Nuts & washers

The size is determined by the screw diameter (e.g., M6) and the thread pitch (usually standard). The easiest way: Measure the screw or check the marking. The washer must match the screw diameter on the inside and be large enough on the outside to distribute the load. For soft materials (fiberglass, wood, plastic), larger washers or backing plates are often recommended.

A2 is sufficient for many applications in dry indoor environments. For marine use, splash water, salty air, and permanently damp environments, A4 is usually the better choice because it is more corrosion-resistant. Important: Even stainless steel can suffer in unfavorable conditions (crevice corrosion)—clean installation and appropriate material combinations help.

Washers distribute the load and protect surfaces. Large washers (e.g., body washers) are ideal for soft materials or slotted holes. Lock washers/spring washers can help prevent loosening, but are not optimal for every application. In cases of strong vibrations, lock nuts (e.g., self-locking) or a suitable threadlocker are often the better solution.

The decisive factors are threads (M size and pitch), standards/dimensions (e.g., washer diameter), material combinations (stainless steel with aluminum/carbon can be galvanically critical), and the installation conditions (tension/shear, vibration, moisture). For safety-critical fittings, you should always ensure a sufficiently large contact surface and, if necessary, use a backing plate.

Keep contact surfaces clean, position washers correctly, and tighten nuts evenly with the appropriate tool (do not “over-tighten”—especially with GRP/wood). For outdoor installations, sealant or sealing may be necessary (e.g., to prevent water ingress). In case of vibration: Use locknuts or threadlocker. Ensure galvanic isolation when mounting stainless steel on aluminum (e.g., suitable insulating washers or paste).

Perform regular visual inspections (for loosening, signs of corrosion, cracks in the substrate), especially after heavy loads or winter storage. In saltwater environments: rinse occasionally to prevent salt from accumulating in crevices. Replace worn nuts/washers early—this prevents consequential damage to fittings and surfaces.

Cap nuts protect the end of the thread, reduce the risk of injury, and look neat—ideal for visible fasteners (railing, brackets, interior fittings). The correct thread length is important: The screw must not be “stuck” inside, otherwise the connection cannot be tightened properly.