Socket Head Cap Screw Guide: DIN 912, Grades, Sizes & Allen Key Selection

What Is a Socket Head Cap Screw?

A socket head cap screw is a high-strength precision fastener with a cylindrical head and an internal hex (Allen) socket drive. It is the workhorse fastener of machine design, used wherever an engineer needs a compact head profile, predictable clamping force, and the option to sit fully recessed below a finished surface in a counterbored hole.

The name describes the geometry exactly. The head is a plain cylinder, slightly larger in diameter than the threaded shank. The socket is a hexagonal recess machined into the top of the head, driven by a hex (Allen) key from above rather than by a spanner from the side. The cap reference is historical — early machine builders called these "cap screws" because they sat as a cap on top of the joint. The shank below the head is fully or partially threaded, depending on the length and grip required.

In Australian workshops you will hear them called by several names — all referring to the same fastener:

• Allen bolt — the most common AU trade term, after the Allen Manufacturing Company that popularised the hex socket drive in the early 1900s.
• Cap screw or cap head screw — short form, used on parts lists and stock cards.
• Allen head screw or Allen key bolt — verbal terms used on the floor.
• Socket bolt or hex socket bolt — used in engineering drawings.
• SHCS — abbreviation that appears on parts lists and stock-keeping systems.
• DIN 912 — used as a stand-alone descriptor in engineering specifications.

If a maintenance fitter asks for "an Allen bolt", they are asking for a socket head cap screw. If a technical drawing calls out "M10 × 50 SHCS Class 12.9", that is also a socket head cap screw. Always confirm thread size, length, grade, and material when ordering — the term alone does not specify the part.

How to Measure a Socket Head Cap Screw

To order or specify a socket head cap screw correctly you need five dimensions. Get any one of them wrong and the screw will not fit, will not clamp correctly, or will fail in service.

1. Thread diameter (nominal size) — the major diameter of the thread, expressed in millimetres for metric screws (M3, M4, M5, M6, M8, M10, M12, M14, M16, M18, M20, M22, M24, M27, M30 and larger). Most AU socket head cap screws are metric. Imperial sizes (1/4", 5/16", 3/8", 1/2") are still encountered on imported American machinery and some agricultural equipment.
2. Thread pitch — the distance between thread crests, in millimetres. DIN 912 socket head cap screws are supplied with coarse pitch as standard (e.g. M8 × 1.25, M10 × 1.5, M12 × 1.75). Fine-pitch versions exist for high-vibration or precision applications and must be specified explicitly.
3. Length — measured from under the head to the end of the thread. The head is not included in the length measurement, because the head sits above (or recessed into) the workpiece while the threaded portion enters the joint. Common stock lengths for an M8 cap screw are 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120 and longer.
4. Head diameter — the outside diameter of the cylindrical head. This dimension is fixed by DIN 912 for each thread size and matters when the head must clear into a counterbored hole or sit within a recess. Example: an M8 cap screw has a 13 mm head diameter and 8 mm head height.
5. Hex socket size (across flats) — the size of the Allen key required to drive the screw, measured across the flat sides of the hexagonal recess. This is also fixed by DIN 912 and varies by thread size. M8 takes a 6 mm hex key; M10 takes 8 mm; M12 takes 10 mm. The full table appears later in this guide.

The standard way to specify a socket head cap screw on a parts list is: M[size] × [length] SHCS, Class [grade], [material/finish]. For example: "M10 × 40 SHCS, Class 12.9, black oxide" — that is unambiguous and orderable from any AU industrial supplier.

The DIN 912 / ISO 4762 Standard

Two standards govern socket head cap screw dimensions. They are dimensionally compatible — a screw made to DIN 912 will fit the same hole and use the same Allen key as one made to ISO 4762 — but you will see both labels in AU supply.

DIN 912 is the German national standard, first published in 1936. For decades it was the global default for socket head cap screws, and most AU distributors still label stock "DIN 912" simply because that is how the manufacturer marks the box.

ISO 4762 is the international successor, first published in 1989 and updated several times since. ISO replaced DIN as the official global standard, and modern engineering drawings tend to specify ISO 4762 for new designs.

The two standards specify identical head dimensions, hex socket sizes, and thread tolerances for sizes M3 through M64. The only practical difference is the documentation — and even that is converging as DIN 912 is now harmonised with ISO 4762.

