Walk into any workshop in Australia and you will find two fastener systems sitting side by side — metric and imperial. Metric has been Australia's official standard since the 1970s, but imperial threads have never fully disappeared. US-manufactured plant equipment, older British machinery, classic vehicles, and some hydraulic systems all run on threads that metric fasteners simply will not fit.
This guide explains how each system works, what you will encounter each one on, and why — despite similar diameters — metric and imperial fasteners are never interchangeable. For a direct conversion table between measurement systems, see the AIMS Fastener Reference Chart.
Why Australia Uses Both Fastener Systems
Australia formally adopted the metric system under the Metric Conversion Act 1970, with the transition largely complete by the mid-1980s. From that point forward, Australian engineering standards, building codes, and manufacturing specifications switched to metric — ISO threads, millimetre dimensions, metric grade designations.
Once you've decided on metric — see the AIMS Metric Bolt Size Guide for the full M3 through M24 reference covering diameter, thread pitch, head dimensions and grade markings across all common head profiles.
But metrication did not erase the installed base. Equipment already in the field kept running on its original threads. New equipment imported from the United States arrived — and continues to arrive — with UNC and UNF fasteners, because the US never adopted metric for most industrial applications. British and Commonwealth machinery manufactured before the 1970s used Whitworth threads (BSW and BSF). That legacy still appears daily in maintenance workshops across Australia.
The result is a practical reality: anyone maintaining plant, vehicles, or machinery in Australia needs to understand both systems. The consequences of misidentifying a thread are not abstract — stripped fasteners, damaged tapped holes, and joints that appear tight but hold no real clamping force. All of these trace back to using the wrong thread system. The good news is that correctly identifying metric and imperial threads is straightforward once you understand how each system is specified.
How Metric Fasteners Are Specified
Metric fasteners follow the ISO standard. The designation uses an "M" prefix followed by the nominal outer diameter in millimetres, then the thread pitch in millimetres, then the length in millimetres. An M10 × 1.5 × 40 bolt has a 10 mm nominal diameter, a 1.5 mm thread pitch, and is 40 mm long. When no pitch is stated — for example, just "M10 × 40" — coarse pitch is assumed by convention.
The thread angle for ISO metric threads is 60°, measured at the flanks of the thread profile. This is the same flank angle as the Unified thread family (UNC and UNF) used in North America, but the pitch tables are entirely different — a metric bolt and a UNC bolt of similar diameter are not interchangeable despite sharing a thread angle.
The table below shows standard metric coarse pitch specifications for common bolt sizes:
| Metric size | Nominal diameter | Coarse pitch | Fine pitch (MF) | Common application |
|---|---|---|---|---|
| M5 | 5.0 mm | 0.8 mm | 0.5 mm | Small machinery, electronics enclosures |
| M6 | 6.0 mm | 1.0 mm | 0.75 mm | General hardware, light structural |
| M8 | 8.0 mm | 1.25 mm | 1.0 mm | Most common general-purpose size |
| M10 | 10.0 mm | 1.5 mm | 1.25 mm | Structural, flanges, brackets |
| M12 | 12.0 mm | 1.75 mm | 1.25 mm | Heavy structural, machinery frames |
| M16 | 16.0 mm | 2.0 mm | 1.5 mm | Steelwork, heavy structural connections |
| M20 | 20.0 mm | 2.5 mm | 1.5 mm | Heavy plant, large structural joints |
| M24 | 24.0 mm | 3.0 mm | 2.0 mm | Crane components, heavy fabrication |
| M30 | 30.0 mm | 3.5 mm | 2.0 mm | Heavy lifting, foundation bolts |
Metric bolts are specified under Australian Standard AS 1110 (precision hexagon bolts) and AS 1111 (commercial hexagon bolts), which are aligned with ISO 4014 and ISO 4018 respectively. For full metric-to-imperial dimension conversion tables, see the AIMS Fastener Reference Chart.
