Socket Head Cap

Match your Allen / hex key AF (across-flats) to the screw size it fits. Our reference chart covers every metric and imperial size we stock.
Cap head socket screws are the standard fastener for precision engineering, machinery assembly, CNC equipment, jigs and fixtures, and any application where you need high clamping force in a compact head profile. The cylindrical head with internal hex (Allen key) socket allows a deep, positive drive connection and high installation torque — significantly more than a Phillips or slot head of the same size.
Also known as: Socket Head Cap Screw, SHCS, Allen Bolt, Hex Socket Cap Screw, Cap Screw.
Fully threaded with uniform diameter throughout — the same thread from end to end, unlike self-tapping or chipboard screws which have tapered threads. They require a pre-tapped hole or a matching nut.
GRADES AND MATERIALS
Class 12.9 Black (plain finish) — The highest strength grade in our cap head range. Unfinished bare steel — no corrosion protection. 'Black' is the old industry term for plain finish. Class 12.9 provides tensile strength of 1220 MPa, making it the choice for high-load structural and mechanical engineering connections.
Class 12.9 Zinc Plated — Same high tensile strength with a bright silver zinc plated finish for light corrosion protection in dry indoor environments.
Class 8.8 Zinc Plated — The standard engineering grade for general machinery and structural applications. Tensile strength 800 MPa. Bright silver zinc finish for internal use.
A2-70 G304 Stainless Steel — The most popular stainless cap head grade. Marked A2, A2-70, 304 or G304. Excellent corrosion resistance in most environments. Largely non-magnetic — A small degree of magnetism from work-hardening is normal and not a sign of the wrong product.
A4-70 G316 Marine Grade Stainless — Maximum corrosion resistance for marine, coastal, food and chemical environments. Largely non-magnetic — Same note on work-hardening magnetism applies.
DRIVE AND INSTALLATION
Cap head socket screws are driven with a hex (Allen) key or a hex socket bit. Metric sizes use metric hex keys: M3 uses a 2.5mm key, M4 a 3mm key, M5 a 4mm key, M6 a 5mm key, M8 a 6mm key, M10 an 8mm key. A full range of hex keys, T-handle hex sets and hex socket bits is available in our Driving section.
A NOTE ON GALLING
Galling can occur with stainless fasteners under excessive tightening pressure. To reduce the risk: always run the fastener on by hand first, tighten slowly, use an anti-seize lubricant on the thread, and avoid power-driving stainless cap heads without careful torque control.
THREAD LENGTH AND MEASUREMENTS
WHY AUSTRALIA TRUSTS BOLT & NUT
With 15+ years selling fasteners online from Brisbane, we hold over 25,000 stock lines and 200 million pieces — 98.75% of online orders placed before 1pm leave same day. Save more as you buy more — we offer up to 13 price breaks plus 3 spend-tier discounts. Read our customer reviews or get in touch if you need help picking the right socket screw.
A cylindrical socket-drive head with an internal hex (Allen) socket - high clamp load in a low-profile head that sits flush in counterbored holes. Stainless and high-tensile carbon, metric and imperial.
TECHNICAL RESOURCES
Need help identifying your socket screw? Check the Allen key sizes for socket products, or our thread size identification guide.
For full sizing data and standards, see our Technical PDFs library.
















