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Contents
A fitting is only as good as the fastener holding it. A 316 stainless cleat mounted with hardware-store screws, no backing plate and a bead of silicone is a leak and a failure waiting to happen. This guide covers what you need to know to choose and install bolts, nuts and washers on a boat: A2 or A4, which standards to look for, what size suits each fitting, how to avoid galling and galvanic corrosion, and how to seal so water stays out.
Short version: A4 (316) fasteners for everything outside; through-bolts with nylon-insert locknuts, fender washers and backing plates on any loaded fitting; anti-seize on the threads, isolation wherever aluminum is involved, and butyl or marine polyurethane in every hole.
A2 vs A4: the first decision
| ISO 3506 class | Equivalent steel | Molybdenum | Use aboard |
|---|---|---|---|
| A2-70 | 304 (1.4301) | No | Dry interiors, furniture, freshwater |
| A4-70 | 316 (1.4401) | 2-3% | Deck, hull, cockpit, exterior hardware |
| A4-80 | Higher-strength 316 | 2-3% | Heavily loaded fittings (chainplates, brackets) |
A2 fasteners stain early in salt air and suffer crevice corrosion in the threads and under the heads. Pairing 316 hardware with A2 fasteners is a common mistake, and the screw ends up being the weak point. The reason molybdenum matters is explained in our guide to 304 vs 316 stainless. The number after the letter (70, 80) gives the minimum tensile strength in tens of MPa, so 700 and 800 MPa. Class 70 is the most common and suits nearly everything. In US listings the same grades appear as "304" and "316," so always check the grade, not just the word "stainless."
DIN and ISO standards worth knowing
Searching by standard avoids surprises with head shape and thread. Most marine-grade fasteners are still sold by DIN or ISO number, metric.
| Standard | Part | Typical use aboard |
|---|---|---|
| DIN 912 / ISO 4762 | Socket head cap screw | Through-bolts for fittings, brackets |
| ISO 7380 | Button head socket screw | Fittings that need a low, rounded head |
| DIN 7991 / ISO 10642 | Countersunk socket screw | Cleats, hinges and countersunk bases |
| DIN 933 / ISO 4017 | Hex head bolt | Engine mounts, structures |
| ISO 14586 / DIN 7982 | Countersunk self-tapping screw | Hinges and small fittings in wood or thick fiberglass |
| DIN 985 | Nylon-insert locknut | Any through-bolt that vibrates |
| DIN 1587 | Acorn (cap) nut | Visible interiors; prevents cuts and snags |
| DIN 125 / DIN 9021 | Flat / fender washer | DIN 9021 under the nut on fiberglass spreads the load |
What size for each fitting
| Fitting | Approximate size | Fastening |
|---|---|---|
| Cleat, 4-6 in (100-150 mm) | M6 (about 1/4 in) | Through-bolt + backing plate |
| Cleat, 8-12 in (200-300 mm) | M8-M10 (5/16-3/8 in) | Through-bolt + backing plate |
| Stanchion base | M6-M8 | Through-bolt + backing plate |
| Boarding ladder | M6-M8 | Through-bolt + fender washer |
| Handrails and grab handles | M6 | Through-bolt or screw into an insert |
| Hatch hinges | M5 or 4.8-5.5 mm self-tapping | Depends on lid thickness |
| Bimini fittings | M5-M6 | Through-bolt or self-tapping in a reinforced area |
| Inspection ports and small accessories | 4.2-4.8 mm self-tapping | With sealant |
These are common sizes; the hole diameter in the fitting always decides. For a through hole with normal clearance (ISO 273, medium series) use 5.5 mm for M5, 6.6 mm for M6 and 9 mm for M8. If you work in inches, the nearest equivalents are 1/4 in bolts in a 9/32 in hole and 5/16 in bolts in an 11/32 in hole.
Nuts, washers and backing plates
- DIN 985 locknut: the nylon insert stops vibration from loosening it. This is the default nut on a boat.
- DIN 9021 fender washer: its outside diameter, roughly three times the nominal size, spreads the pressure over the fiberglass. Essential if there is no backing plate.
- DIN 1587 acorn nut: in interiors it protects heads, lines and skin from thread ends.
- Split lock washer: of little use in stainless; use a locknut instead.