What both standards define for each thread size:

• Head diameter (cylinder OD)
• Head height
• Hex socket size (across flats)
• Hex socket depth
• Thread length and run-out
• Property classes (8.8, 10.9, 12.9 for steel; A2-70, A4-70, etc. for stainless)
• Surface finish requirements

What neither standard mandates is torque — torque values are derived from the property class, thread size, friction coefficient, and joint geometry. We provide an indicative torque table further down this guide, but always check the equipment manual for the specific torque your application requires.

Cap Head, Button Head, Flat (Countersunk) Head — Types Compared

"Socket head cap screw" technically refers to the standard cylindrical-head DIN 912 fastener. In broader trade language, "socket screw" can mean any screw with a hex socket drive, which includes button-head and countersunk variants. Knowing the difference matters because the head profile changes the strength, the bearing surface, and the type of hole you need to prepare. For a wider comparison covering pan, truss, dome, wafer, bugle and other head shapes beyond the socket-driven family, see our Screw Head Types Guide.

The reason cap head outperforms the others on torque is the depth of the hex socket. The deeper the socket, the more contact area between the Allen key and the head walls, and the more torque can be applied without rounding the recess. A button head's socket is typically half the depth of a cap head's, which is why a stripped button head is one of the most common failures on lighter machinery. For the dedicated button head deep-dive — ISO 7380-1 vs 7380-2 flanged, sizes, torque limits and the engineering reasons not to substitute — see our Button Head Socket Screw Guide.

For maximum-strength engineered joints — drives, dies, gearbox covers, structural fixings on vibrating equipment — specify cap head. For appearance-grade applications, light enclosures, or where the head must clear above a panel, button head is appropriate. For flush-fit work, see our Countersunk Screw Guide.

Grades and Strength: 8.8, 10.9 and 12.9 Explained

Steel socket head cap screws are sold by property class — a two-part number (8.8, 10.9, 12.9) that is far more useful than the historical "high tensile" or "low tensile" labels. Each part of the number tells you something specific.

The first number (before the decimal) is approximately the ultimate tensile strength in units of 100 MPa. So Class 8.8 has roughly 800 MPa tensile strength; Class 12.9 has roughly 1220 MPa.

The second number (after the decimal) is the ratio of yield strength to ultimate tensile strength, multiplied by 10. So Class 8.8 has yield = 0.8 × 800 = 640 MPa. Class 12.9 has yield = 0.9 × 1220 ≈ 1100 MPa.

The single most important thing to know about socket head cap screws is that Class 12.9 is the engineering default. When a designer specifies "M10 × 40 SHCS" without giving a grade, they almost always mean 12.9. The very design of the cap screw — narrow head, deep socket, used in tight machined joints — assumes a high-strength grade. If you replace a 12.9 with an 8.8, you have reduced clamping force by roughly 40%, which can fatigue the joint, allow vibration loosening, and ultimately fail.

For a complete breakdown of grade markings, head identification, and full mechanical properties for all bolt grades, see our Bolt Grade Chart.

Material Selection: Steel, Stainless and Bumax

Socket head cap screws come in five common materials at AIMS Industrial. Each has a defined application range and a defined limit. The fastener carton always lists the material — never assume; always read.

Class 12.9 Black Oxide Carbon Steel

The default. Carbon steel heat-treated to Class 12.9, with a black oxide finish that provides mild corrosion resistance and a distinctive matte black appearance. Used for indoor industrial applications: machine bases, gearbox covers, dies, jigs, fixtures, and any precision-engineered joint where the Class 12.9 strength is required and the environment is dry. The black oxide is not a long-term corrosion barrier — for outdoor or wet exposure, choose zinc-plated or stainless.

Class 8.8 / 10.9 Zinc-Plated Carbon Steel

Carbon steel with electroplated zinc finish (typically 5–8 microns), often passivated for an extra layer of corrosion resistance. Lower strength than 12.9 — typically supplied as Class 8.8 or 10.9. Suitable for indoor and light outdoor industrial applications where corrosion exposure is moderate. The zinc plating is decorative-grade only — for genuine outdoor exposure, hot-dip galvanised or stainless is required.

304 (A2-70) Stainless Steel

The standard stainless grade for general industrial work. Property Class A2-70 — approximate tensile strength 700 MPa, yield around 450 MPa. Roughly equivalent to a Class 8.8 carbon steel screw in tensile, but somewhat weaker in yield. Suitable for food processing (non-chloride), light marine (sheltered), pharmaceutical, and most outdoor applications away from salt water. Excellent corrosion resistance to fresh water, mild acids, and atmospheric moisture.

