Driven Rivet Inspection and Rejection Criteria

Rivets are the workhorses of aluminum aircraft structures. Thousands of them hold together skins, frames, stringers, and bulkheads in a typical light aircraft, and the integrity of every one matters. A rivet that was driven incorrectly — or that has loosened or cracked over time — can compromise the load path in a primary structure, sometimes with catastrophic results. That is why the FAA and aircraft manufacturers have established precise inspection criteria and equally precise rejection standards that every Aviation Maintenance Technician (AMT) must understand and apply.

This article walks through the mechanics of how a rivet is properly driven, what a good rivet looks like when finished, and — most importantly — how to identify every common defect that makes a rivet unacceptable. The standards covered here are grounded in the FAA's Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31) and AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, the primary reference document for field maintenance practice.

How a Rivet Is Driven: The Baseline for Inspection

Before you can judge a bad rivet, you have to understand what a correctly driven rivet looks like. A solid shank rivet is inserted into a drilled and deburred hole so that the manufactured head sits flush against (or countersunk into) one sheet and an appropriate length of shank protrudes through the far side. The bucking bar is held against the protruding shank while a pneumatic rivet gun drives the head side, causing the shank to expand radially to fill the hole and the protruding end to be upset into a smooth, dome-shaped shop head (also called a bucked head or driven head).

The shop head dimensions are the first things an inspector examines. AC 43.13-1B provides the accepted standard: the shop head diameter should be approximately 1.5 times the nominal shank diameter, and the shop head height should be approximately 0.5 times the nominal shank diameter, with allowable tolerance bands around these proportions depending on rivet size. For example, a 3/32-inch (AN470) rivet should produce a shop head roughly 9/64 inch in diameter and about 3/64 inch tall. These numbers are the benchmark against which every driven rivet is evaluated.

Tools of the Trade: The Rivet Gauge

A rivet gauge (sometimes called a rivet inspection gauge or shop head gauge) is a small go/no-go tool machined with slots and steps that correspond to the correct height and diameter ranges for standard rivet sizes. The technician places the gauge alongside and over the shop head to verify it falls within tolerance — wide enough, tall enough, and not excessively tall or wide. Visual inspection alone is insufficient for borderline cases; the gauge removes subjectivity. Always use the gauge specified for the rivet diameter you are checking.

Inspection Criteria: What to Look For

A driven rivet inspection covers three areas: the shop head, the manufactured head, and the surrounding skin or structure. Each area has its own set of rejection criteria.

Shop Head Defects

Undersized shop head (rivet not set): If the shop head diameter is less than 1.5 times the shank diameter or the height is less than 0.5 times the shank diameter, the rivet has not been driven enough. The shank may not fully fill the hole, meaning the rivet provides inadequate clamping force and shear strength. This rivet must be removed and replaced.
Oversized or barreled shop head: Excessive driving squashes the shop head too flat and too wide. The shank may have over-expanded, possibly cracking the surrounding material. Reject if the shop head diameter exceeds 1.5 times the shank diameter by more than the allowable tolerance, or if the head shows cracking or splitting.
Off-center (eccentric) shop head: The shop head should be centered on the shank axis. If the bucking bar slipped or was held at an angle, the head will be egg-shaped or offset to one side. Inspectors use a rivet gauge along with visual assessment to judge eccentricity, since AC 43.13-1B does not publish a single numeric offset tolerance. An eccentric shop head means the shank did not upset evenly, and the rivet likely has a bent shank inside the hole — a reject.
Tilted shop head: The shop head should be perpendicular to the material surface. A shop head tilted more than a few degrees indicates the bucking bar was angled, producing uneven shank fill. This is a reject condition.
Cracked or split shop head: Visible cracks anywhere on the shop head indicate the material is work-hardened or defective. Reject immediately.
Smiley face (smile): One side of the shop head is cut into or undercut, forming a crescent-shaped depression. This happens when the bucking bar edge catches the shank. The undercut side has much less material than required — reject.