How Imperial Fasteners Are Specified
Imperial fasteners specify diameter in inches — either as a fraction (1/4", 3/8", 1/2") or, below 1/4" diameter, as a number designation (#4, #6, #8, #10). Thread count is given in threads per inch (TPI), and the thread standard follows: 3/8"-16 UNC is a 3/8 inch diameter bolt with 16 threads per inch in the Unified National Coarse standard.
The thread angle for Unified threads (UNC and UNF) is 60°. Whitworth threads (BSW and BSF) use a 55° flank angle — a fundamentally different thread profile that makes Whitworth fasteners incompatible with both metric and Unified fasteners regardless of pitch.
The table below shows standard imperial sizes and pitches for UNC and UNF:
| Diameter | Decimal (inches) | UNC TPI | UNF TPI | Approx metric equivalent (diameter only) |
|---|---|---|---|---|
| 1/4" | 0.250" | 20 | 28 | ~M6 (6.35 mm vs 6.0 mm) |
| 5/16" | 0.313" | 18 | 24 | ~M8 (7.94 mm vs 8.0 mm) |
| 3/8" | 0.375" | 16 | 24 | ~M10 (9.525 mm vs 10.0 mm) |
| 7/16" | 0.438" | 14 | 20 | ~M11 (no direct metric equivalent) |
| 1/2" | 0.500" | 13 | 20 | ~M12 (12.7 mm vs 12.0 mm) |
| 5/8" | 0.625" | 11 | 18 | ~M16 (15.875 mm vs 16.0 mm) |
| 3/4" | 0.750" | 10 | 16 | ~M20 (19.05 mm vs 20.0 mm) |
| 7/8" | 0.875" | 9 | 14 | ~M22 (22.225 mm vs 22.0 mm) |
| 1" | 1.000" | 8 | 12 | ~M25 (25.4 mm vs 25.0 mm) |
The "approx metric equivalent" column shows only diameter proximity — it does not imply interchangeability. See the near-miss section below for why these diameter similarities are dangerous in practice.
Metric Thread Types: Coarse and Fine
Within the ISO metric system, two thread pitches cover most applications. Understanding when each is used prevents ordering errors and ensures the right fastener reaches the job.
ISO Metric Coarse (MC) is the default for general industrial and structural use. It assembles faster, tolerates slight misalignment, and is less sensitive to contamination in the thread form. When someone says "M10 bolt" without specifying pitch, they almost always mean M10 × 1.5 coarse. Coarse pitch is specified under ISO 261 and covers the vast majority of Australian industrial fastener use.
ISO Metric Fine (MF) uses a smaller pitch — more threads per unit length than coarse at the same diameter. This provides finer adjustment, better resistance to loosening under vibration, and is appropriate for thin-walled tapped sections where a coarse thread would not allow enough thread engagement. M10 fine is typically M10 × 1.25; M8 fine is M8 × 1.0. Fine pitch is more common in automotive, aerospace, and precision mechanical applications than in general structural work.
Coarse and fine metric nuts of the same diameter are not interchangeable — an M10 × 1.5 nut will not correctly engage an M10 × 1.25 bolt. Always confirm pitch when ordering fasteners for fine-pitch applications, as coarse is typically supplied by default.
Imperial Thread Types: UNC, UNF, BSW, BSF and BSP
The imperial world contains several distinct thread standards, each with a specific application history. Understanding the differences between them matters — particularly for anyone maintaining legacy equipment in Australia, where all four standards may be encountered on the same site.
UNC — Unified National Coarse is the most widely used imperial fastener thread in Australia today. It is the standard for US-manufactured industrial equipment, American-brand hand tools, and most hardware imported from North America. UNC uses a 60° thread angle and is defined in ASME B1.1. It is a coarser pitch than UNF at any given diameter, making it faster to assemble and more tolerant of contamination.
