Grub Screw Guide: Types, Drive Styles & Sizing Explained

Socket set screws — called grub screws in most Australian workshops — are among the most widely used fasteners in industrial and trade settings, and among the least understood. They are everywhere: locking pulleys to shafts, securing shaft collars to positioning rods, holding door levers to spindles, fixing mirror brackets to wall studs. Despite this, most tradespeople and engineers select them by habit rather than specification, grabbing whatever is in the parts bin rather than matching the point type, material, and thread form to the job.

That habit works until it doesn't. A cup point socket set screw in a rotating shaft application will eventually fret and loosen under cyclic load where a dog point would have held. A standard alloy steel grub screw in a stainless shaft assembly will corrode and seize. A metric socket set screw in a BSW-tapped hole will cross-thread and strip. These failures are preventable with a basic understanding of how socket set screws work and how to select them correctly.

This guide covers the complete picture: what socket set screws are and how they work, the point type options and when to use each, drive styles, materials, metric and imperial thread systems, sizing, installation, and how to deal with the most common failure mode — the stripped socket.

Socket Set Screw, Grub Screw, Set Screw — What's the Difference?

These three terms all refer to the same fastener type, but they come from different contexts and carry slightly different meanings depending on where you are.

"Socket set screw" is the precise technical product name used in Australian industrial supply. It tells you two things: the fastener is a set screw (fully threaded, headless, used to secure one component against another without a nut), and it is driven by a socket — specifically, a hex (Allen) socket, Torx socket, or square socket in the head. This is the term you will find on AIMS product labels, engineering drawings, and standards documents.

"Grub screw" is the colloquial Australian and British term for the same fastener. It is what tradespeople, maintenance fitters, and most workshops call them. The term has no agreed etymology, but its use is consistent throughout Australia and the UK. If you ask a fitter for a grub screw, they will hand you a socket set screw. The two terms are interchangeable in practice.

"Set screw" is the American term. In US engineering and industrial supply, a set screw (or "setscrew") is exactly what Australians call a grub screw or socket set screw. In some older British and Australian usage, "set screw" could refer to a headed screw used as a locking fastener, which creates occasional confusion — but in modern Australian industrial supply, "set screw" and "socket set screw" are used interchangeably.

In this guide: "socket set screw" is used as the technical term; "grub screw" is used as the shorthand where appropriate. Both are correct in an Australian context.

How Socket Set Screws Work

A socket set screw is fully threaded from tip to top, with no head projecting above the surface it is threaded into. It engages a pre-tapped hole in one component — the collar, hub, or housing — and bears down on a second component — the shaft, surface, or flat — through the action of its tip (point). The threaded engagement holds the screw in place; the point transmits the clamping or locking force to the shaft or surface below.

The mechanism is friction and compression. As the socket set screw is tightened, the point presses into or against the shaft surface. The threads pull the screw upward while the point presses down, creating a clamping force that locks the collar or hub to the shaft. This is not a shear connection — the screw is not taking the load in shear like a bolt through a flange. It is a friction/indentation lock. The holding force comes from the interface between the point and the shaft, not from the screw body.

The implications of this are important:

• A socket set screw with a worn or rounded point has significantly reduced holding force, even if it appears fully tightened
• The point type determines whether the connection indents the shaft surface (cup, cone), sits flat on it (flat point), or engages a machined feature (dog point into a flat or hole)
• Vibration and cyclic loading work against the friction lock — thread locking compound is often needed for grub screws in dynamic applications
• An over-tightened cup point will permanently indent the shaft; this is sometimes intentional (positive location) and sometimes a problem (damaged shaft, difficulty repositioning)

Because socket set screws are driven by a hex key inserted into the socket in the top of the screw (which sits flush with or below the component surface), they provide a clean, unobtrusive fastening — no protruding head to snag or interfere with adjacent components or guards. This is why they are used where space is constrained and where a flush finish is required.

Point Types: The Most Important Selection Decision

The point type is the most consequential choice when specifying a socket set screw. It determines how the screw engages the shaft or surface, what holding force it develops, whether it damages the shaft surface, and whether it can be repositioned after tightening. Most engineers and tradespeople default to cup point without considering the alternatives — this is often the right choice, but not always.

Cup Point

Cup point is the most common socket set screw point type. The tip has a shallow, circular cupped cavity surrounded by a sharp annular rim. When tightened against a shaft or surface, the rim bites into the material, creating a circular indentation that provides positive mechanical location in addition to friction. The cup point delivers high holding force for its size and resists axial and rotational movement under load.

The trade-off is shaft marking. A fully tightened cup point will leave a visible and palpable ring indent in the shaft. On a hardened shaft this indent is slight; on a soft shaft it can be pronounced. This is generally acceptable in fixed-position applications — where the hub or collar is set once and not repositioned. Where repositioning along the shaft is likely, cup point causes progressive surface damage that can affect shaft seating accuracy over time.

