Metric Bolt Size Chart
M3–M48 · Spanner sizes · Hex key sizes · Bolt grades · Torque values · Printable PDF
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Bolt size
Spanner size
17 mm
Hex key (Allen)
8 mm
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Most Common
Workshop Tips
M10 is king
M10 (17 mm spanner) is the most common bolt on UK tractors and farm machinery.
Hex key ≠ bolt size
Hex key size is smaller than bolt diameter — M10 uses 8 mm hex key, not 10 mm.
Grade matters
Never replace a 10.9 bolt with 8.8 — it may fail under load. Match or exceed the original grade.
Torque dry
Torque values assume dry threads. Oiled or greased threads need 20% less torque for same clamping force.
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Metric Bolt Grades Explained — 8.8, 10.9, 12.9
The numbers stamped on a bolt head tell you its strength. Understanding bolt grades is critical on farm machinery — using a weaker bolt where a stronger one is specified can cause joint failure under load. Here is what the markings mean and where each grade is used.
How to read the grade marking
The grade is written as two numbers separated by a point — e.g. 8.8. The first number × 100 = tensile strength in MPa. The second number ÷ 10 × first number × 100 = yield strength in MPa. So 8.8 = 800 MPa tensile, 640 MPa yield. Grade 10.9 = 1000 MPa tensile, 900 MPa yield.
| Grade | Tensile | Yield | Common Use |
|---|---|---|---|
4.6 | 400 MPa | 240 MPa | Non-structural, light duty fixings. Soft — shears easily. Common on cheap imported fasteners. |
4.8 | 420 MPa | 340 MPa | General purpose light fixings. Slightly stronger than 4.6 but still not suitable for critical joints. |
8.8common | 830 MPa | 660 MPa | The standard high-tensile bolt. Used throughout tractors and agricultural machinery. If in doubt, use 8.8. |
10.9common | 1040 MPa | 940 MPa | High-strength applications — wheel bolts, critical structural joints, engine components. Do not substitute 8.8. |
12.9 | 1220 MPa | 1100 MPa | Highest standard grade. Engine internals, hydraulic fittings under high cyclic load. Specialist applications only. |
A2-70 | 700 MPa | 450 MPa | Stainless steel bolt. Corrosion resistant — used on dairy equipment, outdoor fixings. Do not use where high strength is critical (galls easily). |
Always match or exceed
When replacing a bolt, use the same grade or higher — never lower. If the original was 10.9, fit a 10.9 or 12.9. Fitting an 8.8 where 10.9 was specified is a common cause of machinery failures.
Grade 8.8 is your default
If you are replacing a bolt and cannot identify the original grade, 8.8 is the correct default for the vast majority of tractor and implement applications. Low-grade bolts (4.6, 4.8) should not be used on machinery.
Do not mix stainless and steel
Stainless steel bolts (A2-70, A4-80) are weaker than high-tensile steel bolts of the same diameter. Never use stainless where a high-tensile steel bolt is specified. Also — stainless can gall (cold weld) if assembled without anti-seize compound.
Metric Bolt Torque Settings — 8.8, 10.9 and 12.9
Torque values below are for clean, dry threads with no lubrication. If threads are oiled or anti-seize has been applied, reduce these figures by approximately 20%. These are general engineering values — always follow manufacturer torque specifications where available.
These values assume standard coarse pitch threads (e.g. M10 × 1.5 mm pitch). Fine pitch threads of the same diameter require slightly higher torque for the same clamping force.
| Bolt | Spanner | Grade 8.8 (Nm) | Grade 10.9 (Nm) | Grade 12.9 (Nm) |
|---|---|---|---|---|
| M6 | 10 mm | 10 | 14 | 17 |
| M8 | 13 mm | 25 | 35 | 41 |
| M10 | 17 mm | 49 | 70 | 83 |
| M12 | 19 mm | 86 | 120 | 145 |
| M14 | 22 mm | 135 | 190 | 230 |
| M16 | 24 mm | 210 | 295 | 355 |
| M18 | 27 mm | 290 | 405 | 485 |
| M20 | 30 mm | 410 | 580 | 690 |
| M22 | 32 mm | 560 | 790 | 950 |
| M24 | 36 mm | 710 | 1000 | 1200 |
Always use manufacturer specs where available
These are general engineering values. Wheel bolts, cylinder head bolts, hydraulic fittings, and critical structural joints always have specific manufacturer torque values — consult the workshop manual or manufacturer data before torquing critical fasteners.
Torque wrench on Amazon
A 1/2" drive torque wrench covers M8–M24 for most farm machinery work. A 3/8" drive suits M6–M14 for lighter work.
AF (Imperial) to Metric Spanner Conversion
Many UK farms still have imperial AF (Across Flats) spanners in the toolbox — especially on older machinery, American-made equipment, or where someone has bought an imperial set. AF sizes are measured in inches; metric in millimetres. They are not interchangeable exactly, but this table shows the closest metric equivalent for each AF size.
Using the wrong size — even close — risks rounding the flats on the bolt head. Where possible, always use the correct metric spanner for metric bolts.
| AF Size | AF in mm | Closest Metric |
|---|---|---|
| 1/4" AF | 6.35 mm | 6 mm |
| 5/16" AF | 7.94 mm | 8 mm |
| 3/8" AF | 9.53 mm | 10 mm |
| 7/16" AF | 11.11 mm | 11 mm |
| 1/2" AF | 12.7 mm | 13 mm |
| 9/16" AF | 14.29 mm | 14 mm |
| 5/8" AF | 15.88 mm | 16 mm |
| 11/16" AF | 17.46 mm | 17 mm |
| 3/4" AF | 19.05 mm | 19 mm |
| 7/8" AF | 22.23 mm | 22 mm |
| 1" AF | 25.4 mm | 24 mm |
| 1-1/16" AF | 26.99 mm | 27 mm |
| 1-1/4" AF | 31.75 mm | 30 mm |
AF sizes shown are standard Whitworth/BSW AF dimensions as commonly found on older British and American machinery.