316 (A4-70) Stainless Steel

Adds molybdenum to the 304 chemistry, which provides resistance to chloride attack. Property Class A4-70 — similar mechanical properties to 304 but considerably better corrosion resistance in salt water, chlorinated water, food processing brines, and chemical environments. Specify 316 for: marine work (boats, jetties, coastal infrastructure), chlorinated swimming pools, pickling baths, food processing with brine, and any AU coastal industrial site within roughly 1 km of the surf. Cost is around 30% above 304.

Bumax 88 / Bumax 109 — High-Strength Stainless

A specialty stainless grade developed for applications that need both 12.9-equivalent strength and the corrosion resistance of stainless. Bumax 88 has tensile strength around 800 MPa (Class 8.8 equivalent in strength but in stainless); Bumax 109 has tensile strength around 1000 MPa (close to Class 10.9 in strength). Used in oil and gas, defence, subsea infrastructure, motorsport, and high-end food processing where standard 316 lacks the strength but mild steel cannot survive the environment. Available at AIMS Industrial for specification work.

Stainless and galling — the silent failure

The most common failure mode of stainless socket head cap screws is not corrosion or overload — it is galling. When stainless threads are tightened without lubricant, the soft, ductile thread surfaces cold-weld together as friction heats them. The threads seize irreversibly. The screw cannot be removed without drilling out, and often cannot be tightened to specification because the galling occurs partway through the torque.

The fix is simple: always apply a thread lubricant or anti-seize compound to stainless threads before installation. Nickel-based or moly-based anti-seize is the industrial default. PTFE thread paste also works for lower-torque applications. Never install a stainless cap screw dry into a stainless thread.

Socket Head vs Hex Head: Which to Choose

The choice between a socket head cap screw and a hex bolt usually comes down to one factor: clearance.

A hex bolt is driven by a spanner or socket from the side. The spanner needs swing room — typically a clearance arc of around 60° for a ratchet — and the bolt head sits proud of the work surface. Where there is space and where a quick-release joint matters (vehicle wheels, building structural connections, exposed brackets), the hex bolt is the right choice.

A socket head cap screw is driven by an Allen key from above. It needs no side clearance — only a clear path down the centreline of the screw. The head can sit fully recessed in a counterbored hole, completely below the surface of the part. This makes the SHCS the only practical choice for:

• Counterbored holes — gearbox covers, machinery enclosures, motor mounts
• Recessed mounting — die plates, fixture plates, jig bases
• Tight clearances — where a hex spanner would not fit between adjacent components
• Machined assemblies — where surface continuity matters
• High-strength precision joints — where Class 12.9 is required and a hex bolt of equivalent grade is unavailable

The other practical difference is grade availability. Hex bolts are most commonly stocked in Class 8.8 or 10.9; Class 12.9 hex bolts are uncommon and often special-order. Socket head cap screws are stocked in Class 12.9 as the default. If your design calls for 12.9 strength, the SHCS will almost always be more readily available.

For full hex bolt selection guidance, head markings and grade chart, see our Hex Bolt Guide.

Hex Key (Allen Key) Sizes for Metric Socket Head Cap Screws

The single most useful piece of information when working with socket head cap screws is the hex key size — and it is not obvious from the screw's thread size alone. The DIN 912 standard fixes the hex socket size for each thread, so once you know the table, you know the key. Use the wrong size and you will round out the socket.

Two practical points the table will not tell you:

1. Imperial sizes use a different table. An imperial 1/4" socket head cap screw takes a 3/16" hex key — not a metric key of any size. Mixing metric and imperial drivers is one of the fastest ways to round out a socket. If the screw came off American machinery, assume imperial until proven otherwise.
2. Worn keys round out sockets. A used long-arm hex key with a slightly bevelled tip will fit looser than a new one. The looser fit means the corners contact, not the flats — and the corners shear off the socket walls before they shear off the harder hex key. Replace bent or rounded keys before they damage your screws.

For a complete guide to Allen keys, including ball-end vs flat tip, T-handle vs L-handle, torque ratings, and how to choose a hex key set, see our Allen Key & Hex Key Guide.

Torque Values for Metric Socket Head Cap Screws

Torque is what converts a screw into a clamping force. Too little torque and the joint loosens under vibration. Too much torque and the screw yields, stretches, or snaps. The torque required is determined by the screw's grade, thread size, friction coefficient (lubricated vs dry), and the joint geometry.