Common UNC sizes you will encounter on US machinery: 1/4"-20, 5/16"-18, 3/8"-16, 7/16"-14, 1/2"-13, 5/8"-11, 3/4"-10. The number after the dash is the TPI — so 3/8"-16 UNC has 16 threads per inch.
UNF — Unified National Fine uses the same 60° thread form as UNC but with a finer pitch. UNF provides higher tensile strength at a given diameter, better vibration resistance, and finer adjustment range. It is standard in aerospace, automotive precision components, and applications where the joint may be subjected to cyclic loading.
Common UNF sizes: 1/4"-28, 5/16"-24, 3/8"-24, 1/2"-20. UNC and UNF bolts of the same diameter look virtually identical — they can only be reliably distinguished with a thread pitch gauge. Never mix UNC and UNF nuts and bolts even within the same imperial system.
BSW — British Standard Whitworth is the thread standard found on British and older Australian-made equipment manufactured before metrication. The defining characteristic of BSW is its 55° flank angle — different from both ISO metric and Unified threads — combined with rounded thread crests and roots. BSW is identified by diameter in inches and TPI.
BSW is the coarse pitch thread within the Whitworth family. Common BSW sizes encountered on older plant in Australia: 1/4"-20 BSW, 5/16"-18 BSW, 3/8"-16 BSW, 1/2"-12 BSW, 5/8"-11 BSW, 3/4"-10 BSW. Note that some TPI values are shared with UNC — but the 55° Whitworth profile means they are not interchangeable despite having the same TPI.
BSF — British Standard Fine uses the same 55° Whitworth thread form as BSW but with a finer pitch. BSF was used on older British precision applications — classic motorcycles, fine adjusters, vintage vehicles, and industrial machinery where higher clamping force was required from the same bolt diameter. BSF is less commonly encountered than BSW, but is still found on specific older equipment, particularly British motorcycles (Triumph, BSA, Norton) and some vintage agricultural machinery.
AIMS stocks BSF in key sizes, with less common sizes available to order. If you are unsure whether a fastener is BSW or BSF, a Whitworth thread pitch gauge will identify it — the profile will seat correctly on both, but the TPI will tell you which pitch variant it is.
BSP — British Standard Pipe deserves a specific call-out because it is frequently confused with BSW by people who encounter a 55° thread on a fitting and assume it is a fastener thread. BSP is a pipe and fitting thread — used on hydraulic, pneumatic, and plumbing connections — not on nuts and bolts. BSP comes in two forms: BSPP (parallel) and BSPT (tapered, for sealing applications). The 55° thread angle is the same as Whitworth, but BSP's pitch table, diameter designations, and sealing geometry are entirely different from BSW.
The practical rule: if you are working on a hydraulic fitting, a pneumatic manifold, or a fluid system, and the thread has a 55° profile, it is almost certainly BSP, not BSW. Never substitute BSP and BSW fasteners or fittings — the thread forms, pitches, and sealing arrangements are incompatible despite the shared flank angle.
Are Metric and Imperial Fasteners Interchangeable?
No. Metric and imperial fasteners are not interchangeable, and attempting to use them as such is a reliable way to strip threads, damage tapped holes, or create a joint that holds no meaningful clamping force under load.
The incompatibility has two sources. First, thread pitch: even where the nominal diameters of a metric and an imperial fastener are close, the pitch in millimetres does not match the pitch implied by the TPI — so a metric nut tightened onto an imperial bolt of similar diameter will cross-thread within a few turns. Second, for Whitworth threads (BSW and BSF), the thread flank angle is 55° versus the 60° of both metric and Unified threads, making the profiles geometrically incompatible regardless of what the diameter or TPI suggests.
The practical rule is simple: identify the thread specification of the component before selecting a fastener, and match it exactly. If you are unsure what thread a tapped hole uses, identify it with a pitch gauge before inserting any fastener — not after. The cost of proper identification is a few minutes; the cost of a stripped tapped hole in a machine casting or a structural member can be substantial.
