5.2 Sheet-Metal Repairs, Fasteners, and Riveting Judgment

Reading the Rivet Code

Solid rivets are identified by an AN/MS part number that encodes head style, material, diameter, and length. The first number is the head: AN470 is the universal head (the modern general-purpose head that replaced roundhead AN430 and brazier AN455), and AN426 (or MS20426) is the 100° countersunk (flush) head used where aerodynamic smoothness matters. A letter gives the material/alloy:

In a part number like AN470AD3-5: 470 = universal head, AD = 2117-T4, 3 = 3/32-inch diameter (diameter is in 1/32-inch steps), and 5 = 5/16-inch length (length is in 1/16-inch steps). The DD (2024-T4) ice-box rivet is too hard to drive as-received; it is solution heat-treated and stored in a freezer to delay age-hardening, then driven within its allowed time.

Rivet Sizing Math

Three rules from AC 43.13-1B drive almost every rivet question:

• Diameter: the rivet diameter should be about 3 times the thickness of the thicker sheet being joined. Too small a rivet shears; too large a hole weakens the sheet.
• Length: rivet length = total grip (combined sheet thickness) + 1.5D. That extra 1.5 diameters of shank forms the shop head.
• Shop head: a properly driven shop (bucked) head is about 1.5D wide and 0.5D high (1/2 the rivet diameter tall). Too flat = over-driven/thin; too tall = under-driven.

Worked example: joining two sheets each 0.040 in (total grip 0.080 in). Diameter ≈ 3 × 0.040 = 0.120 in, so a 1/8-in (4/32) rivet (AN_AD4) fits. Length ≈ grip + 1.5D = 0.080 + 1.5(0.125) = 0.080 + 0.188 = 0.268 in, round up to the next available length (about 1/4 in plus, i.e. a -4 or -5 length). Always pick the next-longer standard length and let the bucking bar form the head.

Layout: Edge Distance, Pitch, and Transverse Pitch

Rivet placement protects the sheet from tearing out and the joint from buckling:

Measure edge distance to the center of the hole. For a multi-row repair the rows are typically staggered so the rivet load is spread and the sheet is not weakened along one line. A repair patch must restore the original strength — that usually means matching or exceeding the original number of rivets carried across the break, using the same alloy/temper and gauge (or a one-gauge-thicker doubler if the data allows).

Driving Quality and Knowing the Limit

A correct layout still fails if the work is sloppy. Drill holes perpendicular and to the right size (a #30 drill for a 1/8 rivet, #40 for 3/32). Deburr both sides — burrs hold sheets apart and start cracks. Countersink only when the sheet is thick enough (otherwise use dimpling, since a knife-edge countersink in thin sheet is weaker than the rivet head).

When driving, support the work and keep the gun and bucking bar aligned — a tipped, clinched, or cracked shop head, or a smoking/loose rivet later, means drill it out (drill the manufactured head, not the shank) and replace, often one size larger if the hole is enlarged.

Finally, maintenance judgment includes knowing the boundary of standard practices. AC 43.13-1B is acceptable data for a generic repair only if no manufacturer SRM data applies and the repair is truly minor and standard. Negligible-damage limits, control-surface repairs, pressurized-skin repairs, and anything affecting balance, flutter, or primary load paths must follow the manufacturer's SRM or FAA-approved engineering data (DER 8110-3). The exam favors the answer that uses approved data, preserves strength and corrosion protection, and stops short of guessing when the repair leaves standard limits.

Special Fasteners and Hole Quality

Beyond solid rivets, the Airframe test covers blind rivets and special fasteners used where you cannot reach both sides or where higher strength is needed. Blind (pop) rivets such as friction-lock and mechanical-lock Cherry types are set from one side and are not permitted in many primary structural or pressurized locations unless the data specifically allows them.

Hi-Loks, lockbolts (Huck), and Jo-bolts carry higher loads than solid rivets and are installed to specified torque or pin break-off; camloc/dzus fasteners are quarter-turn panel fasteners, not structural. Knowing which fastener the data calls out — and not substituting a weaker blind rivet for a structural solid rivet — is a common judgment question.

Hole quality is as important as fastener choice. The hole must be drilled perpendicular with the correct drill for the rivet (a #40 drill for a 3/32 rivet, #30 for 1/8, #21 for 5/32), then deburred on both faces. An oversized, out-of-round, or tipped hole loses clamp-up and bearing area; if a hole is enlarged during rework the repair moves up to the next rivet size so the new rivet fills it.

Countersinking is limited by sheet thickness: when the sheet is thinner than the rivet head depth, you must dimple instead, because a knife-edge countersink leaves almost no material to carry the head. Edge distance, pitch, and transverse pitch are all measured to hole centers, and on a doubler the new rivet pattern must reproduce the strength the original skin carried across the damage.

These layout and hole-quality rules are where most sheet-metal points are won or lost, because a textbook-correct rivet driven into a poor hole is still an unairworthy repair.