Use cup point for: Fixed shaft/hub locking where shaft marking is acceptable, shaft collars in set positions, sprocket and gear hub retention, general industrial applications where repositioning is unlikely. This is the go-to choice for the majority of socket set screw applications.

Flat Point (Plain Point)

Flat point socket set screws have a flat, ground tip — no raised rim, no indent geometry. The flat end bears against the shaft surface over a broader contact area than cup point, which distributes the load rather than concentrating it at the rim. The flat point does not significantly indent soft shaft materials, which makes it preferable where shaft surface integrity matters or where the screw must not damage a polished or plated surface.

The holding force of a flat point is lower than cup point at the same torque because there is no mechanical interlock from shaft indentation. The connection is purely frictional. Flat points are also used on the end of adjustment screws and pressure pads where the flat face needs to transmit thrust without rotation or side load.

Use flat point for: Locking against finished or plated surfaces where marking is unacceptable, adjustment screws bearing against hardened pads, applications where the component must be repositioned without shaft damage, and as a thrust/pressure point on adjustment assemblies.

Oval Point

Oval point has a convex, rounded dome tip — partway between flat point and cone point. The rounded tip makes light contact with the shaft surface across a small area, produces minimal shaft marking, and seats well on curved or uneven surfaces. It is forgiving of slight angular misalignment between the screw axis and the shaft.

Oval point is less common in standard industrial catalogues than cup or flat, but is useful in fine adjustment applications where a low-friction, low-marking point is needed and where the screw will be adjusted frequently. The rounded tip slides more easily over the shaft surface during adjustment than a flat or cup point would.

Use oval point for: Fine adjustment screws requiring frequent repositioning, applications with curved contact surfaces, and where minimal shaft marking combined with reasonable friction retention is needed.

Cone Point

Cone point has a sharp conical tip designed to be used with a matching conical indent (centre punch mark or drilled dimple) on the shaft. The cone seats into the indent, providing positive location that resists both axial and rotational displacement. Once seated, a cone point grub screw provides higher resistance to rotation than cup point because the engagement is a three-dimensional taper fit rather than a flat rim bite.

The limitation is that the cone point is only fully effective with a matching indent on the shaft. Without the indent, the cone point contacts the shaft on its tip only, which concentrates load on a very small area and can gouge or scratch hardened shafts. Cone point is also permanent in the sense that the shaft dimple becomes the location reference — repositioning to a new location requires a new dimple.

Use cone point for: Permanent or semi-permanent locking into a pre-punched or drilled dimple on the shaft, applications requiring maximum resistance to both axial and rotational displacement, and where the location point on the shaft needs to be defined precisely. Common in precision instruments and spindle applications.

Dog Point

Dog point has a cylindrical pilot projection extending from the tip, smaller in diameter than the screw body. This pilot engages a mating hole or flat ground on the shaft, providing a positive mechanical connection that is significantly stronger in shear than a friction-only cup or flat point connection. The dog point effectively acts as a key — the pilot enters a cross-drilled hole or an axial flat on the shaft and physically prevents rotation of the hub or collar relative to the shaft.

Dog point socket set screws are the correct choice for rotating applications under significant torque — gear hubs, sprocket drives, coupling flanges — where a cup point friction connection would loosen under cyclic load. The pilot diameter is standardised to match common shaft flat dimensions. Dog points require more preparation than other point types (a cross-hole or flat must be machined on the shaft) but provide a mechanically superior connection for demanding applications.

Use dog point for: Rotating shaft/hub connections under torque load, coupling and drive applications where a friction connection is insufficient, applications where the hub must be locked positively against rotation and axial movement, and as a positive locating pin where the point engages a transverse hole.

Half Dog Point

Half dog point (also called half cone or stub dog) is a shortened dog point pilot — approximately half the standard dog point length. It is used where the shaft depth available for the pilot engagement is limited, or where a less aggressive mechanical interlock is acceptable. The shorter pilot provides positive location but less resistance to axial pull-out than a full dog point.

Use half dog point for: Applications with limited shaft engagement depth, where full dog point is specified but space dictates a shorter pilot, and as a cross-pin engagement screw in thinner-walled applications.

Knurled Cup Point

Knurled cup point has a cup-shaped tip with a knurled or serrated rim rather than a smooth rim. The serrations bite more aggressively into the shaft surface than a plain cup point, providing higher resistance to rotation under dynamic load. This increases holding force at the cost of more pronounced shaft surface marking.

Knurled cup is often specified in high-vibration environments where cup point retention has been found inadequate, and where the additional shaft indentation from the serrated rim is acceptable. AIMS stocks Soko M12 knurled cup point socket set screws in this configuration.

Use knurled cup point for: High-vibration applications requiring higher rotational resistance than plain cup, heavy rotating drive components, and applications where dynamic loads have caused plain cup points to loosen.