The values in the table below are indicative dry-thread torques for general industrial use. They assume clean, dry threads with no lubricant or anti-seize. Reduce by approximately 15–20% if threads are oiled, or by 25% if anti-seize compound is applied. Always defer to the equipment manufacturer's specified torque if one is given — these table values are a default, not a substitute for engineering data.

Three things worth knowing about torque on socket head cap screws:

• Lubrication changes everything. A lubricated thread reduces friction by around 20% — which means the same torque produces 20% more clamping force. Apply the dry torque to a lubricated thread and you may yield the bolt. Apply the lubricated torque to a dry thread and you may not develop full preload.
• Re-used cap screws should not be re-torqued to the same value. A Class 12.9 screw that has been torqued to specification once is partially work-hardened and may have begun to yield. For critical joints, replace the screw rather than re-use it.
• The torque wrench must be calibrated. A miscalibrated wrench is worse than no torque wrench at all — it gives you false confidence in a wrong number. See our Torque Wrench Calibration Guide for calibration intervals and methods.

How to Install Socket Head Cap Screws Correctly

Socket head cap screws look simple to install — drop them in and tighten. But the failure modes are predictable, and almost all of them come from the same handful of installation errors.

Step 1 — Verify the screw matches the joint design

Confirm thread size, length, grade, and material against the assembly drawing or original part. If you are replacing a screw that has failed, replace it with the same grade or higher — never lower.

Step 2 — Inspect the threads

Run a finger over the threads. They should be clean and smooth — no burrs, no debris, no rust. A damaged screw should not be installed; a damaged thread in the parent material should be chased with a tap before fitting.

Step 3 — Lubricate where appropriate

• Stainless threads: always apply anti-seize or a thread lubricant. Galling is otherwise inevitable.
• Carbon steel threads in dry indoor environments: light oil or running thread sealant if vibration is a concern. A small amount of thread-locking compound may be specified — see our Thread Locking & Sealing Guide.
• Hot, food-grade or pharmaceutical environments: use a food-grade or high-temperature anti-seize as specified.

Step 4 — Add the correct washer

Always use a washer under the head where vibration is a possibility, where the bearing surface is soft (aluminium, plastic), or where the screw must clamp through a slotted hole. Use a flat washer to spread load and protect the surface; use a spring washer or nylon-insert nut to resist vibration loosening. For a complete washer reference, see our Types of Washers Guide.

Step 5 — Engage the Allen key fully

Push the hex key fully down into the socket before applying torque. A partly-engaged key contacts only the upper portion of the socket and concentrates stress on the shallow walls. This is the single most common cause of stripped sockets — fitting the key under load instead of seating it first.

Step 6 — Tighten in the correct sequence

For multi-bolt joints (gearbox covers, machine bases, flange connections), tighten in a star or cross-pattern sequence to draw the joint down evenly. Never tighten one bolt fully before starting the next on a flange — uneven loading cocks the joint and can crack the casting. Three passes is standard: first pass to roughly 30% of final torque, second to 75%, third to full torque.

Step 7 — Use a calibrated torque wrench for critical joints

For high-strength engineered joints (Class 12.9 dies, gearbox bolts, structural fixings), torque every screw with a calibrated wrench. For non-critical applications, "tight" by feel may be acceptable — but document which joints are which in your maintenance procedure.

How to Remove a Stripped Socket Head Cap Screw

A stripped socket head cap screw — where the hex socket has rounded out and the Allen key spins freely inside — is one of the more common workshop frustrations. There are five removal methods, ordered from least invasive to last resort. Try them in this sequence; do not jump ahead. (For a broken or seized stud rather than a stripped cap screw — different geometry, different tool — see our Stud Extractor Guide.)

Method 1 — Increase grip in the existing socket

The first attempt should always be to grip the rounded socket better. Two field tricks work surprisingly often:

• Rubber band trick: push a wide rubber band into the socket, press the hex key firmly down through it, and turn slowly. The rubber fills the gap between the rounded socket walls and the hex key, increasing friction.
• Steel wool or aluminium foil: same principle — pack a small piece of steel wool or crumpled foil into the socket and engage the key through it.

This works in roughly 30% of cases — particularly where the socket is only lightly rounded.

Method 2 — Use a Torx bit one size larger

If the rubber band fails, the next move is a Torx (star) bit hammered into the socket. The Torx bit's points dig into the rounded hex walls and provide grip. Choose a bit one size larger than the original hex socket — for example, a T30 Torx for an M8 (6 mm hex) cap screw. Hammer the bit firmly into the socket with a soft-faced hammer until it seats, then turn with a wrench or impact driver. This works in another 30–40% of cases.

