Question 1

What torque should I use for an M6 bolt?

Accepted Answer

For a grade 8.8 M6 bolt with clean, lightly oiled threads tightening into steel, the recommended torque is approximately 9.5 Nm. For grade 10.9 M6, use approximately 14 Nm. For grade 12.9 M6, use approximately 16 Nm. M6 bolts are common on hydraulic valve bodies, electrical brackets, and small cover plates on UK farm machinery.

Question 2

What torque should I use for an M8 bolt?

Accepted Answer

For a grade 8.8 M8 bolt with lightly oiled threads, the recommended torque is approximately 23 Nm. For grade 10.9 M8, use approximately 34 Nm. For grade 12.9 M8, use approximately 40 Nm. If the bolt is dry, increase by 25%. M8 bolts are found on linkage brackets, guard fixings, pump mounting, and cab fittings throughout agricultural machinery.

Question 3

What torque should I use for an M10 bolt?

Accepted Answer

For a grade 8.8 M10 bolt with lightly oiled threads, the recommended torque is approximately 46 Nm. For grade 10.9 M10, use approximately 68 Nm. For grade 12.9 M10, use approximately 79 Nm. M10 is a very common size on implement frame joints, hydraulic block mounting, and chassis brackets.

Question 4

What torque should I use for an M12 bolt?

Accepted Answer

For a grade 8.8 M12 bolt with clean, lightly oiled threads tightening into steel, the recommended torque is approximately 79 Nm. For grade 10.9 M12, use approximately 115 Nm. For grade 12.9 M12, use approximately 135 Nm. M12 is one of the most common sizes on UK farm machinery — three-point linkage pins, drawbar bolts, and gearbox mounting. If using copper anti-seize, reduce these figures by approximately 20%.

Question 5

What torque should I use for an M16 bolt?

Accepted Answer

For a grade 8.8 M16 bolt with lightly oiled threads, the recommended torque is approximately 195 Nm. For grade 10.9 M16, use approximately 280 Nm. For grade 12.9 M16, use approximately 330 Nm. M16 is common on main implement frame bolts, PTO guard fixings, and cab mount bolts. A 1/2" drive torque wrench typically reaches 340 Nm — just enough for an M16 grade 10.9.

Question 6

What torque should I use for an M20 bolt?

Accepted Answer

For a grade 8.8 M20 bolt with lightly oiled threads, the recommended torque is approximately 385 Nm. For grade 10.9 M20, use approximately 560 Nm. For grade 12.9 M20, use approximately 650 Nm. At this size a 3/4" drive torque wrench or a torque multiplier with a 1/2" wrench is needed for accurate results.

Question 7

What is the difference between grade 8.8, 10.9 and 12.9 bolts?

Accepted Answer

The numbers refer to the property class (strength) of the bolt per ISO 898-1. Grade 8.8 is medium-strength carbon steel — the most common general-purpose bolt on farm machinery. Grade 10.9 is high-strength alloy steel, used in structural joints, flanges and precision machinery. Grade 12.9 is the highest common grade, maximum tensile strength, usually with a black oxide finish. The grade is stamped on the bolt head. Never substitute a lower grade for a higher grade without recalculating the required torque — the bolt may fail in service.

Question 8

Does using copper slip (anti-seize) change the required torque?

Accepted Answer

Yes — significantly. Copper anti-seize (Copaslip) reduces the friction coefficient, so the same torque produces more clamping force than a dry or oiled bolt. If you apply anti-seize and use the standard oiled torque figure, you risk over-stressing the bolt and potentially stripping threads or fracturing the bolt. Reduce the torque by approximately 20% compared to a lightly oiled condition, or by about 36% compared to a dry condition. The calculator above handles this automatically.

Question 9

What torque wrench do I need for farm machinery?

Accepted Answer

For general farm workshop use you need at minimum a 3/8" drive torque wrench (5–80 Nm) for smaller fasteners and a 1/2" drive torque wrench (40–340 Nm) covering most M8–M20 bolts. For large fasteners like M24+ or PTO flanges, a 3/4" drive torque wrench (100–800 Nm) or a torque multiplier used with a 1/2" drive wrench gives better accuracy at high values. Cheap click-type torque wrenches drift over time — calibrate or replace every 2–3 years.

Question 10

What torque for tractor wheel nuts?

Accepted Answer

Tractor wheel nut torque must always come from the manufacturer's service manual — it varies significantly by make, model, rim size, and stud specification. Typical values range from 390 Nm to 680 Nm for agricultural tractors, but this is only a guide. Do not use a generic bolt torque chart for wheel fasteners. Always refer to the specific torque in the operator's manual or workshop manual, and re-torque after the first 2 hours of use after any wheel removal.

Question 11

How do I read the markings on a bolt head?

Accepted Answer

Metric bolts have the property class stamped on the head — for example "8.8", "10.9" or "12.9". Grade 8.8 bolts are typically plain or silver in colour. Grade 10.9 are often yellow zinc-plated. Grade 12.9 are commonly black (black oxide finish). Bolts without markings should be assumed to be low-grade (4.6 or 4.8) and should not be used as replacements in high-load applications on machinery.

Question 12

Can I reuse bolts after they have been torqued?

Accepted Answer

Standard bolts (not torque-to-yield) can generally be reused if they are in good condition — no corrosion, no thread damage, not stretched. Torque-to-yield (TTY) bolts — which are common on engine internals such as cylinder heads and connecting rods — must never be reused. They are designed to stretch past their yield point during tightening, which provides a precise and consistent clamp load. Attempting to retorque or reuse a TTY bolt is dangerous. TTY bolts are usually identified in the service manual as "one-time use" or by an angular tightening specification (e.g. "tighten to 20 Nm + 90°").

Size	Pitch (mm)	Grade 8.8 (Nm)	Range (Nm)	Common on farm machinery
M5	0.80	5.5	5–6	Small cover plates, sensor mounts, cab trim fixings
M6common	1.00	9.5	9–10	Hydraulic valve bodies, electrical brackets, implement covers
M8common	1.25	23	21–25	Linkage brackets, guard fixings, pump mounting, cab fittings
M10common	1.50	46	41–51	Implement frame joints, hydraulic block mounting, chassis brackets
M12common	1.75	79	71–87	Three-point linkage pins, drawbar bolts, gearbox mounting
M14	2.00	125	113–138	Heavy implement frame joints, larger axle mounting brackets
M16common	2.00	195	176–215	Main implement frame bolts, PTO guard fixings, cab mounts
M18	2.50	270	243–297	Heavy-duty frame joints, counterweight mounting
M20	2.50	385	347–424	Ballast weight frames, large implement hitch points
M22	2.50	520	468–572	Large structural joints, heavy counterweight systems
M24	3.00	660	594–726	Major structural bolts — always check manufacturer spec at this size
M27	3.00	980	882–1078	Large flanges, heavy machinery — manufacturer spec required
M30	3.50	1320	1188–1452	Very large structural joints — always use manufacturer torque figure

Bolt Torque Settings Calculator

Torque Calculator

Reference Table — Lightly Oiled, Coarse Thread

Lubrication Condition Guide

Frequently Asked Questions — Bolt Torque Settings