Near-Misses That Cause Real Problems
The most damaging fastener errors occur not when threads are obviously different, but when they are close enough that a fastener will start threading before seizing. Several metric and imperial combinations are near-misses — similar enough in diameter that someone in a hurry will try them, and similar enough in pitch that the nut advances a few turns before locking solid.
The table below shows the combinations most frequently encountered in Australian workshops:
| Metric | Imperial near-miss | Diameter gap | Pitch difference | What happens |
|---|---|---|---|---|
| M6 × 1.0 | 1/4"-20 UNC | 6.0 mm vs 6.35 mm (0.35 mm) | 1.0 mm vs 1.270 mm | Cross-threads immediately. Damage to nut thread within 1–2 turns. |
| M8 × 1.25 | 5/16"-18 UNC | 8.0 mm vs 7.94 mm (0.06 mm) | 1.25 mm vs 1.411 mm | Closest diameter match. Nut advances 2–4 turns before seizing. High risk of stripped thread in tapped hole. |
| M10 × 1.5 | 3/8"-16 UNC | 10.0 mm vs 9.525 mm (0.475 mm) | 1.5 mm vs 1.588 mm | Appears to thread, locks tight. No clamping force; will fail under load. |
| M12 × 1.75 | 1/2"-13 UNC | 12.0 mm vs 12.7 mm (0.7 mm) | 1.75 mm vs 1.954 mm | Diameter gap is larger but people still attempt. Do not substitute. |
| M6 × 1.0 | 1/4" BSW (20 TPI) | 6.0 mm vs 6.35 mm | 1.0 mm vs 1.270 mm + 55° vs 60° | Thread angle mismatch prevents correct engagement even if pitch were close. |
| M8 × 1.25 | 5/16" BSW (18 TPI) | 8.0 mm vs 7.94 mm (0.06 mm) | 1.25 mm vs 1.411 mm + 55° vs 60° | The most dangerous Whitworth near-miss — diameter almost identical. Profile incompatibility causes hidden thread damage. |
The M8/5/16" combination — in both UNC and BSW variants — is the most commonly encountered near-miss in Australian workshops. The diameter difference is under 0.1 mm, well within the range where threads will engage before the mismatch becomes apparent. The nut or bolt advances far enough to make the assembler think the connection is made, then seizes or strips the parent thread without warning.
The rule to follow, without exception: if a fastener does not run on smoothly by hand for the first several turns with no resistance, stop. A fastener that requires force to start threading is almost certainly the wrong system or the wrong pitch. Apply tool torque only once the fastener has threaded cleanly by hand for at least five to six turns.
Strength Grade Systems Compared
Metric and imperial fasteners use different grade marking systems, and grade values from one system cannot be directly substituted for another. Understanding both systems is essential when replacing fasteners on mixed-standard equipment.
Metric grade markings appear as two numbers separated by a point, stamped on the bolt head — 4.6, 8.8, 10.9, 12.9. The first number multiplied by 100 gives the minimum ultimate tensile strength (UTS) in MPa. The product of the two numbers, divided by 10, gives the yield strength in MPa. So an 8.8 bolt has a UTS of 800 MPa and a yield strength of 640 MPa (80% of 800).
| Metric grade | UTS (MPa) | Yield (MPa) | Common use |
|---|---|---|---|
| 4.6 | 400 | 240 | General hardware, non-structural |
| 5.8 | 500 | 400 | Light structural, general engineering |
| 8.8 | 800 | 640 | Standard engineering/structural — the most common high-tensile metric grade |
| 10.9 | 1000 | 900 | Heavy structural, socket head cap screws, clamped connections |
| 12.9 | 1200 | 1080 | Highest standard grade — critical joints, socket head cap screws in precision machinery |
SAE/ASTM grade markings for imperial fasteners use radial lines on the bolt head. No marks indicates Grade 2 (low strength). Three evenly spaced radial lines indicate Grade 5 (medium — UTS approximately 827 MPa for 3/4" and under). Six radial lines indicate Grade 8 (high strength — UTS approximately 1034 MPa). Grade 5 is broadly comparable to metric 8.8 in tensile strength, and Grade 8 falls between metric 10.9 and 12.9 — but the testing standards differ and direct substitution without engineering sign-off is not appropriate on structural or safety-critical applications.
