When building or repairing aircraft sheet metal structures, getting every rivet hole perfectly aligned before driving a single permanent fastener is absolutely critical. A misaligned rivet hole can weaken a structure, require costly repairs, or even compromise airworthiness. This is exactly why aircraft mechanics rely on Cleco fasteners — spring-loaded, temporary sheet metal clamps that hold drilled workpieces together with consistent, repeatable clamping force during the fitting and drilling process. Understanding how Clecos work, when to use them, and how to use them correctly is a foundational skill for any airframe technician working with metallic structures.
The term "Cleco" is actually a brand name that has become the generic term used throughout the aviation industry for this entire category of temporary fastener, much like "Kleenex" has become a generic term for facial tissues. You will find Clecos referenced throughout FAA maintenance documentation and hear them discussed universally in any sheet metal shop.
How Cleco Fasteners Work
A Cleco fastener consists of a cylindrical steel body containing a spring-loaded expanding mechanism. When you squeeze the Cleco with a special pair of pliers called a Cleco pliers (also called a Cleco gun or sheet fastener pliers), the locking prongs retract into the body, allowing you to insert the Cleco through a drilled hole in aligned sheet metal panels. When you release the pliers, the spring tension expands the prongs on the back side of the sheets, clamping the layers together. The clamping force keeps the sheets firmly aligned so you can continue drilling additional holes, deburring, or preparing for riveting without the parts shifting.
The key to using Clecos effectively is the color-coding system, which indicates what size hole each fastener is designed to fit. The standard color-coding used throughout the industry is:
- Silver (chrome): fits 3/32-inch diameter holes
- Copper (brass): fits 1/8-inch diameter holes
- Black: fits 5/32-inch diameter holes
- Gold (brass or yellow): fits 3/16-inch diameter holes
- Red: fits 1/4-inch diameter holes
The 1/8-inch copper Cleco is by far the most commonly used size in aircraft sheet metal work, since 1/8-inch rivets are extremely common in light aircraft structures. A well-stocked aircraft sheet metal shop will have dozens or even hundreds of Clecos in each common size, since best practice is to Cleco every drilled hole (or at minimum every other hole) before proceeding.
The Temporary Fastening Sequence
Proper use of Clecos follows a logical, methodical process that ensures parts remain properly aligned throughout the entire drilling and fitting operation. The general sequence works as follows:
- Locate and clamp the parts initially. Before any drilling begins, position the sheets or structural members in their correct relationship and hold them with Cleco-compatible clamps, vise grips, or hand pressure. Ensure the parts are flush and aligned before making the first hole.
- Drill the first hole and immediately install a Cleco. Using the appropriate drill bit for the rivet size specified in the repair or assembly drawing, drill your first hole — typically at one end of the row — and immediately insert a Cleco. This anchors that end of the work.
- Drill the far end hole and install another Cleco. By anchoring both ends early, you prevent the sheets from rotating or shifting as you work across the row.
- Drill remaining holes and Cleco as you go. Work progressively from the anchored ends toward the center, installing a Cleco in each hole immediately after drilling. Never drill several holes at once before inserting Clecos — the sheets can shift and later holes will be misaligned.
- Inspect fit-up before proceeding. With all Clecos installed, inspect the assembly for flush fit, proper edge margins, and correct hole spacing before any deburring or countersinking is done.
Once you are satisfied with the fit, you remove Clecos one at a time — replacing each with the permanent rivet — so the assembly stays registered throughout the riveting process.
Why Temporary Fastening Matters
The consequences of skipping or improperly using temporary fasteners can be severe. Sheet metal, even when clamped by hand, can shift by a fraction of an inch during drilling due to drill bit walking, chip pressure, or simple movement. Even a small shift multiplied across a row of 20 or 30 rivets can result in holes that are out of alignment, violating required edge distance or spacing. According to AC 43.13-1B guidance, minimum edge distance for a rivet is 2D (twice the rivet diameter) from the center of the hole to the edge of the sheet, with 2.5D generally preferred/optimum, and minimum spacing between rivets is 3D. Holes drilled too close to an edge or too close together compromise the bearing strength of the joint and may require the entire part to be replaced.
