Metric Bolt Size Guide: M3 to M24 Dimensions, Thread Pitches & AU Standards

Metric bolt sizes use the format M[diameter] x [pitch] x [length] — for example M12 x 1.75 x 50 means a 12mm diameter bolt with a 1.75mm coarse thread pitch, 50mm long. The "M" number is always the nominal outer (major) thread diameter in millimetres. Common sizes run from M3 (3mm) for small fittings up to M24 (24mm) for heavy structural work.

Metric Bolt Size Chart — M3 to M24 Quick Reference

Size	Major Diameter (mm)	Coarse Pitch (mm)	Fine Pitch (mm)	Spanner / AF (mm)
M3	3.0	0.50	0.35	5.5
M4	4.0	0.70	0.50	7
M5	5.0	0.80	0.50	8
M6	6.0	1.00	0.75	10
M8	8.0	1.25	1.00	13
M10	10.0	1.50	1.25	17 (16 in older DIN)
M12	12.0	1.75	1.50 (or 1.25)	19 (18 in older DIN)
M14	14.0	2.00	1.50	22 (21 in older DIN)
M16	16.0	2.00	1.50	24
M18	18.0	2.50	1.50	27
M20	20.0	2.50	1.50	30
M22	22.0	2.50	1.50	32 (34 in older DIN)
M24	24.0	3.00	2.00	36

Pitch is coarse unless the size is specified with a pitch value — e.g. plain "M12" means M12 x 1.75 coarse, while "M12 x 1.25" calls up the fine-pitch variant. Spanner sizes shown are current ISO 4014 / ISO 4017 across-flats (AF) dimensions; some older DIN 933/931 stock uses the values in brackets, which is why an old M10 spanner from a granddad's toolbox often measures 16mm instead of 17mm.

Understanding Metric Bolt Sizing — What "M" Means

The "M" prefix stands for metric — specifically the ISO metric thread system covered by ISO 261 (general purpose metric threads) and ISO 262 (selected sizes for commercial fasteners). The number that follows the M is always the nominal major diameter in millimetres — the diameter measured across the outermost crest of the thread.

An M12 bolt has a 12mm major diameter. An M20 has a 20mm major diameter. The shank diameter (the smooth section under the head on a partially threaded bolt) is the same as the major diameter — they are nominally identical, give or take a few hundredths of a millimetre for tolerance class.

The M number does not tell you:

• Length — that's a separate measurement, in millimetres, called out after the pitch
• Pitch — coarse is the default; fine pitch must be explicitly specified
• Head type — hex, socket cap, button, countersunk, flange, carriage, etc.
• Grade — 4.6, 5.6, 8.8, 10.9 or 12.9 for steel; A2 or A4 for stainless
• Finish — plain, zinc-plated, galvanised, black oxide, stainless

The full call-up format is: M[diameter] x [pitch] x [length] – [grade] – [finish]. Real-world example: "M12 x 1.75 x 50 – 8.8 – Zinc" means a 12mm coarse-thread bolt, 50mm long, in grade 8.8 high-tensile steel with zinc plating. When the pitch is left out, coarse is assumed.

Coarse vs Fine Thread — Which to Use

Every metric bolt size has a default coarse pitch defined by ISO 261. Most sizes also have one or two recognised fine pitches. Coarse is the right answer for most general workshop and construction jobs; fine is reserved for specific engineering reasons.

Coarse Pitch — When to Use It

• General-purpose fastening — workshop, construction, fabrication, machinery assembly
• Faster installation — fewer turns per millimetre of advance, quicker to run down with an impact driver or power tool
• More tolerant of damage — coarser flanks resist galling, cross-threading and dirt better
• Cheaper and more available — coarse is the default stock item for virtually every metric size
• Better for soft materials — aluminium, plastic, timber inserts hold a coarse thread better

Fine Pitch — When to Use It

• Higher tensile strength at the same diameter — fine has a larger minor (root) diameter, so the cross-section that actually takes the load is bigger
• Better resistance to vibration loosening — finer thread = shallower helix angle = lower self-loosening tendency
• Finer adjustment — used in jigs, fixtures, calibration screws, fine-tuning controls
• Thin-walled components — gives more thread engagement in a given depth (more turns per mm)
• Automotive engine and driveline — almost exclusively fine-pitch (cylinder head bolts, conrod bolts, hub bolts, suspension)