BSW and BSF grade markings were not originally standardised in the same way as modern metric or SAE grades. Historical British standards specified material and heat treatment rather than a numerical head marking system. Modern Whitworth replacement fasteners produced for maintenance supply are often manufactured to ISO metric strength levels and marked accordingly — an 8.8-grade BSW bolt is threaded to BSW specification but manufactured to ISO 8.8 tensile requirements. Confirm grade requirements with your supplier when replacing structural BSW fasteners.
For a full breakdown of metric bolt head markings and grade identification, see the AIMS Bolt Grade Chart. For tightening torques across all metric grades, see the AIMS Metric Bolt Torque Chart.
What Equipment in Australia Uses Imperial Fasteners
Knowing where to expect imperial threads prevents wasted time and avoidable damage. The categories below cover the most common sources of imperial fasteners in Australian maintenance workshops.
US-manufactured heavy plant and earthmoving equipment is the primary source of UNC fasteners in Australian industry. Caterpillar, John Deere, Case, Bobcat, Terex, and most American-brand construction, earthmoving, and agricultural machinery use UNC and UNF throughout — engine ancillaries, structural frames, hydraulic mounting brackets, and access panels. This equipment is purchased new in Australia today and is in service on farms, mine sites, and construction projects across the country. If you maintain US OEM equipment, UNC in common sizes (1/4" through 3/4") should be standard stock.
American-designed engines — Detroit Diesel, older Cummins, Continental, Lycoming, and most US-designed diesel and petrol engines — use SAE threads in the block, head, ancillaries, and valve train. Replacement fasteners on these engines must match the original specification. Mixing metric replacements into an imperial engine block will damage the block thread.
Pre-metrication British and Australian machinery — equipment manufactured in Australia or the UK before the mid-1970s will typically carry BSW threads throughout. This includes older industrial lathes, milling machines, presses, compressors, and general workshop machinery still operating in tool rooms and maintenance shops, as well as older British-built vehicles and agricultural equipment. Classic British motorcycles — Triumph, BSA, Norton — and classic Land Rover models (Series I, II, IIA) are predominantly BSW/BSF.
Mining and heavy industry presents a mixed environment. Australian-built process equipment installed from the 1980s onward is typically metric. US and Canadian OEM equipment brought in for mine development, drilling, and materials handling is typically UNC/UNF. It is common for a single machine to have metric fasteners on locally fabricated components and imperial fasteners on OEM components from the US manufacturer. Mixed environments require more discipline in thread identification, not less.
Some hydraulic and pneumatic systems on otherwise-metric Australian machinery use imperial fittings — specifically JIC (37° flare), NPT (National Pipe Taper), and SAE straight thread port connections are common on hydraulic systems even where the machine structure is fully metric. These are pipe and fitting threads, not fastener threads, but they require imperial identification and imperial tooling to service correctly.
Aerospace and defence maintenance in Australia involves both metric (European-origin aircraft) and UNF (US-origin aircraft and defence platforms) threaded fasteners. UNF is preferred in aviation for its vibration resistance and higher strength at a given diameter. Aerospace fasteners are also subject to specific material and certification requirements beyond standard commercial grades.
How to Identify an Unknown Thread
Working on an unfamiliar machine without documentation is a common scenario in maintenance. The following approach identifies thread specification reliably without guesswork.
Step 1 — Measure the diameter. Use a vernier calliper to measure the outer diameter of the bolt or the minor diameter of the tapped hole. Metric bolt diameters will measure close to whole millimetre values: 8.0 mm, 10.0 mm, 12.0 mm. Imperial bolt diameters will measure close to inch fractions: 9.525 mm (3/8"), 12.7 mm (1/2"), 15.875 mm (5/8"). This narrows the candidates to a short list.