Point Type Summary Table

Point Type	Shaft Marking	Holding Force	Repositionable?	Best For
Cup	Moderate (ring indent)	High	Limited	General fixed-position shaft locking
Flat	Minimal	Moderate	Yes	Finished surfaces, adjustment screws
Oval	Very low	Moderate	Yes	Frequent adjustment, curved surfaces
Cone	High (requires dimple)	Very high	No	Permanent precision location
Dog	None (engages hole/flat)	Highest	No (requires prep)	Torque-loaded rotating shafts
Half Dog	None	High	No (requires prep)	Limited depth dog point engagement
Knurled Cup	High (serrated indent)	Very high	No	High-vibration rotating applications

Drive Styles

The drive style refers to the socket type in the top of the screw — the recess that accepts the Allen key or other drive tool. For socket set screws, the dominant drive style is hexagonal socket (Allen socket), which is why "hex key" and "grub screw" are so closely associated.

Hex Socket (Allen Drive)

Hexagonal socket is the standard drive for Australian socket set screws. A hex key (Allen key) is inserted into the socket and rotated to tighten or loosen the screw. The hex socket drive is compact, allows the screw to sit fully recessed below the component surface, and transmits high torque for its small footprint. The socket size is directly related to the screw diameter — see the sizing section for the hex key size per thread size.

For more on hex key types, sizes, and selection — including ball-end keys, T-handles, and the metric/imperial size chart — see our Allen Key & Hex Key Guide.

Torx Socket

Torx (star) drive socket set screws are available in some size ranges. Torx provides better torque transmission than hex socket at small sizes because the star geometry distributes load across six lobes rather than six flats, and is less prone to cam-out under high torque. Torx socket grub screws are more common in precision instrument and electronics applications where small screw sizes (M2–M4) are used and driver engagement is critical.

Slotted Head

Some older-pattern socket set screws use a straight slot rather than a socket drive, engaged by a flat-blade screwdriver. Slotted grub screws are largely obsolete in industrial applications — the torque that can be transmitted is low, cam-out risk is high, and the slot offers no advantage over hex socket. They appear in older British-standard applications and in some domestic hardware (furniture fittings, mirror fixings). Do not confuse with standard grub screws when ordering replacements.

Square Socket (Bristol/Bristo Drive)

Square socket or Bristol-pattern drive is found in some older American and British-standard socket set screws, particularly in larger imperial sizes. The square socket transmits high torque and was widely used before hex socket became dominant. Still encountered in legacy plant and equipment. If you find a grub screw with a square recess that your Allen keys won't fit, it is almost certainly a square-drive (Bristol) socket — Bristol key sets are available.

Materials and Grades

The material and grade of a socket set screw determines its hardness, strength, corrosion resistance, and suitability for the application environment. Selecting the wrong material is one of the most common and consequential errors in socket set screw specification.

High Grade Alloy Steel

High grade alloy steel is the standard material for industrial socket set screws. This covers the ISO property class 45H designation — a medium carbon alloy steel heat-treated to provide hardness suitable for grub screw applications. Class 45H socket set screws are significantly harder than standard grade 8.8 cap screws, which is necessary because the cup or cone point must be harder than the shaft material it is indenting. A soft point will deform on contact with a hardened shaft and lose its holding function.

High grade alloy steel socket set screws are typically supplied with a black oxide or plain (bright) finish. Black oxide provides minimal corrosion resistance (suitable for dry indoor applications with periodic lubrication) and is primarily a cosmetic and anti-galling treatment. Plain finish provides no corrosion protection. Neither is suitable for outdoor, marine, or chemical environments without additional protection.

Stainless Steel (304 and 316)

Stainless socket set screws are specified for applications requiring corrosion resistance — food processing equipment, marine and coastal environments, chemical plant, outdoor installations, and any environment where steel would corrode unacceptably. AIMS stocks both 304 and 316 stainless socket set screws.

The material grade matters:

• 304 stainless (A2): The general-purpose stainless option. Good corrosion resistance in most atmospheric and mildly aggressive environments. Not suitable for chloride-rich environments (coastal, marine, salt spray, chlorinated water systems) — 304 is susceptible to chloride pitting.
• 316 stainless (A4): Contains molybdenum, which significantly improves chloride resistance. The correct choice for marine, coastal, food processing (where CIP cleaning with chlorinated solutions is used), and chemical plant applications. Meaningfully more expensive than 304 but specified correctly in these environments.

Critical note on stainless strength: Austenitic stainless steel (304 and 316) in the annealed condition has lower yield strength than alloy steel socket set screws — approximately equivalent to a grade 4.6 or 5.8 bolt, not a class 45H set screw. Stainless socket set screws are softer than their alloy steel equivalents and should not be used against hardened shafts where the point is expected to indent the shaft material. The stainless point will deform before it indents a hardened shaft.

