Size picker · Countersunk socket · ASME B18.3
UNC and UNF countersunk socket screw sizes.
The inch flat head. Pick a size and length for the head diameter, socket size and the one figure this page exists for: the head is 82°, not the metric 90°, and the two do not mix.
1 Pick a diameter
2 Pick a length
Choose a diameter first.
Your browser could not draw the 3D view. The dimensions beside it are unaffected.
Drawn to proportion from the standard dimensions. Illustration, not a photograph of the item.
Selected
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| Sym | Dimension | Value |
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Job stopped? Ring 01327 877577 and we'll check the stock while you're on.
Read the full ASME B18.3 size chart
| Size | 1/4" | 5/16" | 3/8" | 1/2" | 5/8" | 3/4" | 7/8" | 1" | 1 1/4" | 1 1/2" | 2" | 2 1/2" | 3" |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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| #8 | |||||||||||||
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| 1/4 | |||||||||||||
| 5/16 | |||||||||||||
| 3/8 | |||||||||||||
| 1/2 |
The range shown is what fastener wholesalers commonly list — not a claim about what the standard mandates, and not a stock list. In-between lengths (85, 95, 170…), lengths past 300mm and odd diameters like M7 are made too, and availability varies by finish and grade — ask us for anything you can't see.
82 degrees — the trap on this page
A metric DIN 7991 countersunk head is 90° included; the ASME B18.3 inch flat head is 82°. Sit an imperial screw in a metric countersink and it bears on its rim instead of its cone — it sits proud, it works loose, and the head can crack under load. Same the other way round: a metric screw in an 82° seat touches only at the centre. If you are cutting the seat, use an 82° countersink bit for these.
Fully threaded, and length includes the head
A countersunk screw's length is measured over the head, because the head sinks flush. The listed range here is short enough that everything comes threaded to the head.
Where these figures come from
Head diameter, height and socket size are ASME B18.3, read from two independent published tables that agree to the thousandth. Lengths are what our suppliers actually list.
Working in metric instead?
See the metric countersunk socket chart for DIN 7991 — and remember the angle changes with the system, not just the thread.
Not sure which you need?
Bring the old one in, or send us a photo. We are on Baird Close in Daventry.
More size charts
Every size, drawn to scale.
Hex setscrews
DIN 933 · full thread
Hex bolts
DIN 931 · part thread
Socket cap screws
DIN 912
Low head cap screws
DIN 7984
Button head screws
ISO 7380-1
Flanged button heads
ISO 7380-2
Countersunk socket screws
DIN 7991
Grub screws
DIN 913–916 · five points
Nuts
DIN 934 · nyloc · thin · Stover · flange
Washers
ISO 7089 · BS 4320 · penny
Not sure what the markings on the head mean? Bolt grades explained covers 8.8, 10.9, 12.9, stainless A2 and A4, and the imperial SAE grades.
All of it comes off the shelf at Baird Close — see the full fasteners & fixings range, or every size guide.