Step 2 — Use a thread pitch gauge. A thread pitch gauge is a set of profiled blades, each calibrated to a specific pitch. Place blades against the thread form until one sits flush with no gap at the crests or roots and no rocking. Metric pitch gauge blades are labelled in millimetres of pitch. UN pitch gauge blades are labelled in TPI. Whitworth pitch gauge blades are also labelled in TPI but have a 55° profile — if the Whitworth blade seats correctly where the UN blade does not, the thread is BSW or BSF. This three-way comparison reliably distinguishes all four common thread standards.
Step 3 — Cross-reference with the pitch tables. Once you have diameter and pitch (or TPI), cross-reference against the tables in this article or the AIMS Fastener Reference Chart to confirm the thread designation.
Use a go/no-go gauge for tapped holes. A go/no-go gauge is binary — the "go" end must pass freely through the full depth of the tapped hole, and the "no-go" end must not enter. Go/no-go gauges are the most reliable method for confirming thread specification in production and quality control environments, and for detecting thread damage in a hole that has been previously used.
Bring a sample to AIMS. If you cannot identify a thread from the equipment itself, bring a fastener sample to AIMS. We carry thread gauges across metric, UNC, UNF, BSW, and BSF and can identify threads on the spot. This is faster and less costly than attempting identification by trial and error on the machine — particularly where the tapped hole is in a casting, cylinder head, or other component where thread damage would be expensive to repair.
When to Keep Imperial Fasteners in Stock
For a well-run maintenance workshop, the decision about what imperial stock to carry should follow the equipment you service, not general habit. Maintaining a stock of every thread system in every size wastes space and money and increases the risk of the wrong fastener being selected under time pressure.
If you maintain US-manufactured plant equipment, carry UNC in the sizes most commonly used on that equipment — typically 1/4" through 3/4" in Grade 5 and Grade 8. Grade 5 is the most common working grade on US OEM equipment; Grade 8 for critical joints. Do not substitute metric 8.8 for Grade 5 even when the tensile strength appears comparable — the thread pitch is incompatible, and cross-referencing grades across standards for structural applications requires engineering review.
If you service older British machinery, classic vehicles, or legacy plant, carry BSW in the sizes that recur on your equipment. BSF can typically be sourced on demand unless you regularly work on specific models that require it. Keep BSW and UNC in separate, clearly labelled sections of your fastener storage — their similar TPI values (at some diameters) and nearly identical diameters make them a mix-up risk.
Storage discipline is not optional. Metric and imperial fasteners of similar diameter look identical to the eye at normal working distances. A mixed bin is a liability. Labelled compartments, colour-coded containers, or physically separate storage for each thread system eliminates the problem at the source. The time spent on bin organisation is recovered many times over by the time not spent dealing with stripped threads.
For one-off requirements, unusual sizes, or threads encountered only occasionally, AIMS can supply across all systems without requiring you to hold slow-moving stock. For critical or structural applications involving unusual thread specifications, AIMS has also arranged custom and special fasteners to customer requirement — contact our team to discuss.
AIMS Industrial Fastener Range
AIMS stocks fasteners across all thread systems commonly encountered in Australian industry — metric, UNC, UNF, BSW, and BSF — in standard industrial grades and materials. The range covers bolts, nuts, screws, washers, allthread (threaded rod, also known as Brooker rod), and specialist fastener types including security fasteners and thread inserts.
Metric fasteners cover M5 through M36 in standard grades (4.6 for general hardware, 8.8 for structural and engineering applications). Stainless steel 316 is available for corrosive environments including marine, food processing, and chemical applications. Browse the AIMS bolts range, nuts, screws, and washers.
UNC and UNF fasteners are stocked across common sizes used on US-manufactured equipment — 1/4" through 3/4" in Grade 5 and Grade 8 for most applications. UNF is available alongside UNC in standard sizes. Both are available for immediate dispatch on standard lines.