Galling risk: Stainless fasteners are susceptible to galling (cold welding) when threaded into stainless tapped holes. Stainless-on-stainless threading can seize with only moderate torque, permanently fusing the screw in place. If you are fitting a stainless socket set screw into a stainless tapped hole (stainless shaft collars, stainless housings), apply an anti-seize compound designed for stainless before installation. This is not optional in stainless-on-stainless applications.

Brass

Brass socket set screws are used in applications requiring non-magnetic, non-sparking, or electrically conductive properties — electrical equipment, instrumentation, explosive atmosphere environments, and applications where the screw must not damage soft shafts (brass is softer than most shaft materials, so it will deform before indenting the shaft). Brass is also used in decorative applications where the visible end of the screw needs to blend with brass fittings.

Brass has good corrosion resistance in atmospheric and freshwater environments but should not be used in contact with ammonia solutions or certain acids.

Nylon-Tipped

Nylon-tipped socket set screws have a standard alloy steel body with a nylon or plastic insert at the point. The nylon tip bears against the shaft instead of the steel point, providing a non-marring, electrically insulating interface. They are used in precision instruments and optical equipment where shaft marking is unacceptable, in electrical applications where the screw must not create a conductive path, and in applications where the shaft material is too soft to accept a metal point without damage.

The nylon insert is replaceable in some configurations. The holding force is lower than a metal point because the nylon deforms under load rather than interlocking with the shaft surface. Not suitable for high-torque or vibration-heavy applications.

Thread Systems: Metric and Imperial

Socket set screws in Australia are supplied in metric and three imperial thread systems. The correct thread system must match the tapped hole in the component you are assembling — thread systems are not interchangeable, and using a metric screw in an imperial hole (or vice versa) will cross-thread and damage both the screw and the tapped hole.

Metric

Metric is the default thread system for new plant, machinery, and fabrication in Australia. Metric socket set screws follow the ISO/DIN standard coarse thread pitch for each diameter. The standard range runs from M2 to M20, with M3 through M12 the most commonly stocked sizes in industrial supply. Standard coarse pitch is almost always correct for socket set screw applications — fine pitch metric grub screws exist but are uncommon and usually only specified in precision instrument applications.

BSW — British Standard Whitworth

BSW (British Standard Whitworth) is the old British imperial thread form that was standard in Australian manufacturing and plant from colonial settlement through to metrication in the 1970s. BSW uses a 55° thread form (distinct from the 60° thread form of metric and UNC/UNF) with thread pitches specified in threads per inch.

BSW socket set screws are still actively stocked and used in Australia because a large installed base of older British-origin plant, mining equipment, agricultural machinery, and marine equipment remains in service. If you are servicing pre-metrication machinery — particularly British-manufactured equipment from before approximately 1975 — you are likely to encounter BSW threads. Standard sizes in AU industrial supply run from ¼" to 1".

BSW threads are not interchangeable with UNC or UNF threads of the same nominal diameter. A ½" BSW bolt will not fit a ½" UNC nut. The thread pitch and form are different.

UNC — Unified National Coarse

UNC is the American imperial coarse thread standard, using a 60° thread form with threads per inch pitch specified for each diameter. UNC is the dominant imperial thread in American-specification machinery, equipment, and tooling, and is widely used in Australian industries with American equipment: mining, resources, oil and gas, agriculture (John Deere, Case IH, etc.), and imported American-brand industrial plant.

UNC socket set screws are the correct replacement when servicing American-spec equipment with imperial threads. Standard Australian industrial supply covers sizes from approximately ¼" to 1½".

UNF — Unified National Fine

UNF is the American imperial fine thread standard — more threads per inch than UNC at the same nominal diameter. The finer thread pitch provides higher thread engagement force per turn and better resistance to vibration loosening, at the cost of more turns to assemble and greater sensitivity to thread damage on installation.

UNF socket set screws are used in precision assemblies and where the standard UNC thread is specified as "fine" in the original equipment documentation. Less commonly stocked than UNC but available in the standard size range. AIMS stocks UNF in sizes including 7/16" and ½".

Quick Identification: Which Thread Do I Have?

If you need to identify an existing socket set screw's thread, the practical approach is:

1. Measure the outer (major) diameter with verniers. Metric sizes will be close to a whole millimetre (M6 = 6.0mm, M8 = 8.0mm). Imperial nominal sizes will be close to a fractional inch (½" = 12.7mm, 3/8" = 9.5mm).
2. If imperial, use a thread gauge or pitch gauge to count threads per inch. Compare against BSW, UNC, and UNF pitch charts for the relevant diameter — the pitch differs enough between systems to be distinguishable with a thread gauge.
3. When in doubt on older AU plant: check the machinery plate or manufacturer's specification. Pre-1975 British equipment is almost certainly BSW; post-1975 American equipment is almost certainly UNC/UNF; post-1975 Australian/European equipment is almost certainly metric.