Method 3 — Apply penetrating oil and wait

If the screw is corroded into its thread (common on outdoor or wet-environment installations), the rounded socket may not be the only problem. Apply a quality penetrating oil — see our Penetrating Oil Guide — and wait 24 hours. Tap the head lightly with a hammer to vibrate the oil into the threads. Re-attempt Method 1 or 2 after the wait.

Method 4 — Drill out and use a screw extractor

Where the socket is fully destroyed and grip cannot be re-established, the next step is to drill a small pilot hole down the centre of the screw and drive a screw extractor (a left-hand tapered tool with reverse threads). The extractor bites into the drilled hole and turns the screw out as you turn the wrench anti-clockwise. Use a left-hand drill bit if you have one — sometimes the heat and reverse rotation alone will free the screw before the extractor is even needed.

Method 5 — Drill out completely or weld a nut

The last resorts:

• Drill out: with a series of progressively larger drill bits, drill the screw out completely until only the threaded shell remains in the parent material. The shell can then be picked out or re-tapped to a larger size.
• Weld a nut to the head: for cases where the head is still proud of the surface, weld a hex nut to the top of the cap screw head and turn the screw out using a spanner on the welded nut. The weld heat also helps break thread corrosion. This is a common shop technique on heavily seized cap screws.

Brands of Socket Head Cap Screw at AIMS Industrial

The full AIMS range of socket head cap screws is available at browse the AIMS Industrial socket head cap screw collection here. The key brands stocked, by application:

Bremick

Australian-owned fastener supplier — broad range of metric DIN 912 socket head cap screws in Class 8.8 zinc-plated and Class 12.9 black oxide. Reliable stock availability for general industrial work, sized M3 through M30. The default choice for most workshop and maintenance applications where quality and price both matter.

Hobson Engineering

Specialist fastener supplier with engineering-grade stock. DIN 912 cap screws in carbon steel and stainless, including 304 and 316 in metric and imperial sizes. Strong choice for precision engineering and applications where certified material and traceability are required.

Inox World

Stainless-only specialist — A2 (304) and A4 (316) socket head cap screws across the full metric size range. Used where corrosion resistance is the primary requirement: marine, food processing, pharmaceutical, and outdoor coastal applications. Proper stainless property class marking on every part.

SOKO

European-manufactured high-quality socket head cap screws, particularly strong in Class 12.9 black oxide for precision engineering. Used where consistent metallurgy and dimensional accuracy matter — die work, jig and fixture building, gearbox manufacture.

Bumax

Swedish high-strength stainless specialist. Bumax 88 and Bumax 109 grades provide tensile strength approaching Class 8.8 and 10.9 carbon steel respectively, in a fully stainless body. Used in offshore, defence, motorsport, and any application where standard 316 lacks the strength and carbon steel cannot survive the environment. Specified by name on engineering drawings.

For full stock availability, sizes, and pricing across all five brands: browse the AIMS Industrial socket head cap screw collection. For pairing with the right nut, see our Types of Nuts Guide; for the right washer, see our Types of Washers Guide.

Frequently Asked Questions

What is a socket head cap screw?

A socket head cap screw is a high-strength precision fastener with a cylindrical head and an internal hex (Allen) socket drive. It is also called an Allen bolt, cap screw, or socket bolt. Manufactured to DIN 912 (or the equivalent ISO 4762), it is used wherever a low-profile head, high-strength clamping, or recessed installation is required — machine bases, gearbox covers, dies, jigs, and engineered joints.

What is a socket head cap screw also known as?

In Australian workshops, the most common names are "Allen bolt", "cap screw", "Allen head screw", and "socket bolt". On engineering drawings and parts lists, you will see "socket head cap screw", "SHCS", "DIN 912", or "ISO 4762". All terms refer to the same fastener — a cylindrical-head screw driven by a hex (Allen) key.

What is the difference between a socket head cap screw and a hex bolt?

A socket head cap screw has a cylindrical head with an internal hex socket — driven by an Allen key from above. A hex bolt has a six-sided external head — driven by a spanner or socket from the side. Socket head cap screws fit into recessed or counterbored holes where a spanner cannot reach, and are typically supplied at higher property classes (Class 12.9 standard). Hex bolts are most commonly Class 8.8 or 10.9 and require side clearance for the spanner.

What does DIN 912 mean on a fastener?