Coarse vs Fine — Side-by-Side

Size	Coarse (mm)	Fine (mm)	Common Application
M6	1.00	0.75	Fine used in clock/instrument work
M8	1.25	1.00	Fine on engine ancillaries
M10	1.50	1.25 or 1.00	1.25 = engine bolts; 1.00 = very fine
M12	1.75	1.50 or 1.25	1.25 common on cylinder heads, hubs
M14	2.00	1.50	1.50 on spark plugs & hubs
M16	2.00	1.50	Fine on bearing locknuts
M18	2.50	1.50	Fine on hydraulic ports
M20	2.50	1.50	Fine on automotive drivetrain
M22	2.50	1.50	Specialist use
M24	3.00	2.00	Heavy machinery fine

Rule of thumb: if the original fastener that came out was fine pitch, replace with fine. Mixing a coarse bolt into a fine-pitch tapped hole will strip the female thread on the first turn — the pitches are incompatible.

Bolt Length — How to Measure Correctly

Length is measured in millimetres, and where you measure from depends on the head type. Get this wrong on a drawing and a 50mm countersunk screw will sit 5–7mm deeper than expected.

Hex Head, Hex Flange, Cap (Socket Head), Button Head

Measure from under the head to the tip of the thread. The head sits on the surface; the length is what goes through the hole and into the joint. This is by far the most common case.

Countersunk Head (Socket or Slotted)

Measure overall, from the tip of the thread to the top of the head. Because countersunk heads sit flush in a chamfered hole, the head is part of the length you're specifying.

Half-Countersunk / Raised Countersunk

Measure from the largest diameter of the conical head to the tip. The domed portion above is not included in the length call-up.

Common Stock Lengths

Most metric bolt and screw lines are stocked in standard length increments — typically 5mm increments up to 30mm, then 10mm increments to 100mm, then 20mm increments beyond that. AIMS stocks practical working ranges by size:

• M3, M4, M5: 6, 8, 10, 12, 16, 20, 25, 30, 35, 40mm
• M6, M8: 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100mm
• M10, M12: 20 to 200mm in standard increments
• M16, M20: 30 to 300mm
• M24: 50 to 300mm and longer to order

Thread reach: partially threaded bolts (DIN 931 / ISO 4014) have a smooth shank under the head, then thread for the remainder. The thread length is set by the standard — typically 2D + 6mm for short bolts, 2D + 12mm for medium, 2D + 25mm for long. Fully threaded bolts (DIN 933 / ISO 4017) are threaded right up to the head — used when the joint design needs thread engagement through the full clamp length.

Australian and International Standards for Metric Bolts

Australian workshops see a mixed standards landscape — ISO, DIN, and AS/NZS are all in circulation, and you'll find them stamped on the heads of bolts in the same tray. Understanding which is which prevents bad assumptions.

ISO 261 — Metric Thread, General Purpose

The umbrella standard for metric threads. Defines the M-series basic profile, pitch values, and the coarse/fine pitch series. Every legitimate metric bolt in the world is based on this thread form.

ISO 262 — Selected Sizes for Screws, Bolts, Nuts

The "shopping list" subset of ISO 261 — the practical commercial range from M1 up. M3, M4, M5, M6, M8, M10, M12, M16, M20, M24 are the preferred-series sizes; the rest are second-preference (M14, M18, M22) or specialist.

ISO 4014 — Hex Bolts, Partial Thread (Replaces DIN 931)

Hexagon head bolts with a smooth shank under the head and thread for the remaining length. The international successor to DIN 931. Differences are subtle — head dimensions and thread-length formulas are essentially the same.

ISO 4017 — Hex Bolts, Full Thread (Replaces DIN 933)

Hexagon head bolts threaded right up to the head — no smooth shank. The international successor to DIN 933. Use when the clamp depth needs thread the whole way, or when the bolt passes through plain holes and engages in a tapped section deep in the joint.

AS 1110.1:2015 — ISO Metric Hexagon Bolts and Screws — Product Grades A and B

The Australian Standard for high-quality metric hex bolts. Aligns with ISO 4014/4017 but specifies the Australian acceptance and marking requirements. Most engineering-grade metric bolts sold in Australia are nominally compliant with this standard.

AS 1112.1:2015 — ISO Metric Hexagon Nuts — Style 1

The Australian Standard for matching metric hex nuts. Style 1 is the standard-height nut (approximately 0.8D thick). Tall nuts (Style 2) and thin nuts (jam nuts) are covered in subsequent parts of AS 1112.