Metric Sizing: Dimensions and Allen Key Reference

Metric socket set screws are specified by thread diameter (M-size) and length. Length is measured as the full screw body length from tip to top — because there is no head, the screw is entirely within the tapped hole when installed, and the length is simply the thread engagement depth.

Thread Size	Hex Key Size (AF)	Common Lengths (mm)	Typical Applications
M2	0.9mm	2, 3, 4, 5	Precision instruments, small mechanisms
M2.5	1.3mm	3, 4, 5, 6	Instruments, electronics
M3	1.5mm	3, 4, 5, 6, 8, 10	Small shaft collars, light duty
M4	2mm	4, 5, 6, 8, 10, 12	Shaft collars, light mechanical
M5	2.5mm	5, 6, 8, 10, 12, 16	General mechanical, small pulley hubs
M6	3mm	6, 8, 10, 12, 16, 20	Common industrial — shaft collars, couplings
M8	4mm	8, 10, 12, 16, 20, 25	Medium industrial — drive hubs, sprockets
M10	5mm	10, 12, 16, 20, 25, 30	Medium-heavy drive components
M12	6mm	12, 16, 20, 25, 30, 35	Heavy industrial shafts and hubs
M16	8mm	16, 20, 25, 30, 35, 40	Large shaft locking
M20	10mm	20, 25, 30, 35, 40	Heavy machinery shafts

The hex key size relationship: For metric socket set screws, the hex socket size (across-flats, AF) is approximately half the thread diameter — M6 takes a 3mm key, M8 takes a 4mm key, M10 takes a 5mm key. This is a useful rule of thumb but not universally precise at small sizes (M2, M2.5, M3). When in doubt, use the table above or check the manufacturer's specification.

For the full hex key size reference across metric and imperial, including ball-end key dimensions and long-arm key sizes, see our Allen Key & Hex Key Guide.

Imperial Sizing Reference

Thread	System	Nominal Diameter	Hex Key Size	Common Context in AU
3/16"	BSW / UNC	4.76mm	3/32"	Light British/American equipment
1/4"	BSW / UNC / UNF	6.35mm	1/8"	Common — legacy AU plant, American equipment
5/16"	BSW / UNC / UNF	7.94mm	5/32"	Common — drives, shaft collars
3/8"	BSW / UNC / UNF	9.53mm	3/16"	Common — medium shafts
7/16"	BSW / UNC / UNF	11.11mm	7/32"	Medium — American equipment
1/2"	BSW / UNC / UNF	12.70mm	1/4"	Common heavy — conveyors, drives
5/8"	BSW / UNC	15.88mm	5/16"	Heavy machinery
3/4"	BSW / UNC	19.05mm	3/8"	Large shaft locking
1"	BSW / UNC	25.4mm	1/2"	Heavy plant
1-1/2"	UNC	38.1mm	3/4"	Heavy American-spec plant

Note on BSW vs UNC at the same nominal size: BSW and UNC share the same nominal diameter (both use fractional inch designations) but have different thread pitches and thread forms. A ½" BSW has 12 TPI; a ½" UNC has 13 TPI. A ½" UNF has 20 TPI. They will not interchange. Always verify the thread system before ordering replacements.

Applications

Shaft and Hub Locking

The most common industrial application for socket set screws is locking a hub, collar, or sleeve to a shaft — preventing both axial movement (along the shaft) and rotational movement (around the shaft). This covers: sprocket and timing pulley hubs, coupling halves, shaft collars used as mechanical stops, encoder and sensor mounting collars, impeller hubs on pumps, and fan hub assemblies.

For static applications with light to moderate load, a cup point socket set screw provides adequate holding force. For rotating applications under significant torque — drive sprockets, coupling flanges, high-speed pulleys — dog point into a machined flat or cross hole provides a mechanically stronger connection. Two socket set screws offset 90° or 120° around the collar circumference distribute the load and reduce the risk of the collar walking around the shaft.

Shaft Collars

Shaft collars are a specific and important application. There are two collar types: set-screw collars (one or two socket set screws through the collar bore clamping against the shaft) and clamp collars (the collar is split and compressed around the shaft by cap screws tightening the split gap). Set-screw collars are simpler and less expensive; clamp collars distribute load more evenly around the shaft circumference and are preferred for precision positioning and for shafts where surface damage is unacceptable.

For set-screw shaft collars, cup point is standard. Dog point into a machined flat is used where higher axial and rotational resistance is required. The shaft collar is a common context where the socket set screw is doing the entire job of locating and locking the collar — the screw selection directly determines whether the collar stays put under load.

