Countersunk Connecting Screw (Female)

What a post head countersunk connector is

Hobson sells this part under two names: SS316 female connecting screw, post head countersunk, and 316 barrel nut CSK Phillips. It is a short internally threaded post with a countersunk head and a Phillips #3 recess. It does the job of a nut: Hobson's sheet says it acts as a nut in timber and furniture connections, and that used with a countersunk metal thread screw, the same head is seen from both sides of the connection.

Hobson lists it for fastening timber, furniture and joinery, and, on the current sheet, as a replacement for the nut on TygaBolt anchors to give a countersunk finish; the earlier sheet put that as replacing the nut in masonry fixings, and listed electrical among the trades that use it. It is the same part as the countersunk Phillips barrel nuts on our barrel nut page.

Hobson publishes no load rating for it. For a joint that carries a structural or safety load, such as a balustrade, play equipment or a fixing overhead, use the fixings the designer or equipment maker specifies.

Post head countersunk sizes

Select a row to open that size in the selector. The thread length is how far the internal thread runs inside the post, which limits how much of the screw can engage.

Choosing the screw that goes with it

Pair it with a countersunk machine screw of the same thread, in 316 stainless so both parts are the same steel; a Phillips countersunk screw gives the matching look on both faces. The post goes in from one face and the screw from the other, and the screw has to reach into the post's internal thread without running past it.

Working it out (simple arithmetic on Hobson's dimensions, with both parts measured overall): the joint should be at least as thick as the post is long, or the post stands proud. Then screw length = joint thickness minus post length plus the thread engagement you want, and the engagement can be no more than the post's thread length. Example: an M6 post (23mm long, 16.5mm of thread) through a 30mm joint. 30 minus 23 leaves 7mm, so a 20mm M6 countersunk screw engages 13mm of thread, inside the 16.5mm available; a 25mm screw would need 18mm and run past the end of the thread.

How to fit it

1. Drill the hole through the parts to clear the post's shank (the shank diameter in the table) and countersink both faces for the heads.
2. Push the post in from one face and start the countersunk screw from the other.
3. Hold one head with a Phillips #3 bit or driver and tighten the other until both heads sit flush. Drive steadily: Hobson's stainless steel article notes that galling of stainless threads is worse with tight clearances, lack of lubrication, dirt and over-tightening.
4. On a TygaBolt sleeve anchor, Hobson lists the post as a replacement for the anchor's nut to give a countersunk finish. Match the post's thread to the anchor's bolt thread and the fixture hole to the post. The anchor's own setting torque, edge distance, spacing and member thickness limits still apply (see the sleeve anchor page); Hobson gives no torque for setting it through the post's Phillips #3 head and no load data for the anchor with the post fitted, so do not use it on a loaded fixing unless an engineer specifies it.

316 stainless: where it suits

Both the post and Hobson's matching screws are 316 stainless (A4). Hobson's corrosion guide for metal connectors lists 304 or 316 stainless for sheltered and exposed positions in every zone, including seaspray, and 316 within 2m of a pool edge, so the finish suits outdoor furniture and joinery near the coast (for any loaded fixing, see the no-rating note above). Keep them clean: Hobson's stainless article says salt deposits allowed to build up can visibly corrode even 316, and neither 304 nor 316 is made to sit in seawater.

Post head connectors in Adelaide

Allmat supplies 316 post head countersunk connectors, countersunk machine screws, barrel nuts and Phillips bits to carpenters, joiners, cabinet makers and builders across Adelaide and South Australia. Add the posts and screws to a quote and we will price the lot.

Questions