Dome Nut

What is a dome nut?

A dome nut is a hex nut with a closed, rounded top. It screws onto the bolt like an ordinary nut, but the dome covers the cut end of the thread, which gives a neat finish, protects the thread and leaves no sharp bolt end to catch hands or clothing. The same nut is called an acorn nut, a cap nut or a capping nut.

They are used wherever a bolt end is on show or within reach: furniture, shopfitting and display work, balustrades and handrails, gates and fences, playground and outdoor equipment, and boat fittings. Hobson publishes no proof load or tightening torque for these dome nuts, so where the joint is structural or safety related (a balustrade or a playground frame, for example), use the nut grade and fixing the designer or engineer specifies.

Which dome nut finish for indoors, outdoors and the coast?

• •304 stainless (DIN 1587, A2): the everyday stainless dome nut, metric M3 to M16 and imperial #8 to 5/8 inch UNC and UNF. Hobson's corrosion guide puts exposed outdoor fixings in stainless or hot dip galvanised, so 304 suits outdoor work away from the sea.
• •316 stainless (DIN 1587, A4), M4 to M36: molybdenum gives better resistance near the sea, and Hobson's corrosion guide calls for 316 within 2m of a pool edge. Neither 304 nor 316 is made to sit in seawater, and even 316 can visibly corrode where salt spray builds up, so rinse coastal fittings.
• •Hot dip galvanised class 5 steel, M8 to M36: Hobson rates hot dip galvanising as high corrosion resistance for long term outdoor use, a match for galvanised bolts and threaded rod on fences, posts and farm work. Hobson notes galvanising weathers to a matt grey and that its brightness has no effect on how long it lasts.
• •Chrome plated class 5 steel, M6 to M16: a bright decorative finish for furniture, shopfitting and indoor work. Hobson's finish comparison gives no corrosion rating for chrome, so use stainless or galvanised outdoors.
• •Natural nylon (DIN 1587), M3 to M16, plus an M20 Form A and imperial UNC: light, non-metallic and electrically insulating. Hobson's standard nylon is PA6.6, with a working temperature of -30 to +100°C; it absorbs moisture, which lowers its strength, and Hobson notes carbon black improves its outdoor weathering, while these are natural (uncoloured) nylon.
• •Two-piece 304 dome nuts (M16 and 1/2 inch BSW): Hobson lists these as dome 2-piece hex nuts, a welded two-piece stainless dome nut.

Metric dome nut sizes

Every metric dome nut we carry, smallest to largest, with the pack size for each material. The size is the bolt or rod the nut fits (M10 fits a 10mm bolt), and all are coarse pitch. Select a size to open it in the selector.

Imperial dome nuts: UNC, UNF and BSW

Imperial dome nuts are sized by gauge (#8, #10) or inch diameter and threads per inch (TPI). UNC is the coarse thread, UNF the fine and BSW the older Whitworth thread; they do not mix, so check the bolt before you order.

Fitting dome nuts

1. Check the bolt length first. A dome nut is closed, so the bolt must stop short of the inside of the dome: if the bolt end reaches the dome before the nut is tight, the nut cannot clamp. Cut long bolts or threaded rod back, and clean up the cut thread.
2. Use a flat washer under the nut on timber and soft or painted surfaces.
3. Tighten with a spanner or socket on the hex, not on the dome.
4. For stainless dome nuts, run them on slowly and lubricate the thread to avoid galling (threads seizing), as Hobson recommends. A different stainless grade for the bolt and nut also helps.
5. Nylon dome nuts are for light duty: tighten them gently by hand or with a spanner.

Dome nuts in Adelaide

Allmat supplies dome nuts, bolts, threaded rod and washers to builders, fabricators, shopfitters and trade across Adelaide and South Australia. Add the dome nuts to a quote with the bolts and washers they go on, and we will price the lot.

Questions