Door Hardware and Domestic Fittings

Door lever handles, knobs, and pull handles are almost universally locked to their spindles with one or two socket set screws — the small hex socket screw you find on the underside or back of the handle rose or on the handle shank. These are typically metric M4 or M5 in residential hardware, and M6 in commercial hardware. When a door lever loosens or spins on its spindle, a stripped or loose grub screw is the first thing to check.

Bathroom and kitchen tapware uses socket set screws to lock handles to valve spindles. Stainless steel grub screws are often specified here to prevent corrosion in wet environments.

Mirror and Shelf Bracket Fixings

Frameless mirror mounting systems, shelf bracket systems, and some rail mounting hardware use socket set screws to clamp components to mounting rods or rails. These are typically small metric sizes (M4–M6) with flat or cup point, where the screw must hold the component in a set position on the rod without damaging the rod surface excessively.

Electronics and Instrument Enclosures

Panel mount connectors, BNC and SMA RF connectors, instrument shaft encoders, and potentiometers often use very small socket set screws (M2–M3) to lock components to shafts or to secure covers. Torx or hex socket drive at these small sizes. Nylon-tipped or brass point types are common where shaft damage must be avoided and where electrical isolation between the screw and the shaft is required.

Installation: Getting It Right

Check Thread System and Size First

Before installing any socket set screw, confirm the thread system (metric, BSW, UNC, or UNF) and the nominal diameter match the tapped hole. If a screw starts easily by hand for the first two or three turns and then suddenly becomes stiff, stop — this is the symptom of a thread mismatch. Forcing a mismatched screw will cross-thread and damage the tapped hole. The correct fit is smooth hand threading for the full depth.

Hex Key Quality and Size

Using a worn, undersized, or wrong-system hex key is the single most common cause of stripped sockets. A metric 3mm key in a 3mm metric socket sets correctly; an imperial 1/8" key (3.175mm) is slightly too large and will not seat fully, creating corner contact that rounds out the socket when torque is applied. Always verify metric vs imperial before applying force.

Quality hex keys with hardened tips and accurate dimensions make a significant difference to socket longevity, particularly at small sizes (M3–M6) where the socket wall is thin. A chrome vanadium or S2 steel hex key will transmit full torque without deforming; a cheap key will round its own corners before rounding the socket. Ball-end hex keys are convenient for reaching at angles but should only be used to start and run down the screw — apply final tightening torque with the straight end fully seated, not the ball end, which contacts the socket at an angle and transfers torque less efficiently.

See our Allen Key & Hex Key Guide for a full breakdown of key types, sizes, and selection for different applications.

Tightening Torque

Socket set screws should be tightened to the torque value specified for the thread size and grade. Over-tightening a cup point in a soft shaft will indent the shaft excessively; over-tightening in a hard shaft can shear the screw. Under-tightening will allow the connection to loosen under vibration or load. Indicative tightening torques for class 45H alloy steel socket set screws:

Thread Size	Torque (Nm)
M3	0.5–0.8
M4	1.2–1.5
M5	2.0–2.5
M6	3.5–4.5
M8	9–12
M10	18–22
M12	30–38

These are indicative figures for alloy steel screws. Stainless socket set screws should be tightened to lower values (approximately 70–80% of the alloy steel torque) to reduce the risk of galling.

Thread Locking Compound

Socket set screws in vibrating machinery should be secured with a thread locking compound to prevent loosening. Loctite 243 (medium strength, blue) is the standard choice for most socket set screw applications — it allows disassembly with hand tools when needed. Apply a single drop of thread locker to the thread before installation and tighten immediately; do not allow to cure before tightening. Full cure strength is reached after approximately 24 hours at room temperature.

Loctite 271 (high strength, red) is used where the set screw must never loosen — precision position-critical applications — but requires heat (approximately 230°C) for disassembly. Use 243 unless permanent lock is specifically required.

Do not use thread locking compound on stainless-on-stainless assemblies without also applying anti-seize — the combination of galling risk and thread locker can make a stainless set screw effectively permanent without heat.

Removing a Tight or Stripped Socket Set Screw

The stripped socket is the most common grub screw problem encountered in practice. Once a socket rounds out, the hex key turns without engaging the socket walls and the screw cannot be turned. This happens most often from: using a worn or incorrect key, applying torque at an angle with a ball-end key, using a metric key in an imperial socket or vice versa, or simply applying too much force on a small socket.

Step 1 — Check the other system first. If a metric key rounds out at a given size, try the next imperial size (or vice versa). A moderately stripped M5 socket may respond to a 5/32" imperial key (3.97mm) which is slightly smaller than the worn metric socket opening and can bite on remaining material. This is the simplest fix and works more often than expected.

Step 2 — Apply penetrating oil and heat. If the screw is seized in addition to being stripped, apply penetrating oil (CRC, WD-40 Specialist Penetrant) to the thread area and allow to soak. For steel screws in steel or aluminium, a brief application of heat from a soldering iron or heat gun to the surrounding material will cause thermal expansion that can break the thread seizure. Do not use an open flame near thread locking compounds or lubricants.