AS/NZS 1252:2016 — Structural Bolts

The Australian/New Zealand standard for high-strength structural bolts — Grade 8.8 with matching nuts and washers, supplied as an assembly. This is the standard for structural steel connections in construction and is referenced by AS 4100 (steel structures) and AS 5100 (bridge design). Stamped "8.8/S" or "8.8/TF" to distinguish from general-purpose 8.8 fasteners.

DIN 933 and DIN 931 — German Standards Still in Circulation

The original German standards for fully threaded (933) and partially threaded (931) hex bolts. Still widely stocked in Australia because European machinery and equipment is built with them, and because legacy DIN-stamped product remains in the supply chain. Functionally interchangeable with the ISO replacements for almost all applications. The most visible difference is the across-flats spanner sizes for M10, M12, M14 and M22 — older DIN stock uses one size smaller (M10 = 16mm, M12 = 18mm, M14 = 21mm, M22 = 34mm) while current ISO product uses 17, 19, 22 and 32mm respectively.

Common Metric Bolt Sizes by Application

M3, M4, M5 — Small Fittings, Electronics, Light Hardware

The small end of the range. M3 and M4 dominate electronics enclosures, switchgear panels, instrument cases, small motor mountings, and model engineering. M5 turns up in light cabinet hardware, computer chassis, white-goods assembly, and small bracketry. Spanner sizes are 5.5, 7 and 8mm respectively — well within hex key territory.

M6, M8 — General Workshop, Cabinet Hardware, Light Machinery

The bread-and-butter sizes for workshop work. M6 and M8 cover general bracketing, equipment mounting, light machinery, automotive ancillaries (alternator brackets, coil mounts, accessory drives), light steel fabrication, and most cabinet hardware over 50kg. Available in every head type, every grade, every finish.

M10, M12 — Structural Fastening, Automotive, Machinery

The crossover into serious structural work. M10 and M12 are the workhorse sizes for steel-on-steel fabrication, automotive chassis and suspension, machine baseplates, equipment foundations, and the lighter end of structural steel connections. M12 in particular is the most-stocked size at AIMS by volume — it's the default sizing for medium-duty industrial work across virtually every sector.

M14, M16 — Heavy Machinery, Light Structural Steel

M14 is the "in-between" size — less commonly stocked than M12 and M16, mostly used for specific applications like spark plugs (M14 x 1.25), automotive hub bolts, and some hydraulic port sizes. M16 is a major structural size — steel beam connections, large equipment baseplates, crane and hoist mounting, and the lighter end of mining and construction work.

M18, M20, M22 — Heavy Structural and Industrial

M20 dominates this range. M18 and M22 are second-preference sizes that turn up in specific equipment (drivetrain, hydraulic equipment, mining components) but aren't a default workshop stock. M20 structural bolts to AS/NZS 1252 are the standard for medium-to-heavy steel building construction.

M24 and Larger — Heavy Structural, Civil, Marine

M24 is the upper boundary of "common" metric bolting. Above this — M27, M30, M36, M42 and beyond — is heavy structural, civil engineering, large industrial machinery, wind tower bolting, and marine work. Usually special-order, supplied with matching nuts and washers as a structural assembly.

Bolt Head Types — Hex, Socket Cap, Button, Countersunk

The bolt head decides how you drive the fastener, how flush it sits, and how much head-room you need around it. Choose the head type that matches the joint geometry and the tool access you'll have.

• Hex Head (DIN 933/931, ISO 4014/4017) — driven with a spanner or socket. Highest torque capacity for a given grade. Sits proud of the surface. The default for general engineering.
• Socket Head Cap Screw (DIN 912, ISO 4762) — cylindrical head driven by an internal hex key. Used where a low profile is needed and where socket access is reliable. The flagship of high-tensile fastening (typically supplied 10.9 or 12.9 grade).
• Button Head Socket Screw (ISO 7380) — domed head, internal hex drive. Lower head height than a cap screw, decorative appearance. Lower clamp force capacity than a cap screw of the same size.
• Countersunk Socket Screw (DIN 7991, ISO 10642) — conical head with internal hex drive, sits flush in a chamfered hole. Used where nothing must project above the surface.
• Hex Set Screw — fully threaded hex-head bolt (a.k.a. "set bolt" or "tap bolt") — same as ISO 4017. The Australian/British term "set screw" can confuse — in metric work it refers to a fully threaded hex bolt, not the headless grub screws Americans call set screws.
• Headless Socket Set Screw (DIN 913/914/915/916, ISO 4026–4029) — no head; internal hex drive at one end; cup, flat, dog, cone or oval point at the other. Used to lock collars, gears and hubs onto shafts.

