Size picker · Grub screw · ASME B18.3
UNC and UNF grub screw sizes.
What the American standard calls a socket set screw — headless, driven by a hex key, cup point biting the shaft. Pick a size and length for the key size and point diameter.
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 | 1/16" | 1/8" | 3/16" | 1/4" | 5/16" | 3/8" | 1/2" | 5/8" | 3/4" | 7/8" | 1" | 1 1/4" | 1 1/2" | 2" |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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| #8 | ||||||||||||||
| #10 | ||||||||||||||
| 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.
"Set screw" means the opposite thing over there
In the UK a set screw is a fully-threaded hex head; in the States a set screw is this — the headless grub. Reading an American parts list, "socket set screw 1/4-20 x 3/8 cup pt" is the part on this page, not a hex setscrew. Both trades at least agree on the cup point.
Cup point is the shelf; the rest is to order
Nearly everything listed imperial is cup point — the general-purpose form that bites the shaft — and that is what this chart offers. Flat, cone and dog points exist in the standard and we can get them; ask us rather than hunting a listing that is not there.
The key is the figure to check
A 1/4 inch grub takes a 1/8 inch hex key, and imperial keys are not metric keys — a 3mm key in a 1/8 socket (3.175mm) rounds the socket the moment it slips. If the key wobbles, it is the wrong system.
Where these figures come from
Key size, key engagement and cup diameter 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 grub screw chart for DIN 913–916, where all four points are stocked.
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.