DIN 912 is the German national standard that defines the dimensions, tolerances, and material properties of socket head cap screws — head diameter, head height, hex socket size across flats, thread tolerance, and grade designations from M1.6 through M64. It is the most widely cited socket head cap screw standard in industrial supply. ISO 4762 is the equivalent international standard and is dimensionally compatible with DIN 912.

How do I measure a socket head cap screw?

Five measurements identify a socket head cap screw: thread diameter (e.g. M8), thread pitch (typically coarse, 1.25 mm for M8), length (measured from under the head to the end of the thread, NOT including the head), head diameter (across the cylindrical body), and hex socket size (across flats). The standard parts-list format is "M[size] × [length] SHCS, Class [grade], [material]" — for example, "M10 × 40 SHCS, Class 12.9, black oxide".

What is the difference between Grade 8.8, 10.9 and 12.9 socket head cap screws?

The two-part grade number indicates strength. The first digit relates to ultimate tensile strength in 100-MPa units; the second relates to the yield-to-tensile ratio. Class 8.8 has 800 MPa tensile, 640 MPa yield. Class 10.9 has 1040 MPa tensile, 940 MPa yield. Class 12.9 has 1220 MPa tensile, 1100 MPa yield. Class 12.9 is the standard grade for socket head cap screws and is the engineering default — never substitute a lower grade without re-engineering the joint.

Can I use a stainless socket head cap screw instead of a steel Class 12.9?

Not as a direct substitute. Standard 304 (A2-70) and 316 (A4-70) stainless socket head cap screws have tensile strength around 700 MPa — closer to Class 8.8 carbon steel than Class 12.9. Replacing a Class 12.9 with stainless reduces clamping capacity by approximately 40%, which can cause vibration loosening, joint fatigue, or failure. For high-strength stainless applications, specify Bumax 88 (≈ Class 8.8 strength) or Bumax 109 (≈ Class 10.9 strength) — both available at AIMS Industrial.

What size Allen key do I need for an M8 socket head cap screw?

An M8 socket head cap screw to DIN 912 takes a 6 mm Allen key (hex key) across flats. Other common metric sizes: M3 = 2.5 mm, M4 = 3 mm, M5 = 4 mm, M6 = 5 mm, M8 = 6 mm, M10 = 8 mm, M12 = 10 mm, M16 = 14 mm, M20 = 17 mm. Always use the correctly sized key — undersized or worn keys round out the socket. Imperial socket head cap screws use a different table and require imperial hex keys.

What is the torque spec for an M10 socket head cap screw?

For an M10 Class 12.9 socket head cap screw, indicative dry torque is approximately 84 Nm. For Class 10.9, around 72 Nm. For Class 8.8, around 49 Nm. These are dry-thread values — if the threads are lubricated or have anti-seize applied, reduce torque by approximately 15–25% to avoid over-stressing the fastener. Always defer to the equipment manufacturer's specified torque if one is given.

What is the difference between a cap head and a button head socket screw?

A cap head (DIN 912) has a tall cylindrical head and deep hex socket — designed for maximum strength and high-torque applications, the standard SHCS form. A button head (DIN 7380 / ISO 7380) has a low-profile rounded head and shallower socket — used where head clearance is limited or where a softer cosmetic finish is preferred. Button heads have approximately 30–40% lower torque rating than cap heads. Specify cap head for engineered joints; specify button head only where clearance or appearance matters more than maximum torque.

How do I remove a stripped socket head cap screw?

Try methods in order, starting least invasive: (1) pack a rubber band, foil or steel wool into the rounded socket and re-engage the Allen key for additional grip; (2) hammer a Torx (star) bit one size larger than the hex socket into the head — the points bite into the rounded walls; (3) apply penetrating oil and wait 24 hours if corrosion is suspected; (4) drill a pilot hole and drive a screw extractor with a tap wrench; (5) for the most severe cases, drill out the screw entirely or weld a hex nut to the head and turn out with a spanner. The most common prevention: use the correct hex key size, replace worn keys, and never mix metric and imperial drivers.

What is the difference between Grade 304 and Grade 316 stainless socket head cap screws?

Grade 304 (A2) stainless contains chromium and nickel — suitable for general indoor use, food processing without chlorides, and most outdoor applications away from salt water. Grade 316 (A4) adds molybdenum, providing resistance to chloride attack — required for marine work, coastal industrial sites, chlorinated swimming pools, food processing brines, and chemical environments. Grade 316 is approximately 30% more expensive than 304. For any AU coastal application within 1 km of the surf, specify 316.