For a deeper breakdown of head styles and their applications, see our Screw Head Types Guide. For the engineering detail behind socket head cap screws specifically — including DIN 912 dimensions, grade markings, and Allen key selection — see the Socket Head Cap Screw Guide.

Metric Bolt Grades — Marking and Strength

Metric bolts are graded by a two-number marking stamped on the head — for example "8.8", "10.9", or "12.9". The numbers are not arbitrary — they encode the bolt's mechanical properties.

The first number x 100 = the nominal tensile (ultimate) strength in MPa. The two numbers multiplied together x 10 = the nominal yield strength in MPa.

• 4.6 — tensile 400 MPa, yield 240 MPa. Mild steel, general-purpose, "commercial bolt". The lowest commonly-stocked grade.
• 5.6 — tensile 500 MPa, yield 300 MPa. Slightly stronger commercial grade, less common.
• 8.8 — tensile 800 MPa, yield 640 MPa. The workhorse "high-tensile" grade for engineering and structural work. The most-stocked grade above 4.6.
• 10.9 — tensile 1040 MPa, yield 940 MPa. High-strength bolt for heavily loaded joints, machine tools, automotive driveline.
• 12.9 — tensile 1220 MPa, yield 1100 MPa. The strongest commonly-available grade. Almost always supplied as socket head cap screws in black-oxide finish. Used where joint design requires maximum clamp force per bolt.

For stainless steel, the marking system changes — bolts are stamped A2-70 or A4-80, where A2/A4 is the alloy (304 / 316) and the number after the dash is tensile strength in MPa divided by 10. A4-80 is the standard marine-grade stainless bolt.

For the full grade marking reference including imperial SAE grades, see the Bolt Grade Chart. For stainless specifically — A2 vs A4, -70 vs -80, when to step up to A4 — see the Stainless Steel Fastener Grades Guide.

Torque Values for Metric Bolts

Tightening torque depends on the bolt size, grade, lubrication state, and the friction coefficients of the surface and thread. The table below is the AIMS quick-reference for dry assembly of plain-finish bolts — for lubricated assembly, reduce by approximately 25%; for zinc-plated bolts, reduce by approximately 10%.

Size	Grade 4.6 (Nm)	Grade 8.8 (Nm)	Grade 10.9 (Nm)	Grade 12.9 (Nm)
M6	4	10	14	17
M8	10	25	35	41
M10	19	49	69	83
M12	34	86	120	145
M14	54	140	190	230
M16	83	210	295	355
M18	115	290	405	485
M20	165	410	580	690
M22	225	550	780	930
M24	285	710	1000	1200

Values shown are typical engineering reference values at 90% of yield clamp load with K = 0.20 (dry, plain finish). Always defer to the equipment manufacturer's torque specification where one is provided — particularly for engine, suspension, and structural connections.

For the full torque chart including fine pitch, lubricated, and structural bolt values, see the Metric Bolt Torque Chart.

Common Mistakes When Specifying Metric Bolts

Mixing Metric and Imperial in the Same Assembly

An M10 bolt and a 3/8" UNC bolt look almost identical at a glance — diameters are 10.0mm vs 9.525mm and pitches are 1.5mm vs ~1.587mm — but they are not interchangeable. Forcing an M10 nut onto a 3/8" bolt (or vice versa) strips threads on the first half-turn. Australian workshops handling both legacy imperial machinery and modern metric equipment need clear bin labelling and separation. See the Metric vs Imperial Fasteners Guide for the full incompatibility picture.

Wrong Pitch (M12 x 1.25 Where Coarse Was Needed)

Fine and coarse pitch bolts share the same diameter but cannot mate to each other's nuts. Cross-threading a fine bolt into a coarse hole damages both. Always confirm pitch when sourcing replacements — particularly on automotive engine, hydraulic port, and bearing locknut work, where fine pitch is the default and the standard "M12" coarse from the workshop bin will not fit.

Using High-Strength Bolts in Galvanic-Mismatch Joints

A grade 10.9 carbon-steel bolt clamped through aluminium plate in a damp marine environment will accelerate galvanic corrosion on the aluminium and seize itself in place within months. Match material to environment — stainless A4 for marine, hot-dip galvanised for outdoor structural, plain steel for sheltered indoor industrial.