Manufactured Head Defects

Loose manufactured head: The head should be tightly seated against the skin with no rocking or movement. A loose rivet — one that can be felt to rock when pressed — has lost clamping force and must be replaced.
Damaged manufactured head: Gun marks, chipped edges, or digs on the manufactured head from a misaligned rivet set can weaken the head or allow corrosion entry. Minor cosmetic marks within manufacturer tolerances may be acceptable; significant deformation or cracks are not.
Countersunk rivet not flush: On flush (countersunk) rivets, the head must be flush with the skin surface or within a very small tolerance (typically not protruding more than a few thousandths of an inch and not recessed more than a few thousandths). A head that stands proud can disrupt airflow or catch on other structures; one that is set too deep has an under-driven shank.

Surrounding Structure Defects

Skin dimpling or pillowing: The skin between rivets should lie flat. Bulging or pillowing between rivet rows indicates the rivets are loose or the structure has been overstressed.
Cracked skin around the hole: Driven cracks radiating from the rivet hole indicate the hole was undersized, or the material was not properly supported during driving. This is a structural defect requiring repair beyond simple rivet replacement.
Corrosion at the rivet: White powdery deposits (aluminum oxide) around the manufactured head or seeping from beneath the head indicate galvanic or crevice corrosion. The rivet and surrounding material must be evaluated for continued airworthiness.

Why It Matters: Structural Load Paths

Aircraft structures are designed so that loads are shared among many fasteners. When even a small number of rivets in a row are defective, the remaining sound rivets must carry proportionally higher loads. This progressive overload can cause additional rivets to fail in a cascade — a phenomenon well-documented in fatigue analysis. Primary structure such as wing spars, fuselage frames, and control surface attachment brackets are especially critical: a single rejected rivet left in place is not merely a paperwork violation, it is a structural risk.

The FAA's AC 43.13-1B establishes repair standards partly because improperly driven rivets in repairs are a leading source of structural weakness. Technicians who skip the gauge step or accept borderline shop heads are not saving time — they are compressing the safety margin the designer built in.

Key Numbers and Rules

Shop head diameter: approximately 1.5 × nominal shank diameter.
Shop head height: approximately 0.5 × nominal shank diameter.
Edge distance: AC 43.13-1B specifies a minimum edge distance of about 2 times the rivet shank diameter, with 2.5 times the shank diameter preferred as a more conservative standard; closer spacing risks edge tear-out.
Rivet pitch (spacing): Typically about 4× to 6× shank diameter between rivet centers along a row.
Hole size: The drilled hole should be approximately 0.002 to 0.004 inch larger than the rivet shank, per the standard oversize drill number for the rivet diameter, for a proper slip fit before driving.
Material clamping: No gap should exist under the manufactured head after driving; the head should be tightly and uniformly seated against the skin surface with no perceptible rock when checked by hand.
Replacement rule: When a rivet is removed and replaced, the replacement rivet must be the same type and alloy or a stronger approved substitute; the next larger diameter rivet may be used if the hole has been enlarged, but hole enlargement must not violate edge distance or pitch rules.

Memory Aid: CLOTS

CLOTS is a practical checklist some technicians use when inspecting shop heads:

C — Centered: Is the shop head centered on the shank?
L — Level (perpendicular): Is the head perpendicular to the skin surface?
O — On size: Does the shop head meet the 1.5D diameter and 0.5D height requirements?
T — Tight: Is the manufactured head tight against the structure with no gap or rock?
S — Smooth: Is the shop head free of cracks, splits, smiles, and undercutting?

Work through CLOTS for every rivet you inspect, and you will catch all the primary rejection criteria before signing off a repair.

Common Test Traps

Confusing the 1.5 and 0.5 ratios: Test questions frequently swap the diameter and height ratios. Remember: the shop head is wider (1.5×) than it is tall (0.5×), just like a flattened dome.
Assuming cosmetic damage is automatically rejectable: Minor gun marks on the manufactured head that do not affect structural integrity may be within tolerance. The test distinguishes between cosmetic and structural defects — read questions carefully.
Forgetting edge distance when upsizing: If a hole is drilled oversize and a larger rivet is substituted, the new edge distance and pitch must still be checked. Simply using a larger rivet does not automatically fix the repair.
Thinking a loose rivet can be re-driven: A rivet already driven cannot be re-driven in place. It must be drilled out and replaced. Attempting to re-drive causes additional material damage and is not an approved technique.
Overlooking the surrounding skin: Inspection questions may describe a structurally sound shop head but cracked or pillowed skin. The rivet itself may be within tolerance, but the skin condition still constitutes a reject or repair situation. Always inspect the full joint, not just the fastener.