Step 3 — Use a diamond-tipped or knurled hex key. Some manufacturers produce hex keys with a diamond-coated or knurled working surface specifically for extracting rounded sockets. The abrasive surface bites into partially rounded socket walls and can transmit enough torque to turn the screw. Try this before drilling.

Step 4 — Torx key in a stripped hex socket. Selecting a Torx key one size up from the socket dimensions and tapping it lightly into the stripped hex socket with a small hammer can create enough engagement to turn the screw. The Torx star geometry bites into the remaining socket material.

Step 5 — Screw extractor. Left-hand spiral extractors (EZ-Out type) can be driven into the stripped socket with a centre punch or small hammer and then turned anticlockwise with a tap wrench or socket. As the extractor bites the socket walls and is turned, it simultaneously loosens the screw. This works well on screws that are not fully seized.

Step 6 — Drill out. If all else fails, the screw body must be drilled out, leaving the thread in the housing intact. Use a drill bit slightly smaller than the screw's minor (root) diameter to remove the screw body without damaging the thread. After removing the body, the remaining thread can often be wound out with a dental pick or sharp probe. This is the most reliable method of last resort but requires patience and accurate drilling to avoid destroying the tapped hole.

Socket Set Screw Selection Guide

Application	Recommended Point	Material	Thread System	Notes
Fixed shaft collar, general use	Cup point	Alloy steel	Metric (new plant)	Standard choice for most applications
Rotating drive hub under torque	Dog point	Alloy steel	Metric / UNC	Machine flat or cross-hole on shaft required
High-vibration rotating application	Knurled cup	Alloy steel	Metric	Use thread locker (Loctite 243)
Precision instrument location	Cone point	Alloy steel or brass	Metric	Pre-punch or drill dimple on shaft
Finished surface, no shaft marking	Flat point	Brass or alloy steel	Metric	Lower holding force — verify adequacy
Soft shaft, no marking allowed	Nylon-tipped	Alloy steel (nylon tip)	Metric	Reduced holding force
Door hardware / domestic fitting	Cup point	Zinc plated steel or SS	Metric (M4–M6)	Replace with stainless in wet areas
Food processing / wash-down	Cup or flat	316 stainless	Metric	Anti-seize on SS-on-SS assembly
Marine / coastal environment	Cup or flat	316 stainless	Metric or BSW	316 not 304 in chloride environments
Legacy British plant (pre-1975 AU)	Cup point	Alloy steel	BSW	Verify with thread gauge before ordering
American-spec machinery	Cup point	Alloy steel	UNC or UNF	UNF for fine-thread specification
Explosive / non-sparking environment	Cup or flat	Brass	Metric	Verify Ex classification requirements

Frequently Asked Questions

What is a socket set screw?

A socket set screw — commonly called a grub screw in Australia — is a fully threaded, headless fastener used to secure one component against another without a nut. It threads into a tapped hole in one component (a collar, hub, or housing) and presses its point against a second component (typically a shaft or surface), locking the two together through friction and point engagement. Because there is no projecting head, the screw sits fully flush with or below the surface of the component it is threaded into. The "socket" in the name refers to the hex, Torx, or square recess in the top face that accepts the drive key.

What is the difference between a grub screw and a socket set screw?

Nothing — they are the same fastener. "Socket set screw" is the precise technical product name used in Australian industrial supply. "Grub screw" is the colloquial Australian and British term for the same thing. Both terms are in common use in Australian workshops and on engineering drawings. The American equivalent term is "set screw."

What is the most common grub screw point type?

Cup point is the most common point type for general industrial socket set screw applications. The cup point has a sharp annular rim that bites into the shaft surface on tightening, creating both a friction lock and a mechanical indentation that resists axial and rotational movement. It provides a good balance of holding force, ease of installation, and availability across all sizes and thread systems. Dog point is specified when a cup point friction connection is insufficient — typically in rotating drive applications under significant torque.

What size Allen key do I need for a grub screw?

For metric socket set screws, the hex key size is approximately half the thread diameter — an M6 takes a 3mm key, an M8 takes a 4mm key, an M10 takes a 5mm key, an M12 takes a 6mm key. This ratio is a reliable guide for M4 and larger. At smaller sizes (M2, M2.5, M3), check the table rather than assuming the half-diameter rule. For imperial socket set screws, the hex key size is specified in fractional inches — a ½" BSW or UNC set screw typically takes a ¼" hex key. Always verify metric vs imperial before applying torque — using a metric key in an imperial socket (or vice versa) is the most common cause of stripped sockets. For full hex key sizing across all systems, see our Allen Key & Hex Key Guide.

What is the difference between BSW, UNC, and metric socket set screws?