Specifying Coarse Where Fine Was Required

If you're replacing fasteners on engine ancillaries, drivetrain components, or precision equipment and the original was fine pitch, the replacement must be fine. Defaulting to the coarse bin because "M12 is M12" will damage the female thread on installation.

Confusing Nominal Diameter with Shank Diameter

The M number is the nominal major thread diameter. The plain shank under a partially threaded bolt head is the same nominal diameter — they are not different specifications. A drilled clearance hole sized for an M12 bolt accommodates both the thread and the shank because both are 12mm nominal.

Substituting Grade Without Re-Checking Torque

Swapping an 8.8 bolt for a 10.9 of the same size without adjusting torque puts the joint under-clamped. Swapping a 10.9 for an 8.8 without reducing torque snaps the bolt. The clamp load — not the torque value — is what holds a joint together, and the right torque depends on the grade.

AIMS Industrial Metric Bolt Range

AIMS holds deep metric fastener stock across all the common head types, grades and finishes. Sydney warehouse pickup at Milperra; freight nationally.

By Head Type

• Hex Bolts — DIN 933/931 and ISO 4014/4017, partial and fully threaded, M5 to M24, grades 4.6 and 8.8, plain, zinc-plated and galvanised.
• Hex Set Screws — fully threaded hex-head bolts (ISO 4017 / DIN 933), the workhorse for general assembly work.
• Socket Head Cap Screws — DIN 912 / ISO 4762, internal hex drive, typically grade 12.9 black oxide. M3 to M24.
• Button Head Socket Screws — ISO 7380, domed head, internal hex drive, M3 to M16.
• Socket Countersunk Screws — DIN 7991 / ISO 10642, flush mounting, internal hex drive.
• Countersunk Screws — slotted, Phillips and Pozidriv head countersunk machine screws.
• Machine Screws — full range of pan, button, countersunk and cheese-head metric machine screws.

Nuts and Lock Nuts

• Hex Nuts — AS 1112.1 / ISO 4032 Style 1 standard hex nuts, M3 to M24, plain and zinc.
• Nylon Lock Nuts (Nyloc) — DIN 985 / ISO 10511 self-locking nuts with nylon insert, the standard solution for vibration-prone joints.
• Hex Lock Nuts — all-metal locking nuts for high-temperature service where nylon inserts can't be used.
• All Lock Nuts — full range including Nyloc, Stover, K-nut, and serrated flange lock nuts.
• All Nuts — every metric nut style in one place: standard hex, lock nuts, dome nuts, wing nuts, T-nuts and weld nuts.

Stainless and High-Tensile

• Stainless Fasteners — A2-70 (304 stainless) and A4-80 (316 marine-grade stainless) hex bolts, cap screws, nuts and washers across the metric range.
• Bumax — high-tensile stainless bolts (Bumax 88, Bumax 109) bringing 10.9-equivalent strength in 316L stainless. The right answer when stainless corrosion resistance and high clamp load are both needed.

Family Categories

• Bolts — every bolt style and head type
• Screws — machine screws, self-tappers, wood screws
• Fasteners — the full fastener category (1,400+ products)

Specialty Bolt Guides

Building a specific application? These deep-dive guides cover the engineering detail:

• Socket Head Cap Screw Guide — DIN 912 dimensions, grades, Allen key selection
• Coach Bolt & Coach Screw Guide — round-head square-neck bolts for timber work
• Eye Bolt Guide — lifting eye bolts, working load limit, angle de-rating
• Shoulder Bolt Guide — ISO 7379 precision shoulder bolts for jigs and bearings
• Threaded Rod Guide — full-length all-thread, grades and applications
• Bolt Grade Chart — metric, imperial and stainless markings explained
• Metric Bolt Torque Chart — tightening reference for 4.6, 8.8, 10.9, 12.9
• Fastener Thread Size Chart — metric, imperial and BSP comparison
• Tap Drill Size Chart — what to drill before you tap

Need Help Sizing?

Bring the old bolt or component to our Milperra trade counter, or call (02) 9773 0122 and the AIMS team will match it — diameter, pitch, length, head type, grade and finish. For trade and account customers, online ordering with same-warehouse-day picking on in-stock items.

Frequently Asked Questions

What does M12 mean on a bolt?

