This is a general, instructive guide to how the system works and what to look for. It is not a service manual, and it is not specific to your truck. Procedures, clearances, pressures, torque figures and service limits differ between makes, models and build dates, and the manual for the machine you are actually working on is the only authority on what is correct for it. Work to that manual, and to your employer's safety procedures.
Every threaded fastener on a forklift is made to a specification that fixes its strength and hardness, and that specification is stamped on the head. A capscrew that looks identical to the one you removed may be half its strength. This covers how to read the markings, what the grades mean, and what torque to apply.
A replacement fastener must be the same strength or greater than the original, and the correct size. Fasteners from different national standards are often dimensionally similar and are not interchangeable. A Grade 8 bolt in a Grade 2 nut is a Grade 2 joint — the nut strips first.
Reading a fastener description
Both systems put the shank diameter first, but they differ in what comes next.
- Inch: 1/2 x 13 UNC x 1-1/2
- 1/2 in shank diameter, 13 threads per inch, Unified National Coarse thread form, 1-1/2 in shank length
- Metric: M12 x 1.75 x 50
- 12 mm thread size, 1.75 mm pitch, 50 mm length
The difference in the middle number catches people out. The inch figure is threads per inch — a count. The metric figure is pitch — the distance from one thread crest to the next, in millimetres. A bigger inch number means a finer thread; a bigger metric number means a coarser one.
Inch descriptions often omit the thread count entirely and just say UNC or UNF, because each diameter has one standard count for each. Metric always states the pitch, because several are common at the same diameter — M12 exists as 1.75 coarse and 1.25 fine.
Strength markings
Inch fasteners: SAE grades
Grade is shown by radial lines on the head, arranged like spokes. Count them and add two:
- No marks
- Grade 2 — the lowest common grade. Unmarked does not mean ungraded, but it also gives you no assurance.
- Three lines
- Grade 5 — the general-purpose structural fastener
- Six lines
- Grade 8 — high strength
Grades 5.2, 7 and 8.1 also exist with their own patterns, and socket head capscrews are frequently unmarked regardless of strength. Nuts follow the same idea with lines or dots, and hex nuts may instead carry marks on the flats.
Metric fasteners: property class
Metric strength is a printed number, not a pattern — 8.8, 10.9, 12.9 and so on, stamped on the head or, for nuts, on the face or flats. Classes below 8.8 are often left unmarked. Marks are generally applied only from M5 upward; below that, sizes M5 to M11 may carry an optional geometric symbol instead of a number.
It encodes the actual strength. The first number is tensile strength in hundreds of megapascals; the two multiplied together, times ten, give yield strength in megapascals. So 8.8 means roughly 800 MPa tensile and 640 MPa yield, and 10.9 means 1000 MPa tensile and 900 MPa yield. Once you know that, the numbers stop being codes and start being data.
Rough equivalence between the systems
Useful for judging what you are looking at, not for substituting one for the other — the threads do not interchange regardless.
- SAE Grade 2
- approximately metric property class 5.8
- SAE Grade 5
- approximately metric property class 8.8
- SAE Grade 8
- approximately metric property class 10.9
Nut markings
Nuts carry their own strength marks, and the systems mirror the bolts: inch nuts use a count of dots or dashes, metric nuts use a printed number. A nut must be at least the grade of the bolt it runs on, because the joint fails at whichever is weaker.
Marking practice varies between manufacturers, and plenty of legitimate fasteners carry no marks at all — socket head capscrews and smaller metric sizes especially. An unmarked fastener in a structural joint should be replaced with one of known grade rather than assumed to match.
Prevailing torque nuts
These resist loosening by deliberately being hard to turn all the way down, and they come in two families.
All-metal prevailing torque nuts use a deformed section of thread. They tolerate heat and can usually be reused a limited number of times, though the prevailing torque falls with each use.
Nylon insert nuts use a polymer collar that the thread cuts into. They grip well, they are cheap, and they have two limits worth respecting: heat destroys the insert, and reuse is poor because the thread has already cut its path. Both types carry their own marking schemes, and on inch sizes markings are frequently not required at all.
Torque values
These are general values for standard fasteners in the grades shown. Where the service data for a specific joint gives a torque, that figure wins — designed joints frequently specify something other than the generic value for the fastener size.
A torque figure is only meaningful for the friction condition it was derived for. Oil, anti-seize or thread locker on threads that were meant to be assembled dry produces substantially more clamp load at the same torque reading — enough to yield or snap the fastener. If a procedure calls for a thread treatment, it has been accounted for. If it does not, do not add one.