Which Bolt Sizes Are Used Where on Farm Machinery?
Knowing which bolt size is typical for a given application on a tractor or implement saves time when you are working in the field without a manual. This is a general guide — always verify with the actual fastener before ordering replacements.
Tractor bodywork & guards
Non-structural panels, cab guards, PTO covers, inspection covers. M8 and M10 dominate. Grade 8.8 standard.
Three-point linkage
Top link pins, lower link pins, linkage brackets. M12 and M16 most common. Critical joints — check grade.
Wheel bolts (tractors)
Front axle wheel bolts are typically M16 or M18. Rear wheel weights use M20 or M24. Always 10.9 minimum. Torque to manufacturer spec.
Engine & gearbox
Internal engine bolts are often grade 10.9 or higher. Sump plug is typically M14 or M16. Always use manufacturer torque values here.
Hydraulic connections
Banjo bolts on hydraulic pipes are typically M12 or M14. These seal under pressure — correct torque is critical. Do not overtighten.
Implement attachment
Plough beam bolts, mower blade bolts, fertiliser spreader discs. M12 and M16 most common. Replace blade bolts at service intervals.
Trailer chassis
Drawbar pins and bolts M20–M24. King pins on grain trailers M24–M30. Leaf spring U-bolts often M16. High dynamic load — check regularly.
PTO shafts & drivelines
PTO shield retaining bolts M8. Universal joint clamp bolts M10–M12. Replace rather than re-use if removed in service.
Frequently Asked Questions
- What spanner size fits an M10 bolt?
- An M10 bolt takes a 17 mm spanner (across flats). This is the single most common bolt size on UK tractors and farm equipment — a 17 mm combination spanner lives permanently in most farm toolboxes. Note that some older European-manufactured equipment uses a 16 mm spanner for M10 bolts due to older DIN standards, so if your 17 mm won't fit, try 16 mm.
- What spanner size fits an M12 bolt?
- A standard M12 hex head bolt uses a 19 mm spanner. The hex key (Allen key) size for an M12 socket head cap screw is 10 mm. M12 is extremely common on implement attachment points, three-point linkage pins, and PTO guards.
- What does 8.8 mean on a bolt?
- The marking 8.8 indicates the bolt's strength grade. The first number (8) multiplied by 100 gives the ultimate tensile strength in MPa — so 8.8 = 800 MPa tensile strength. The second number (8) multiplied by the first number and by 10 gives the yield strength — so 8 × 8 × 10 = 640 MPa yield. Grade 8.8 is the standard high-tensile bolt used throughout tractor and agricultural machinery manufacture. If you're replacing a bolt and aren't sure of the grade, 8.8 is the correct default choice for most farm machinery applications.
- What is the difference between 8.8 and 10.9 bolts?
- Grade 10.9 bolts are approximately 25% stronger than 8.8 in both tensile and yield strength. 10.9 bolts are used where the joint is under high dynamic load — wheel bolts on tractors and trailers, engine mounts, critical structural connections. You should never substitute an 8.8 bolt where a 10.9 is specified, as the lower yield strength means the bolt may stretch and fail under load. 10.9 bolts are often black oxide finished and may have the 10.9 marking on the bolt head.
- Why won't my 17 mm spanner fit the M10 bolt?
- A few possible reasons: the bolt may be an older DIN standard that uses 16 mm across flats rather than 17 mm. The bolt head may be corroded or burred, making it appear larger. It may be an imperial AF (across flats) bolt — 11/16" AF is 17.46 mm, just slightly over 17 mm metric. On some imported machinery, bolts may be from different national standards with slightly different across-flats dimensions. Try a 16 mm, then check whether the bolt is metric at all by measuring the thread pitch.
- What bolts are most common on a John Deere tractor?
- John Deere tractors manufactured from the 1990s onwards use metric fasteners throughout. M8, M10, M12 and M16 cover the vast majority of bolts on the tractor body, guards, and linkage. Older pre-1990 John Deere machinery (especially US-manufactured) may use imperial UNC or UNF threads — if a metric bolt does not thread in easily, check the thread with a thread gauge before forcing it. Wheel bolts on most modern John Deere tractors are M18 or M20 depending on the rim specification.
- What is the difference between a hex key and an Allen key?
- They are the same tool — Allen key is a brand name (from the Allen Manufacturing Company) that became a generic term in the UK and Ireland. The correct technical term is hex key or hexagonal key. They are used to drive socket head cap screws (SHCS), also known as Allen bolts or socket bolts. The size refers to the distance across the flats of the hexagonal drive, which is smaller than the bolt diameter — an M10 socket head cap screw uses an 8 mm hex key, not a 10 mm.
- Should I torque bolts dry or lubricated?
- Torque values on most manufacturer torque charts are specified for dry, clean threads. If threads are oiled or greased, the actual clamping force produced at a given torque is higher — meaning you can over-stress the bolt even if the torque wrench shows the right figure. If you must assemble with lubricated threads, reduce the specified torque by approximately 20-25%. Anti-seize compound (copper slip) on stainless or dissimilar metal threads also requires a reduction in torque — typically around 15-20% depending on the compound.