M12 means a metric bolt with a nominal major (outer) thread diameter of 12 millimetres. The "M" indicates metric ISO thread, and the number that follows is always the diameter in millimetres. M12 by itself implies coarse pitch (1.75mm) — fine pitch would be called out as "M12 x 1.5" or "M12 x 1.25".

What size spanner do I need for an M12 bolt?

A current-standard M12 hex bolt to ISO 4014/4017 (or AS 1110.1) takes a 19mm spanner across the flats. Older DIN 933/931 stock uses 18mm — so if you have an older European-spec bolt and your 19mm spanner is loose, try 18mm. Socket head cap screws (DIN 912) are driven by a 10mm hex key, not a spanner.

What's the diameter of an M12 bolt?

The nominal major diameter is 12.0mm — measured across the outermost crest of the thread. The shank (smooth section on a partial-thread bolt) is also nominally 12.0mm. The minor (root) diameter — the smallest cylindrical section at the thread root — is approximately 9.85mm for coarse pitch and 10.16mm for fine pitch.

What's the thread pitch of an M12 bolt?

Coarse M12 has a pitch of 1.75mm — meaning each thread is 1.75mm apart, and one full turn of the nut advances it 1.75mm along the bolt. Fine M12 is typically 1.5mm (most common in general engineering) or 1.25mm (common on automotive cylinder heads, wheel hubs, and hydraulic ports). Always confirm pitch when sourcing replacements.

What's the difference between coarse and fine metric thread?

Coarse has a larger pitch (more advance per turn, fewer threads per millimetre) — it's faster to install, more tolerant of damage and dirt, and is the default for general engineering. Fine has a smaller pitch (less advance per turn, more threads per millimetre) — it gives higher tensile strength at the same diameter, better vibration resistance, finer adjustment, and more thread engagement in thin-walled components. Coarse is M12 x 1.75; fine is M12 x 1.5 or M12 x 1.25. The two are not interchangeable — forcing a fine nut onto a coarse bolt strips both.

Are M-sized bolts compatible with imperial sizes?

No. Metric and imperial fasteners share the same general shape but have incompatible thread forms — different pitches, different thread angles in some cases, and different diameters. An M10 bolt is 10.0mm, while the closest imperial sizes are 3/8" UNC (9.525mm) and 7/16" UNF (11.11mm) — neither will mate with an M10 nut. Mixing metric and imperial fasteners in the same assembly damages both threads on the first turn.

What's the strongest metric bolt grade?

For carbon steel: grade 12.9 is the strongest commonly-available grade — 1220 MPa tensile, 1100 MPa yield. Almost always supplied as socket head cap screws in a black oxide finish. Grade 10.9 (1040 / 940 MPa) is the next step down. For stainless steel: Bumax 109 in A4-grade stainless gives 10.9-equivalent strength with marine corrosion resistance, and is the right answer when both maximum clamp load and stainless are needed.

How do I identify metric bolt grade?

The grade is stamped on the head — for hex bolts, look for a two-number marking like "8.8", "10.9", or "12.9" on the top face. For socket head cap screws, the grade is usually stamped on the side of the head or top. For stainless, the marking is "A2-70" or "A4-80". If there is no head marking at all, treat it as commercial-grade 4.6 — never use unmarked bolts for structural, lifting, or safety-critical work.

What's the typical length range for an M12 bolt?

AIMS stocks M12 hex bolts in standard increments from 20mm up to 200mm — typically 20, 25, 30, 35, 40, 45, 50, 60, 65, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 180, 200mm. Longer M12 bolts are available to order. Socket head cap screws follow similar increments. Length is measured from under the head to the tip of the thread.

Can I use M10 bolts in an M12 hole?

Only as a clearance fit, not a thread engagement. An M10 bolt will pass through a hole drilled for M12 clearance (typically 13mm) but will be loose — the joint relies on the bolt's clamp force, not the bolt's bearing against the hole wall. For a structural connection where bolts are loaded in shear, the bolt must match the hole. For a clamp joint where bolts are loaded in tension, undersized bolts can work but you've designed in slop that wasn't intended. Don't substitute without checking the joint design.

What's the Australian standard for metric bolts?

AS 1110.1:2015 covers ISO metric hex bolts and screws (Product Grades A and B). AS 1112.1:2015 covers the matching hex nuts. AS/NZS 1252:2016 covers structural high-strength bolts (Grade 8.8) for steel building construction. These align closely with ISO 4014/4017 (bolts) and ISO 4032 (nuts), so most ISO-compliant product sold in Australia meets the AS requirement.