Inch fasteners
| Size and pitch | Grade 2 (lbf ft) | Grade 2 (N·m) | Grade 5 (lbf ft) | Grade 5 (N·m) | Grade 8 (lbf ft) | Grade 8 (N·m) |
|---|---|---|---|---|---|---|
| 1/4-20 UNC | 4 | 6 | 6 | 9 | 9 | 12 |
| 1/4-28 UNF | 5 | 6 | 7 | 10 | 10 | 14 |
| 5/16-18 UNC | 8 | 11 | 13 | 18 | 18 | 25 |
| 5/16-24 UNF | 9 | 13 | 14 | 20 | 20 | 28 |
| 3/8-16 UNC | 15 | 20 | 23 | 31 | 33 | 44 |
| 3/8-24 UNF | 17 | 23 | 26 | 36 | 37 | 50 |
| 7/16-14 UNC | 24 | 33 | 37 | 50 | 52 | 71 |
| 7/16-20 UNF | 27 | 36 | 41 | 56 | 58 | 79 |
| 1/2-13 UNC | 37 | 50 | 57 | 77 | 80 | 110 |
| 1/2-20 UNF | 41 | 56 | 64 | 86 | 90 | 120 |
| 9/16-12 UNC | 53 | 72 | 82 | 110 | 115 | 155 |
| 9/16-18 UNC | 59 | 80 | 91 | 125 | 130 | 175 |
| 5/8-11 UNC | 73 | 99 | 115 | 155 | 160 | 215 |
| 5/8-18 UNF | 83 | 110 | 130 | 175 | 180 | 245 |
| 3/4-10 UNC | 130 | 175 | 200 | 270 | 280 | 380 |
| 3/4-16 UNF | 145 | 195 | 225 | 300 | 315 | 425 |
| 7/8-9 UNC | 125 | 170 | 320 | 435 | 455 | 615 |
| 7/8-14 UNF | 140 | 185 | 355 | 480 | 500 | 680 |
| 1-8 UNC | 185 | 255 | 485 | 655 | 680 | 925 |
| 1-14 UNF | 210 | 285 | 540 | 735 | 765 | 1,040 |
| 1-1/8-7 UNC | 265 | 360 | 595 | 805 | 965 | 1,310 |
| 1-1/8-12 UNF | 300 | 405 | 670 | 905 | 1,080 | 1,470 |
| 1-1/4-7 UNC | 375 | 510 | 840 | 1,140 | 1,360 | 1,850 |
| 1-1/4-12 UNF | 415 | 565 | 930 | 1,260 | 1,500 | 2,050 |
| 1-3/8-6 UNC | 490 | 665 | 1,100 | 1,490 | 1,780 | 2,420 |
| 1-3/8-12 UNF | 560 | 760 | 1,250 | 1,700 | 2,040 | 2,760 |
| 1-1/2-6 UNC | 650 | 885 | 1,460 | 1,980 | 2,370 | 3,210 |
| 1-1/2-12 UNF | 735 | 995 | 1,650 | 2,230 | 2,670 | 3,620 |
Metric fasteners
| Size and pitch | Class 5.8 (N·m) | Class 5.8 (lbf ft) | Class 8.8 (N·m) | Class 8.8 (lbf ft) | Class 10.9 (N·m) | Class 10.9 (lbf ft) |
|---|---|---|---|---|---|---|
| M3 x 0.5 | 0.62 | 0.5 | 0.99 | 0.7 | 1.34 | 1.0 |
| M3.5 x 0.6 | 0.97 | 0.7 | 1.55 | 1.1 | 2.11 | 1.6 |
| M4 x 0.7 | 1.44 | 1.1 | 2.30 | 1.7 | 3.13 | 2.3 |
| M5 x 0.8 | 2.91 | 2.1 | 4.65 | 3.6 | 6.33 | 4.7 |
| M6 x 1 | 4.94 | 3.6 | 7.90 | 6 | 10.8 | 8 |
| M8 x 1.25 | 12.0 | 9 | 19.2 | 14 | 26.1 | 19 |
| M8 x 1 | 12.8 | 9 | 20.5 | 15 | 27.9 | 21 |
| M10 x 1.5 | 23.8 | 18 | 38.0 | 28 | 52 | 38 |
| M10 x 1.25 | 25.1 | 19 | 40.1 | 30 | 55 | 41 |
| M12 x 1.75 | 41.4 | 31 | 66 | 49 | 90 | 66 |
| M12 x 1.25 | 45.3 | 33 | 72 | 53 | 98 | 72 |
| M14 x 2 | 66 | 49 | 105 | 77 | 145 | 105 |
| M14 x 1.5 | 72 | 53 | 115 | 85 | 155 | 115 |
| M16 x 2 | 105 | 77 | 165 | 122 | 225 | 165 |
| M16 x 1.5 | 110 | 81 | 175 | 130 | 240 | 175 |
| M20 x 2.5 | 200 | 150 | 320 | 235 | 435 | 320 |
| M20 x 1.5 | 225 | 165 | 355 | 260 | 485 | 360 |
| M24 x 3 | 345 | 255 | 555 | 410 | 755 | 560 |
| M24 x 2 | 375 | 275 | 605 | 445 | 820 | 605 |
| M27 x 3 | 505 | 370 | 810 | 600 | 1,100 | 810 |
| M27 x 2 | 550 | 405 | 875 | 645 | 1,190 | 880 |
| M30 x 3.5 | 690 | 510 | 1,100 | 810 | 1,500 | 1,100 |
| M30 x 3 | 715 | 530 | 1,140 | 840 | 1,550 | 1,140 |
| M30 x 2 | 765 | 565 | 1,220 | 900 | 1,660 | 1,230 |
| M33 x 3.5 | 940 | 695 | 1,500 | 1,100 | 2,040 | 1,510 |
| M33 x 2 | 1,030 | 760 | 1,640 | 1,210 | 2,240 | 1,660 |
| M36 x 4 | 1,200 | 885 | 1,930 | 1,430 | 2,620 | 1,940 |
| M36 x 3 | 1,280 | 945 | 2,040 | 1,510 | 2,780 | 2,050 |
| M39 x 4 | 1,560 | 1,150 | 2,490 | 1,840 | 3,390 | 2,500 |
| M39 x 3 | 1,640 | 1,210 | 2,630 | 1,940 | 3,570 | 2,640 |
| M42 x 4.5 | 1,930 | 1,430 | 3,080 | 2,280 | 4,200 | 3,100 |
| M42 x 3 | 2,070 | 1,530 | 3,320 | 2,450 | 4,510 | 3,330 |
| M45 x 4.5 | 2,410 | 1,780 | 3,850 | 2,840 | 5,240 | 3,870 |
| M45 x 3 | 2,580 | 1,910 | 4,120 | 3,040 | 5,610 | 4,140 |
| M48 x 5 | 2,900 | 2,140 | 4,630 | 3,420 | 6,300 | 4,650 |
| M48 x 3 | 3,160 | 2,330 | 5,040 | 3,720 | 6,860 | 5,060 |
Unit conversions
- Torque
- lbf ft × 1.356 = N·m • N·m × 0.7376 = lbf ft • lbf in × 0.113 = N·m