BSW fasteners are stocked in the sizes most frequently required for maintenance of older British and Australian machinery — the sizes you will encounter most often on older plant, classic vehicles, and legacy industrial equipment. AIMS also carries BSF in key sizes; less common BSF sizes can be sourced on request. BSW and BSF are not always readily available from general hardware suppliers, so AIMS's stocking of the Whitworth range is a practical advantage for workshops maintaining older equipment.
Allthread and threaded rod is available in metric and imperial specifications, in common diameters and standard lengths. Allthread is used for threaded anchors, through-bolt assemblies, suspension systems, and custom fastening solutions where standard bolt lengths are insufficient. Browse the AIMS allthread range. For full coverage of allthread grades, sizes, the nut trick for cutting, joining with coupling nuts and acme thread, see our Threaded Rod Guide.
Specialist fastener products include security fasteners, thread inserts (Recoil and standard), washers across metric and imperial, rivets, and anchors. For the full range, see AIMS Industrial fasteners — over 1,400 products across all fastener categories.
Custom and special fasteners — non-standard lengths, unusual grades, specific materials, or thread specifications outside the standard range — can be arranged through AIMS. Contact our team via the AIMS contact page or call (02) 9773 0122 to discuss requirements.
For screw head types and drive patterns across both metric and imperial fasteners, see the AIMS Screw Head Types Guide. For socket head cap screws specifically, see the Socket Head Cap Screw Guide For metric pin fasteners — including roll pins (spring pins, sellock pins) in DIN 1481 sizing — see the Roll Pin Guide.
Frequently Asked Questions
Are metric and imperial fasteners interchangeable?
No. Metric and imperial fasteners are not interchangeable. Even where diameters appear similar, thread pitches differ, and Whitworth threads (BSW/BSF) use a 55° flank angle versus 60° for metric ISO and Unified threads. Attempting to mix systems will cross-thread or strip the fastener, often with no visible warning until the joint fails.
What does M10 × 1.5 mean on a metric bolt?
M10 × 1.5 is an ISO metric designation. 'M10' means the nominal outer diameter is 10 mm. '1.5' is the thread pitch — the distance in millimetres between adjacent thread crests. When a length follows (e.g. M10 × 1.5 × 40), the final number is the bolt length in millimetres. If pitch is not stated, coarse pitch is assumed by convention.
What is the difference between UNC and UNF?
UNC (Unified National Coarse) and UNF (Unified National Fine) both use a 60° thread angle. UNC has fewer threads per inch — it assembles faster and tolerates contamination better. UNF has more threads per inch, providing finer adjustment and better vibration resistance. Common UNC: 3/8"-16. Common UNF: 3/8"-24. They are not interchangeable even at the same nominal diameter.
Is 3/8" the same as M10?
No. 3/8" is 9.525 mm in diameter; M10 is 10.0 mm. More importantly, their pitches differ: M10 coarse is 1.5 mm pitch and 3/8"-16 UNC is approximately 1.588 mm pitch. An M10 nut will not correctly engage a 3/8" UNC bolt. This is one of the most common near-miss combinations in Australian workshops — the diameter similarity makes it tempting to try, and the pitch mismatch ensures thread damage results.
What does 8.8 mean on a metric bolt?
8.8 is the ISO property class for a medium-high strength metric fastener. The first digit (8) multiplied by 100 gives the minimum UTS in MPa: 800 MPa. The second digit (8) indicates the yield-to-UTS ratio as a percentage: 80%, giving a yield strength of 640 MPa. 8.8 is the most common high-tensile metric grade for general engineering and structural applications in Australia.
Which fastener system is standard in Australia?
Metric (ISO) is the Australian standard for fasteners under AS 1110, AS 1111, and related standards. All new engineering, construction, and manufacturing in Australia specifies metric. However, UNC is common on US-manufactured equipment imported into Australia, and BSW/BSF appears on pre-metrication British and Australian machinery. All three systems are regularly encountered in maintenance environments.
