Countersunk Coach Screw

What a countersunk coach screw is

Hobson describes it as a coach screw with a gimlet point, countersunk for a flush finish. Like a hex coach screw it has a plain shank and a coarse wood thread that screws into timber, but the head is countersunk and driven with a hex key or hex bit in the socket, so it sits flush instead of standing proud. They are 316 stainless (A4).

Hobson lists them for timber as the base material: fencing, brackets, hinges, timber decking, landscaping and retaining walls, other timber connections, and with wall plugs into concrete and brick. Hobson publishes no load rating for them, so where the connection is structural (deck and pergola framing, a retaining wall holding back soil, anything holding people or loads) the screw size, number and spacing must come from the designer, an engineer or the building code for that job.

Countersunk or hex coach screw?

• •Countersunk (this page): flush head, driven with a 5mm (M8) or 6mm (M10) hex bit. Use it where a head must not stand proud, such as on a face people walk past or brush against, or where a hinge or bracket has countersunk holes. For a flush finish the hole in the part being fixed needs a countersink to suit the 16mm (M8) or 20mm (M10) head.
• •Hex head: driven with a socket or spanner, usually with a washer under the head. Our hex coach screws come in zinc, galvanised and 316 stainless from M6 to M20 on the coach screw page.
• •Going right through the timber? A cup head (coach) bolt with a nut does that; a coach screw grips in the timber it screws into.

Countersunk coach screw sizes and dimensions

Every countersunk coach screw we carry, with the dimensions from Hobson's data sheet. Select a row to open that size and length.

Pilot hole sizes

These screws are not self-drilling. Hobson's data sheet says pre-drilling into the timber is required to serve as a pilot hole, with the size set by the type of timber. Softwood means timber such as pine and hardwood timber such as merbau or jarrah (common examples, not from Hobson's sheet). Select a row to open that size.

How to fit a countersunk coach screw

1. Drill through the part being fixed so the screw passes through it, and countersink the hole to suit the head.
2. Drill the pilot hole into the timber at the size in the table: 4mm (M8) or 5mm (M10) in softwood, 6mm (M8) or 7.5mm (M10) in hardwood.
3. Drive the screw with a hex bit of the right size: 5mm for M8, 6mm for M10. Hobson recommends its S2 alloy hex insert and power bits, or its black impaX power bits.
4. Stop when the head is flush. Do not keep driving once it is home.
5. Into concrete or brick, use a wall plug sized for the screw and follow the plug's own hole size; if the plug has no published load, don't use it for a load-bearing fixing.

Where 316 stainless suits

• •Exposed outdoor timber such as decks, pergolas and fences: Hobson's corrosion guide (written for metal connectors) lists stainless steel (304 or 316) or hot dip galvanising for exposed positions in low, coastal and industrial zones, and stainless in the exposed sea spray zone. These are 316. For deck and pergola framing or any load-bearing connection, Hobson publishes no load rating for these screws, so use the fixing the designer, engineer or building code specifies.
• •Within 2m of a pool edge, Hobson's guide calls for 316 stainless.
• •Not for seawater: Hobson's stainless article (from the Australian Stainless Steel Development Association) explains seawater has about 20 times the chloride 316 can withstand, and in severe coastal spots where salt spray builds up, 316 can visibly corrode. Rain washing or regular cleaning keeps it bright.

Countersunk coach screws in Adelaide

Allmat supplies countersunk and hex coach screws, coach bolts, driver bits and wall plugs, with the timber, to builders, carpenters, landscapers and fencers across Adelaide and South Australia. Add them to a quote with the rest of your list and we will price the lot.

Questions