The locking method is specified because someone determined it was the correct locking method for that load, vibration, and temperature environment. "It's the same bolt" is not an acceptable rationale for omitting the retention feature.

Identifying Drilled vs Undrilled in the Field

When you have hardware in hand and need to determine what you're looking at:

• Part number: AN4H-7 = drilled shank (H suffix). AN4-7 = undrilled. AN4-7A = undrilled, aluminum. If the "H" suffix is not present, the shank is not drilled.
• Visual check for drilled shank: Look at the unthreaded portion of the bolt shank near the thread runout. A cross-drilled shank has a visible hole through the shank diameter, typically 0.062"–0.094" diameter depending on bolt size. This hole is in the grip region, not through the threads.
• Visual check for drilled head: Look at the hex head faces. Two small holes through opposite faces (or at 60 degrees apart on some NAS configurations) identify a drilled head bolt. The holes are small — 0.040"–0.063" — and are easy to miss if you're not looking for them.
• Box label: Reputable hardware packaging will state "drilled" or "cross-drilled shank" in the description field. The part number suffix is definitive — if the label and the part number conflict, trust the part number and inspect the hardware.