A fiberglass deck a few millimeters thick, or a sandwich with foam or balsa core, cannot take the concentrated load of a nut. A backing plate spreads the load over a wide area. It can be stainless (the thinnest for a given strength), aluminum (light, but isolate it from the stainless), G10 or fiberglass-epoxy laminate (will not corrode or rot) or marine plywood sealed in epoxy (cheap and effective if well sealed).
Galvanic corrosion and galling
Stainless on aluminum: always isolate
On aluminum masts, booms, platforms, arches and engine brackets, if stainless touches them in salt water the aluminum corrodes: white powder, stripped threads, seized bolts. Put isolating paste on the thread and use nylon washers under heads and bases.
Stainless on stainless: anti-seize
Stainless cold-welds to itself when it is threaded dry and under pressure. This is galling. Prevent it with anti-seize on the thread (see marine lubricants and anti-corrosion products), tightening by hand or at low speed and never using an impact driver. If a bolt galls halfway home, do not force it; you will snap it.
Sealing fittings: butyl or polyurethane
Every hole in the deck is a potential leak that also soaks the sandwich core and rots it. Good sealing rests on three things: product, preparation and tightening.
| Sealant | Behavior | When to use it |
|---|---|---|
| Butyl (tape or rope) | Never cures, stays pliable, follows movement | Portlights, stanchions, tracks, fittings you will remove later |
| Marine polyurethane | Cures flexible and also bonds | Fixed fittings and joints you will not touch for years |
| MS polymer / hybrid | Cures flexible, good UV resistance | Versatile alternative, also on plastics |
| Acetic-cure silicone | Releases acetic acid, poor adhesion | Never on stainless fittings or gelcoat |
For product options, see marine sealants and silicones.
Installing a fitting step by step
- Position and mark the fitting, checking you can reach the underside for the nut and backing plate.
- Drill with a sharp bit, starting with a pilot. In a cored sandwich, overdrill, remove some core, fill with thickened epoxy and redrill the next day so the core is sealed from water.
- Chamfer the hole edge slightly: the sealant forms a gasket around the bolt and the gelcoat does not crack.
- Clean with isopropyl alcohol or the cleaner the sealant maker recommends.
- Apply sealant under the base, around each hole and on the shank, plus anti-seize on the thread.
- Tighten in two stages: first by hand, without squeezing out all the sealant; once it has cured (polyurethane) or the next day (butyl), do the final tightening by turning the nut, not the bolt, so you do not tear the seal.
- Remove the excess with a plastic scraper and pull the masking tape before the sealant cures.
Watch out: sheet-metal screws are not for cleats, stanchions or ladders. If the fitting works under load, use a through-bolt, backing plate and locknut.
Common mistakes
- Buying "stainless" with no grade: if it does not say A4 or 316, assume A2 or worse.
- No backing plate or fender washer: the nut sinks into the fiberglass and the fitting works loose.
- Acetic-cure silicone as a sealant.
- Impact driver on stainless: galling is almost guaranteed.
- Final tightening from the head after the sealant has cured: you tear the seal and open a leak.
All of this applies to any installation, from cleats to bimini hardware. Before you start, also go over the essential tools to carry aboard.
Recommended gear for this guide
Frequently asked questions
What is the difference between A2 and A4 screws?
A2 and A4 are ISO 3506 designations for austenitic stainless fasteners: A2 belongs to the 304 family and A4 to the 316 family, which contains molybdenum. On a boat, use A4 for anything outdoors or in contact with salt water.
What does A4-70 or A4-80 mean?
The number is the strength class: 70 means a minimum tensile strength of 700 MPa and 80 means 800 MPa. Most commercial A4 hardware is A4-70, while A4-80 is used where loads are higher. Both resist corrosion equally well.
What bolts should I use to mount a cleat?
Use A4 stainless through-bolts, typically M6 (about 1/4 in) for cleats up to 6 in and M8 or larger for 8 in and up, with fender washers, locknuts and a backing plate underneath. Never use sheet-metal screws on a fitting that works under load.
Butyl or polyurethane for bedding hardware?
Butyl never cures and stays pliable, which suits fittings that move or that you want to remove later. Marine polyurethane cures and also bonds, so it is more permanent but harder to take apart. Both work; the one thing you should never use is acetic-cure silicone.
How do I stop stainless bolts from galling?
Galling happens when stainless is threaded against stainless dry, at speed and under pressure. Prevent it with anti-seize on the thread, tightening by hand or at low speed and avoiding impact drivers.