Fasteners and adhesives

01 — Wood screws, machine screws, and self-tapping screws: three different jobs

A wood screw's tapered shank pulls itself deeper into wood fibers as it turns, creating increasing grip the further it goes. A machine screw has a uniform diameter built for a hole that's already threaded, or for use with a nut. A self-tapping screw is built to create its own threads in a material that started with none.

The wood screw's tapered shape is the entire point: as the screw advances, the increasing diameter compresses surrounding wood fibers against the threads, which is what generates the screw's holding power. Drive a wood screw into the wrong material, or drive a machine screw straight into bare wood expecting it to grip the way a wood screw would, and the mismatch between the fastener's design and the material's behavior is exactly what causes splitting, stripping, or a fastener that simply pulls out under load.

• Self-tapping screws split into two genuinely different mechanisms, worth distinguishing. "Thread-forming" self-tapping screws (common in softer materials like plastic or thin sheet metal) displace material outward to create their own threads without removing anything, similar to how a wood screw's point works in wood. "Thread-cutting" self-tapping screws have actual flutes machined into the threads near the tip, similar to a metalworking tap, that physically remove material to cut threads, which is what lets them work in harder substrates like steel.
• A genuinely useful practical fact: a self-tapping machine screw only cuts new threads on its first installation. If you remove and reinsert the same screw into the same hole, it follows the threads it already cut rather than cutting fresh ones, which is worth knowing if you're reusing a hole rather than starting fresh.
• Using the wrong screw type is a common, avoidable cause of failed repairs. A wood screw forced into metal will not cut clean threads and will likely strip. A machine screw driven into untapped wood has nothing to grip and will simply spin or pull free under any real load.

02 — Finishing nails versus common nails: head design follows purpose

A common nail's wide, flat head exists specifically to resist pulling through the material it's driven into, which matters enormously for hidden structural work where strength is everything and appearance is irrelevant. A finishing nail's small, barrel-shaped head exists to be driven below the surface, filled, and hidden entirely, which is exactly what makes it the right choice anywhere the nail itself needs to disappear.

The head size tradeoff is direct: a larger head distributes force across more surface area and resists pulling through the material under stress, which is genuinely valuable in framing and other structural connections hidden behind drywall or siding. A finishing nail trades away some of that pull-through resistance for a head small enough to be set below the wood surface with a nail set and concealed with wood filler, the right tradeoff for trim, baseboards, and any surface meant to be seen.

• Use common nails for structural framing and anywhere the connection is hidden and strength matters most. Use finishing nails for trim, molding, and any visible surface where a flush, concealable nail head matters more than maximum pull-through resistance.
• Ring-shank nails add a different kind of holding power: ridges along the shank itself that resist the nail backing out over time, particularly valuable for decking and subflooring, surfaces that experience repeated flexing and moisture cycling that can gradually work a smooth-shank nail loose.

03 — Bolt sizing: where metric and SAE bolts look nearly identical and quietly aren't

Three measurements determine a correct replacement bolt: diameter, thread pitch, and length, plus knowing whether the bolt is metric or SAE (imperial), since these two systems are never truly interchangeable even when a pair happens to look extremely close. Several specific metric-and-SAE pairs are close enough in diameter to be genuinely dangerous, since a wrong bolt can start threading and feel correct for several turns before binding and damaging both the bolt and the hole.

Metric bolts specify pitch as the actual millimeter distance between adjacent thread crests (an M8 x 1.25 bolt has threads spaced 1.25mm apart). SAE bolts specify pitch as threads per inch, or TPI (a 5/16"-18 bolt has 18 threads in one inch of length). These are fundamentally different ways of describing thread spacing, and a bolt from one system forced into a hole tapped for the other will not seat correctly, regardless of how close the diameters happen to be.

• The most deceptive lookalike pairs, worth knowing by name since they cause most of the cross-threading damage that happens in home shops: M8 (8.00mm) versus 5/16" SAE (7.94mm), a difference of only 0.06mm, thinner than a sheet of paper, yet their pitches (1.25mm versus 18 TPI) are entirely incompatible. M10 (with a 1.5mm pitch) versus 3/8" SAE is similarly close, with a pitch difference small enough that a mismatched bolt can thread in two to three full turns before binding, by which point real thread damage has typically already occurred.
• A correct-size bolt threads in smoothly with finger pressure alone. If a bolt requires force, a wrench, or noticeable resistance to start threading, it is very likely the wrong size or the wrong system entirely; never force a bolt that doesn't want to go in by hand.
• Coarse threads (fewer threads per inch or a larger millimeter pitch) install faster and tolerate dirt, debris, and minor cross-threading attempts better, making them the default for most general repair work. Fine threads pack more threads into the same length, offering greater clamping force and meaningfully better resistance to loosening under vibration, which is why they show up in automotive and precision applications where vibration resistance genuinely matters.
• Bring the old bolt to the hardware store for direct comparison, or use a thread gauge or thread checker tool, rather than trusting a visual estimate or a single test turn, given how close some of these lookalike pairs genuinely are.

04 — Matching the adhesive to the materials, not the other way around

No single adhesive works well across every material and every job; each of the four common household adhesives is built around a different bonding mechanism suited to specific material combinations and stress conditions.

• Wood glue (PVA) creates a bond that, on a clean wood-to-wood joint, is genuinely stronger than the wood itself, meaning the surrounding wood will typically fail before a properly glued joint does. Its real limitation is scope: it works specifically on porous wood-to-wood joints and performs poorly on non-porous or dissimilar materials.
• Two-part epoxy bonds metal, ceramic, glass, and dissimilar material combinations that wood glue cannot handle, since epoxy cures through a chemical reaction between its two components rather than relying on absorption into a porous surface the way wood glue does.
• Construction adhesive (commercial products like Liquid Nails) is built to fill gaps and bond heavier materials with real structural demands, the kind of job where the surfaces being joined may not fit together perfectly cleanly.
• Cyanoacrylate (superglue) bonds clean, close-fitting surfaces almost instantly, which makes it excellent for small parts and precise repairs, but genuinely unsuited to gap-filling or heavy structural loads, since its strength depends on a very thin, tight bond line rather than bulk material between the surfaces.
• Choose based on both the materials being joined and the actual strength the joint needs to bear, rather than defaulting to whichever adhesive happens to be on hand; the wrong adhesive for the material combination frequently fails well before the repair was actually stressed to its limit.