- Length
- in × 25.4 = mm • mm × 0.03937 = in • ft × 0.3048 = m
- Mass
- lb × 0.4536 = kg • kg × 2.205 = lb
- Pressure
- psi × 6.895 = kPa • psi × 0.006895 = MPa • MPa × 145.0 = psi
- Volume
- US gal × 3.785 = litres • litres × 0.2642 = US gal • US qt × 0.9464 = litres
- Power
- hp × 0.7457 = kW • kW × 1.341 = hp
- Temperature
- (°F − 32) × 0.556 = °C • (°C × 1.8) + 32 = °F
Frequently asked questions
The bolt I removed has no markings. What grade is it?
On an inch fastener, no radial lines normally means Grade 2 — the lowest common grade. But absence of marks is not proof: socket head capscrews are often unmarked whatever their strength, and metric classes below 8.8 are frequently unmarked too. If the fastener is doing real structural work and you cannot identify it, replace it with a known grade at or above what the application requires rather than guessing.
What does 8.8 actually mean?
It is the strength, written out. The first number is tensile strength in hundreds of megapascals and the two multiplied by ten give yield strength in megapascals — so 8.8 is about 800 MPa tensile and 640 MPa yield, and 10.9 is 1000 and 900. It is roughly equivalent to SAE Grade 5, with 10.9 close to Grade 8.
Can I fit a stronger bolt than the one that came out?
The standard rule is that a replacement must be of equal or greater strength and the correct size, so going up a grade is normally acceptable. Two cautions. The nut must match — a Grade 8 bolt in a Grade 2 nut gives you a Grade 2 joint, because the nut strips first. And if you fit a higher grade, torque it to the value for the grade actually installed, not the one you removed.
Metric and inch bolts look the same size. Will one substitute for the other?
No. Fasteners made to different national standards are frequently similar enough to start into a thread and are not interchangeable. A close-but-wrong thread damages the tapped hole as it goes in, and the joint holds at a fraction of its rated load. This is the reason to order by part number rather than by eye.
Should I oil the threads before torquing?
Not unless the procedure says so. Torque figures assume a friction condition, and a torque wrench measures the resistance to turning rather than the clamp load you are actually creating. Lubricated threads reach far higher clamp load at the same reading — enough to yield or break the fastener. Where a procedure calls for oil, anti-seize or a thread locker, it has been allowed for in the figure given.
Can I reuse a nylon insert lock nut?
Poorly. The thread cuts a path through the nylon on first use, so the locking effect drops sharply afterwards. The insert is also destroyed by heat, which rules them out near an exhaust or a brake. All-metal prevailing torque nuts tolerate both reuse and heat far better, though their prevailing torque also falls with each cycle.
Why does the middle number mean different things on inch and metric bolts?
Because inch fasteners count threads and metric fasteners measure them. In 1/2 x 13 UNC the 13 is threads per inch, so a higher number is a finer thread. In M12 x 1.75 the 1.75 is the pitch in millimetres — the distance between thread crests — so a higher number is a coarser thread. They run in opposite directions.
Before you order
Have the size, thread pitch or count, length, and grade or property class. Any one of those wrong makes the fastener useless, and three of them are invisible once the part is in a bag. For anything structural — mast, axle, overhead guard, counterweight — order by the part number from the parts manual rather than matching from a bin, because those joints often use fasteners with specified torques and specified grades that are not the generic value for their size.