A bolt and nut work as a pair: the bolt passes through both parts and the nut clamps them together. The right pairing depends on thread size, thread pitch, head style and the job. Mixing the wrong thread types is the most common beginner mistake. This guide covers the common types, how threads are described, a worked fitting example and the problems that crop up most often.
Common Bolt Types
- Hex bolts: six-sided heads turned with a spanner or socket, the general-purpose choice.
- Carriage bolts: a domed head with a square neck that bites into wood so the bolt does not spin while you tighten the nut.
- Machine screws and bolts: threaded the full length, often with slotted or cross heads, for metal parts.
- Lag (coach) bolts: coarse threads that screw directly into wood, used without a nut.
- Eye bolts and U-bolts: shaped for attaching ropes, chains or pipes.
- Anchor bolts: made to fix into masonry or concrete.
Common Nut Types
Hex nuts are the standard. Lock nuts (nylon insert type) resist working loose from vibration. Wing nuts can be turned by hand where you need to adjust often. Flange nuts have a built-in washer face that spreads the load. Cap or acorn nuts cover the exposed thread end for a smooth, tidy finish. Castle nuts have slots for a retaining pin in demanding assemblies.
Thread Matters
A nut only works on a bolt with the same diameter and pitch. Metric sizes are written like M8, where the number is the diameter in millimetres, and imperial sizes use fractions of an inch. Coarse and fine pitches are different and do not interchange. Check by threading the nut on by hand; it should run smoothly with no force. If it binds, stop.
Reading a Size Label
Metric bolts are often written as diameter, then pitch, then length. An invented example: M8 x 1.25 x 40 would mean an 8 millimetre diameter, a thread pitch of 1.25 millimetres between thread crests, and a 40 millimetre length. Imperial bolts use a fraction for diameter, a number for threads per inch, and a length, for example a quarter inch bolt in a coarse or fine thread series. Many packs and hardware counters have a thread gauge or a size board where you can test a loose bolt or nut against marked holes.
Length is measured differently by head style. For most hex and machine bolts, length runs from under the head to the tip. For flat or countersunk heads, the whole head sits flush, so length includes the head. If you are unsure, check the maker’s chart.
Quick Pairing Table
| Situation | Suitable combination |
|---|---|
| Joining two wooden boards | Carriage bolt, washer and nut |
| Metal bracket in a high-vibration area | Hex bolt with lock nut |
| Tool-free adjustment | Bolt with wing nut |
| Fixing into masonry | Anchor bolt as the maker directs |
Fitting a Bolt Properly
- Drill a hole just slightly larger than the bolt diameter.
- Slide in the bolt and add a washer under the nut, and under the head if it will bear on wood.
- Thread the nut on by hand first.
- Tighten with a correctly sized wrench, holding the head still if needed.
- Stop when snug. Where the maker gives a tightness value, use a torque wrench and follow that figure.
For anything that carries load, such as swing frames, ladders, vehicle parts or structural connections, follow the maker’s instructions and building codes, and ask a qualified professional when in doubt. Do not assume a bolt that looks the same is the same strength.
Worked Walkthrough: Bolting a Wooden Bench Frame
This is a general example for a light garden bench. For any seat that must carry people, follow the plan or kit maker’s fixing list and ask a qualified person if you are unsure.
- Dry fit. Clamp the two timbers together in their final position and check the alignment.
- Mark the hole. Mark where the bolt will pass, away from end grain and edges so the wood does not split.
- Drill through. Keep the drill straight, ideally with a guide block. Use a bit slightly larger than the bolt as noted above.
- Insert the carriage bolt from the side where the domed head will be visible. Tap it gently so the square neck seats into the wood.
- Add washer and nut on the other side. Thread by hand until it meets the washer.
- Tighten gradually with a wrench until the joint is snug and the washer begins to bite slightly. Do not crush the wood.
- Recheck after a few days. Wood moves with moisture, so joints may loosen a little and need another turn.
Common Scenarios
A nut that spins freely on the bolt
Either the thread is stripped or the sizes differ. Compare with a matching nut or a thread gauge and replace both parts if damaged.
A bolt that turns while you tighten the nut
Hold the head with a second wrench, or use a carriage bolt in wood so the neck grips. Where the head is hard to reach, a hex head with a wrench from both sides works.
Outdoor use
Fasteners left outside can rust. Look for coatings or materials the maker recommends for outdoor use, and do not mix very different metals in damp conditions.
Reusing old hardware
Check threads and heads for damage, and do not reuse lock nuts whose insert has worn. When a part is critical, use new fasteners as directed.
Quick Troubleshooting
| Symptom | Likely cause | What to try |
|---|---|---|
| Nut will not thread on | Wrong pitch or damaged thread | Back off, check with a gauge, clean the thread |
| Joint loosens over time | Vibration or wood movement | Add a lock nut or washer, retighten |
| Head rounds off | Wrong tool size | Use the exact socket or wrench size |
| Wood splits at the hole | Hole too close to edge or end | Move the hole inward and drill a pilot first |
| Rust or seized threads | Moisture and time | Use a rust-loosening product as directed and work gently |
Mistakes to Avoid
- Over-tightening. Too much force can stretch the bolt, strip the thread or crush wood.
- Skipping washers on wood. Without a washer, the nut or head can sink into the surface and the joint loosens.
- Mixing metric and imperial. They may begin to thread but jam, damaging both parts.
- Assuming all bolts are equally strong. Strength depends on material and marking, so use the grade the plan or maker calls for.
Glossary
- Pitch: the distance between neighboring threads, or the count of threads per inch.
- Shank: the unthreaded part of a bolt below the head.
- Washer: a flat ring that spreads pressure under a head or nut.
- Torque: the twisting force applied when tightening.
- Grade: a marking showing the strength class of a bolt.
FAQ
Why does my nut loosen over time?
Vibration and movement can do this. A lock nut, spring washer or thread-locking product made for the purpose can help; follow the maker’s guide.
Can I cut a bolt shorter?
You can, but file the end smooth and check that the thread still accepts the nut before you rely on it. Cutting can also remove coatings, so protect the bare end where it will sit outdoors.
What is the difference between a bolt and a screw?
A bolt normally pairs with a nut and clamps parts. A screw threads into a tapped hole or directly into material. The line is not always sharp, as machine screws and some bolts overlap.
Do I always need a washer?
Not always, but washers help on soft materials such as wood and where a hole is larger than the head. Follow the plan or maker for the load-bearing parts.
How tight is tight enough?
Snug without damage is the general rule. Where the maker gives a torque value, use a torque wrench and follow it.
The Short Version
Match diameter, pitch and head style, use washers to spread the load, and add a lock nut where vibration is expected. Check threads by hand before tightening. For load-bearing work, follow the maker’s instructions and seek professional advice.