These are three different thread systems that are not interchangeable. Metric uses the ISO thread form (60° thread angle, pitch in mm). BSW (British Standard Whitworth) uses a 55° thread form with pitch in threads per inch — found in older British and Australian-heritage plant and equipment. UNC (Unified National Coarse) uses a 60° thread form with pitch in threads per inch — the standard American imperial thread, used in American-specification machinery. Screws from one system will not thread correctly into a hole tapped for another system, even at the same nominal diameter. When replacing a socket set screw, always confirm the thread system before ordering.

Should I use stainless steel socket set screws?

Use stainless where corrosion resistance is required — food processing, marine, coastal, wash-down environments, and outdoor installations. Select 316 stainless for chloride-rich environments (coastal, salt spray, CIP cleaning with chlorinated solutions); 304 is adequate for general atmospheric and mild environments. Two important limitations: first, stainless socket set screws are softer than alloy steel equivalents and should not be used against hardened shafts where the point must indent the shaft material. Second, stainless-on-stainless thread assemblies are susceptible to galling (cold welding) — always apply anti-seize compound designed for stainless when threading a stainless set screw into a stainless tapped hole.

When should I use a dog point instead of a cup point?

Use a dog point when the connection must resist torque or axial load that exceeds what a cup point friction connection can reliably hold. The dog point has a cylindrical pilot that engages a machined flat or cross-hole on the shaft, providing a positive mechanical interlock rather than a friction-only connection. This is the correct specification for rotating drive hubs — sprockets, pulleys, coupling flanves — under significant transmitted torque, where a cup point set screw may loosen over time under cyclic load. Dog point requires a matching machined feature on the shaft (a flat or a drilled hole to suit the pilot diameter); it cannot be used on an unmodified round shaft.

Can I use thread locker on socket set screws?

Yes, and it is recommended in vibrating machinery applications. Loctite 243 (medium strength, blue) is the standard choice — it prevents vibration loosening and allows disassembly with hand tools when needed. Apply a single drop to the thread before installation. For permanent locking where the screw should never loosen, Loctite 271 (high strength, red) can be used, but requires heat for disassembly. For stainless-on-stainless assemblies, apply anti-seize first and then thread locker on top if vibration resistance is needed — do not rely on thread locker alone to prevent galling in stainless assemblies.

Why does my grub screw keep coming loose?

The most common causes of socket set screw loosening are: vibration in the assembly without thread locking compound; insufficient tightening torque on initial installation; a worn or rounded cup point that has lost its shaft indentation and provides only residual friction; and a cup point on a shaft that is too hard for the point to indent (giving only metal-to-metal contact without bite). The fix for vibration loosening is Loctite 243. The fix for a worn point is replacement. The fix for a hard shaft with no bite is to switch to a dog point with a machined engagement feature, or to use knurled cup point which bites more aggressively than plain cup.

How do I remove a stripped socket set screw?

Work through these options in order: (1) Try the other thread system's key — a slightly smaller imperial key in a stripped metric socket (or vice versa) can bite remaining socket material. (2) Apply penetrating oil and, if the screw is seized, heat the surrounding material with a soldering iron or heat gun to break thread seizure. (3) Use a diamond-tipped or knurled hex key designed for stripped socket extraction. (4) Drive a Torx key slightly larger than the socket into the rounded recess and turn anticlockwise — the Torx star geometry bites into remaining material. (5) Use a left-hand spiral screw extractor driven into the socket. (6) Drill out the screw body with a bit slightly smaller than the thread minor diameter, then pick out the remaining thread material. Taking the time to use the correct key at the correct size prevents stripped sockets — most stripping is caused by metric/imperial mix-up or worn keys.

What grade are standard socket set screws?

Standard alloy steel socket set screws are supplied to ISO property class 45H, which specifies a minimum Vickers hardness of 45 HRC. This is significantly harder than standard structural bolts (8.8 grade has approximately 24 HRC equivalent) because the point of the set screw must be hard enough to indent or bear against shaft materials without deforming. Stainless steel socket set screws are supplied to A2-70 (304 SS) or A4-70 (316 SS) — equivalent to approximately 23 HRC, which is considerably softer than class 45H alloy steel. Stainless set screws should not be used in applications where the point must penetrate a hardened shaft surface.

What is a knurled cup point socket set screw?

A knurled cup point socket set screw has a cup-shaped point with a serrated or knurled rim instead of a smooth rim. The serrations bite more aggressively into the shaft surface on tightening than a plain cup rim, increasing resistance to rotational displacement under dynamic load. This makes knurled cup point the preferred choice in high-vibration applications or where a plain cup point connection has been found to loosen under operating conditions. The trade-off is more pronounced shaft surface marking than plain cup point.

Our Tap Types guide covers every cutting and forming tap variant with material-specific selection rules.