What is BSW?
BSW stands for British Standard Whitworth — developed by Sir Joseph Whitworth in the 1840s. BSW uses a 55° thread flank angle (versus 60° for metric and UN threads), with diameter specified in inches and pitch in threads per inch. It is found on older British and Australian machinery manufactured before metrication, classic British vehicles, and some legacy industrial equipment. BSW is the coarse pitch thread in the Whitworth family.
What is the difference between BSW and BSF?
Both BSW and BSF are Whitworth threads with a 55° flank angle. BSW (British Standard Whitworth) is the coarse pitch thread. BSF (British Standard Fine) uses a finer pitch — more threads per inch at the same diameter — for applications requiring greater clamping force or vibration resistance. BSW nuts and BSF bolts of the same nominal diameter are not interchangeable.
Is BSP the same as BSW?
No. Both share a 55° thread angle but are completely different standards. BSP (British Standard Pipe) is a pipe and fitting thread for hydraulic, pneumatic, and plumbing connections — not a fastener thread. BSP comes in parallel (BSPP) and tapered (BSPT) forms. The pitch tables, diameter designations, and sealing arrangements are entirely different from BSW. Never substitute BSP fittings for BSW fasteners or vice versa.
How do I tell if a bolt is metric or imperial?
Measure the outer diameter with a vernier calliper. Metric diameters will be close to a whole millimetre (8.0 mm, 10.0 mm, 12.0 mm). Imperial diameters will be close to inch fractions (9.525 mm for 3/8", 12.7 mm for 1/2"). Then use a thread pitch gauge to confirm pitch — metric blades read in mm, UN blades in TPI, Whitworth blades in TPI with 55° profile. If the Whitworth blade seats where the UN blade does not, the fastener is BSW or BSF.
Can I use an M10 nut on a 3/8" bolt?
No. M10 and 3/8" are close in diameter but their thread pitches are different. An M10 nut started on a 3/8"-16 UNC bolt will initially appear to thread, then seize and strip the nut thread within a few turns. Always match thread system, not approximate diameter.
What is allthread or Brooker rod?
Allthread — also called threaded rod or Brooker rod — is a length of bar stock threaded continuously along its full length, with no unthreaded shank. It is used in through-bolt assemblies, anchor bolt applications, suspension systems, and custom fastening solutions where standard bolt lengths are insufficient. Allthread is available in metric and imperial thread specifications and in materials including mild steel, high tensile, and stainless steel.
What US equipment in Australia uses UNC fasteners?
Most US-manufactured heavy plant and machinery uses UNC throughout — Caterpillar, John Deere, Case, Bobcat, Terex, and similar brands. US-designed diesel engines (Detroit Diesel, older Cummins) also use SAE/UNC threads. If you maintain American OEM equipment, carry UNC in common sizes (1/4" through 3/4") in Grade 5 and Grade 8 as standard stock.
Why can't I just use the closest metric bolt to the imperial size I need?
Because thread compatibility requires matching diameter, pitch, AND — for Whitworth threads — flank angle. Diameter proximity is not sufficient. A metric fastener of similar diameter to an imperial one has a different pitch, meaning threads will not engage correctly. In the best case it will cross-thread immediately; in the worst case it will appear to hold under hand tightening before stripping or failing under load. Always match thread specification exactly.
Does AIMS stock BSW and other imperial fasteners?
Yes. AIMS Industrial stocks UNC, UNF, BSW, and BSF alongside a full metric range. UNC and BSW are stocked in common sizes for immediate supply. UNF and BSF are available across the range, with BSF in more limited stock. Allthread is available in metric and imperial. Custom and special fasteners — unusual lengths, grades, materials, or thread specifications — can also be arranged. Call (02) 9773 0122 or contact AIMS to discuss your requirements.