What's the difference between an M6 bolt and an M6 screw?

In Australian and British usage, the terms are largely interchangeable for threaded fasteners — but there is a technical distinction. A bolt is fully or partially threaded and is intended to be assembled with a nut. A screw is fully threaded and is intended to engage a tapped female thread directly (without a nut). In commercial supply, you'll see "M6 bolts" and "M6 machine screws" both referring to similar parts — the head type is usually what distinguishes them: hex head = bolt, slotted/Phillips/socket head = screw, in general usage. Both use the same M6 thread.

Are DIN bolts the same as AS bolts?

For practical purposes, yes — DIN 933 (fully threaded hex bolt) and ISO 4017 / AS 1110.1 are functionally interchangeable in 99% of applications. Both use the same ISO 261 metric thread form, the same major diameters and pitches, and the same nominal head dimensions. The notable difference is across-flats spanner sizes on M10, M12, M14 and M22 — older DIN stock is one millimetre smaller (16, 18, 21, 34mm) than current ISO/AS (17, 19, 22, 32mm). For structural and engineering work in Australia, AS 1110.1 compliance is the safer specification, but DIN-stamped product remains widely accepted in general workshop use.

What's a partial-thread vs fully-threaded bolt?

A partial-thread bolt (DIN 931 / ISO 4014) has a smooth shank under the head and thread for the remaining length — used when the smooth shank passes through a clearance hole and provides shear bearing, while the thread engages the nut or tapped hole on the far side. A fully-threaded bolt (DIN 933 / ISO 4017) is threaded right up to the head — used when the joint depth requires thread engagement the whole way, or when shorter bolts (where there isn't room for both shank and thread) are needed. For clamp joints loaded in tension, either works; for joints loaded in shear, the partial-thread (with the smooth shank in the shear plane) is the engineered choice.

What does "M12 x 1.25 x 50" mean?

It calls up a metric bolt with a 12mm nominal major diameter, 1.25mm fine thread pitch, and 50mm length. The "x 1.25" is the fine-pitch specification — without it, the bolt would be assumed to be M12 coarse (1.75mm pitch). The "x 50" is length measured from under the head to the tip of the thread. The grade and finish would be called out separately — e.g. "M12 x 1.25 x 50, Grade 10.9, Zinc Plated".

Why does my M10 bolt fit a 17mm spanner but my mate's fits 16mm?

Both are M10 — same diameter, same thread. The head size across flats (AF) is the difference. DIN 933/931 specified 17mm AF as the traditional standard, and that's what most Australian general stock still uses. ISO revised the recommendation from 1992 onwards toward a 16mm head to reduce material weight, and some European-built equipment and aftermarket parts now ship with the smaller 16mm head. Stock both spanner sizes in your kit if you work on a mix of older and newer metric gear. The same split exists at M12 (19mm DIN/AS vs 18mm older ISO) and M14 (22mm vs 21mm).

Why do automotive M12 bolts often need an M12 x 1.25 thread instead of M12 x 1.75?

Automotive engineering uses fine pitch (typically 1.25 or 1.5) on most engine, suspension, hub and brake fasteners because fine pitch gives higher tensile strength at the same diameter, better resistance to vibration loosening, and more thread engagement in shallow hub or knuckle bosses. General workshop "M12" almost always means M12 x 1.75 coarse — but if you grab a coarse bolt for a Subaru wheel stud or a brake calliper bracket, it won't engage and will damage the female thread. Forum discussions on Practical Machinist and MR2 Owners Club consistently flag this as the most common metric fastener mistake. Always verify the pitch on automotive work.

Can I tell if a bolt is grade 8.8 or 4.6 if there's no marking?

If there is no head marking, assume it is grade 4.6 commercial — that is the default for unmarked metric fasteners and is roughly half the strength of grade 8.8. The grade marking on metric bolts is a two-number stamp on the top of the head ("4.6", "8.8", "10.9", "12.9" for carbon steel; "A2-70" or "A4-80" for stainless). A blank head means either (a) a low-grade commercial bolt or (b) a no-name import where you simply don't know — both should be treated as 4.6 and never used for structural, lifting, brake, suspension or safety-critical applications. If grade is critical, source bolts from a known supplier with traceable batch certification rather than buying loose from generic bins.