Evidence grade: B1 — Trade practice Effort: Intermediate · 30 min · drill, small bit, center punch, deburring tool, eye protection
Drill a small hole at the very tip of the crack. The hole replaces the crack's sharp tip with a blunt round edge, which kills the stress concentration that drives the crack forward — and buys you time. This is a delaying tactic, not a repair. It restores zero strength.
This move is called a stop-drill, and it is standard practice in aircraft sheet-metal work: stop-drill the crack first, then plan the real repair (AvWeb/Aviation Consumer, source 1).
Why it works
A crack grows because stress concentrates at its razor-sharp tip. Every load cycle — vibration, flexing, thermal expansion — focuses force on that one point, and the crack creeps forward one microscopic step at a time.
A drilled hole interrupts this. The round edge of the hole distributes stress around a curve instead of focusing it on a point. If the peak stress at the hole drops below the metal's threshold for crack growth, the crack stops. If it doesn't — if the part is still being overloaded — the crack will start a new tip from the hole's edge and keep going.
That is the honest physics behind the technique: a stop-drill lowers the local stress; it does not fix whatever caused the crack.
Where it applies
Stop-drilling is appropriate for sheet metal and non-structural panels: fairings, cowlings, covers, brackets, drip trays, and similar thin components. Aviation technicians use it routinely on aluminum airframe skins, and also on fiberglass, molded plastic, and acrylic panels, which can be stop-drilled and patched in a similar way (source 1).
Cracks love to start at creases, sharp bends, dents, and anywhere a thin panel flexes repeatedly — the "oil-canning" phenomenon, where a dented panel flexes in and out until a corner radius gives up (source 1).
The procedure
- Find the true tip of the crack. Clean the area. The visible end of the crack is not always the real end — use magnification and raking light. If you have dye penetrant available, use it.
- Center-punch the exact tip. The hole must be past the crack's end. A hole drilled short of the tip leaves the sharp tip intact and the crack will keep running straight through your hole.
- Drill one small hole. For thin sheet metal, a small bit is enough — aviation guidance on acrylic windshields specifies a #30 drill (1/8 inch), which suggests the scale involved (FAA AC 43.13-1B, Par 3-40 — source 2, opened 2026-09-30). Drill perpendicular, from one side, in a single pass.
- Deburr both sides. A ragged hole edge is itself a stress riser. Remove burrs so the hole edge is smooth.
- Record it. Note the exact location and length of the crack, photograph it, and re-inspect after the next hours of use, then periodically (aviation practice: check after 10–20 hours, then every 100 hours — source 1). If the crack grows past the hole, the stop-drill failed and the real repair is overdue.
Then the real repair follows
A stop-drilled crack that is small and in a low-vibration area may stay stable for a long time — but not always (source 1). The aviation rule of thumb: if the crack was more than about 2 inches long to begin with, or it keeps growing after stop-drilling, it needs a proper patch (source 1). A sound patch is at least twice as long as the stop-drilled crack, carries multiple fasteners per side, and is made from the same alloy and gauge as the parent metal (source 1).
The discipline is: stop-drill to arrest, patch (or replace) to repair. In that order.
⚠️ Stop rules
- Load-bearing structure — STOP. Cracks in frame rails, suspension arms, steering components, pressure vessels, brake parts, or anything where failure means injury: do not drill, do not patch, do not "stabilize." Mark the crack, photograph it with a scale reference, and hand the part to a professional for inspection. Drilling a hole in a loaded member removes material from the exact place that needs it most.
- Can't find the tip — STOP. A hole drilled short of the crack tip is worse than no hole: it weakens the part without arresting the crack.
- Crack keeps growing after stop-drilling — STOP drilling more holes. Escalate to patch or replacement.
- Never treat a stop-drill as a repair. It adds no strength and removes no damage. Anyone relying on it as a fix is one vibration cycle away from the crack resuming.
PPE and handling
- Eye protection is mandatory when drilling metal — chips are sharp and fast.
- Deburr every hole edge; sharp drilled edges cut gloves and hands and act as new stress risers.
- If drilling near fuel lines, wiring, or hydraulics, stop and reassess — know what is behind the panel before the bit goes through.
Honest limits
- Stop-drilling does nothing about the cause of the crack. If the part is overloaded, misaligned, or fatigued, the crack will restart — from the hole, from a neighboring crease, or somewhere new.
- The "2 inches → patch" threshold comes from aircraft sheet-metal practice, not universal engineering law. Thicker, stiffer, or hotter components follow different rules.
- It is a technique for thin, ductile materials. It is not appropriate for castings, glass, ceramics, or anything brittle — drilling those typically extends the crack.
- One hole per crack tip. Drilling a row of holes "just in case" turns the panel into perforation and invites it to tear.
Sources
- "Airframe Cracking? Drill It, Patch It Right," AvWeb / Aviation Consumer, July 2022. https://avweb.com/features/airframe-cracking-drill-it-patch-it-right/ — stop-drill as first remedy; monitor location/length with photos; patch when crack exceeds ~2 inches or grows after stop-drilling; same technique applied to plastics/Plexiglas.
- FAA Advisory Circular AC 43.13-1B, Par 3-40 (opened and read 2026-09-30 via full-text mirror: https://m.rexresearch1.com/FlightMechanicsLibrary/AircraftInspectionRepairAlterationsAcceptableMethod.pdf) — acrylic windshield crack repair: "stopdrilling the ends of the crack with a # 30 or a 1/8-inch drill" to prevent stress concentration continuing the crack.
Repair Lab · Release 1 · Evidence last checked September 29, 2026 · A project of AOJEL