Beyond hole spacing, misaligned holes create a condition called hole misalignment or "out-of-round" holes, where the drill walks between layers and produces an oblong or offset hole that cannot be properly filled by the intended rivet. A rivet driven into an oversized or misaligned hole will not develop its designed shear and bearing strength, which directly affects the structural integrity of the aircraft.
Cleco fasteners also prevent the problematic condition of oil-canning — the tendency of unsupported thin sheet metal to buckle or distort away from the mating surface under the pressure of a drill bit. The consistent clamping force of evenly spaced Clecos keeps both sheets tight against each other, ensuring clean, true holes through both layers simultaneously.
Other Temporary Fastening Techniques
While Cleco fasteners are the most commonly used temporary fastening device in aircraft sheet metal work, a trained airframe technician should also be familiar with other temporary holding methods:
- Sheet metal screws: Temporary screws can be used in some situations to align and hold structural members when Cleco-sized holes are not yet drilled or when more clamping force is required than Clecos provide.
- C-clamps and edge clamps: Used to hold parts in alignment before the first holes are drilled, or to supplement Clecos on flanges and edges where Cleco placement is impractical.
- Vise-grip pliers: Useful on thicker material or in tight spaces; must be padded or used with care to avoid marring finished surfaces.
- Pins and locating fixtures: On production or repetitive repairs, tooling fixtures with locating pins are used to establish and maintain precise part relationship before drilling begins.
Key Numbers and Rules
- Install a Cleco in every drilled hole (or at minimum every other hole) before proceeding to the next — do not drill the entire row before Clecoing.
- Color-code reminder: Silver = 3/32", Copper = 1/8", Black = 5/32", Gold = 3/16", Red = 1/4".
- Minimum edge distance: 2D minimum from hole center to sheet edge, 2.5D preferred (where D = rivet diameter).
- Minimum rivet spacing: 3D center-to-center between adjacent rivets.
- Rivet replacement sequence: Remove one Cleco, drive one rivet, then move to the next — never remove all Clecos before riveting begins.
- Cleco pliers are required to install and remove Clecos — never try to force a Cleco in or out by hand or with substitute tools, as this can damage the locking mechanism.
Memory Aid
For the Cleco color-code sequence, many experienced sheet metal technicians use the phrase: "Some Copper-colored Bears Go Running" — Silver (3/32), Copper (1/8), Black (5/32), Gold (3/16), Red (1/4). Say it once out loud and the size-to-color relationship tends to stick, which is particularly useful when working in a busy shop and grabbing Clecos quickly from a storage board.
Common Test Traps
- Confusing Cleco colors with rivet sizes: FAA knowledge test questions may describe a specific hole diameter and ask which Cleco to use. Remember: copper = 1/8 inch, the most common; do not guess that "gold" means the largest or most common.
- Reversing the drilling sequence: A question may ask about the correct order of operations. Drilling all holes first and then Clecoing is wrong — Clecos must follow immediately after each hole is drilled to maintain alignment.
- Forgetting edge distance applies to temporary setups too: The minimum edge distance rule applies to where you locate and drill the holes, not just to finished rivets. Improperly located pilot holes are just as problematic as final holes drilled off-spec.
- Assuming Clecos provide structural strength: Clecos are temporary alignment devices only — they provide no permanent structural strength and must be replaced with approved fasteners before the aircraft is returned to service.
- Using the wrong Cleco size: Inserting a Cleco that is too small for the hole will not provide adequate clamping force and the sheets may shift; too large a Cleco can enlarge or deform the hole. Always match Cleco color (size) to the drilled hole diameter exactly.