Choosing wrenching tools
The list above is what exists. This is how to decide which of it you actually need, what separates a tool worth keeping from one that ends up at the back of a drawer.
How many flats the tool actually holds
An open ended spanner touches two flats of a hexagon. All the force goes through those two faces and it wants to spread open, so on a tight fastener it slips and rounds the corners. It exists because sometimes there is no room for anything better.
A ring spanner surrounds all six. A six point socket does too, and bears on the flats rather than the corners. A twelve point socket fits in twice as many positions and bears closer to the corners, which is faster in tight spaces and more likely to round a tight nut.
- Open end, two flats
- Fastest to use, most likely to slip. For access, not for tight fasteners.
- Ring, all six
- Surrounds the nut so it cannot spread. Use this whenever it will go on.
- Six point socket
- Bears on the flats. The right choice on anything seized or already marked.
- Twelve point socket
- Twice the engagement positions and closer to the corners. Convenient, less secure.
Size, and the near misses
Metric and imperial sizes overlap closely enough to be dangerous. A 1/2 inch spanner is 12.7mm and will go onto a 13mm nut with a little play, which is exactly the play that rounds it. 19mm and 3/4 inch differ by a third of a millimetre.
The rule is that the correct spanner goes on with no perceptible rock. If it rocks, it is the wrong size, and no amount of care makes up for that. This is the single commonest cause of a damaged fastener.
- No rock, no play
- The right size goes on snug. Any rock at all and you are on the wrong tool.
- The dangerous pairs
- 13mm and 1/2 inch, 19mm and 3/4 inch, 16mm and 5/8 inch. All close enough to fit badly.
- Whitworth exists
- Anything British and pre 1970 may be Whitworth, which matches neither set.
- Check before force
- The moment to find the wrong size is before the pull, not during it.
Getting a seized fastener moving
Force is the last thing to try, not the first. Penetrating oil and time, then heat, then shock from a hammer, then leverage. Each of those has a better chance of breaking the corrosion bond than simply pulling harder, and none of them rounds the nut.
When leverage is needed, a breaker bar with a six point socket spreads the load across all six flats. Sharp shock helps more than steady pull: an impact driver or a few hammer blows on the end of a bar break a bond that constant force will not.
- Oil and wait
- Penetrating oil needs hours, not seconds. Applied and left overnight it does most of the work.
- Heat expands
- Warm the nut, not the bolt. It grows away from the thread and breaks the rust bond.
- Shock over pull
- Sharp blows crack corrosion that steady force only tightens against.
- Tighten first
- A fraction tighter often breaks the bond, and then it comes loose. Counterintuitive and it works.
Tightening, and why torque is a number
Tight enough is not a feeling. A bolt is a spring: the correct torque stretches it into its elastic range so it stays clamped, and past that it yields and loses clamping force even though it feels tighter than ever.
That is why a torque wrench matters on anything structural, on alloy castings, and on wheels. Alloy strips at surprisingly low figures, and a wheel nut over torqued can crack the stud. Under torqued is not safer, just a different failure.
- A bolt is a spring
- Correct torque stretches it elastically. Past that it yields and the clamp is lost.
- Alloy strips early
- Aluminium threads take far less than the steel bolt going into them. Look the figure up.
- Click and stop
- A click wrench releases once. Pulling through it again applies more than the setting.
- Wind it back
- Store a click wrench at its lowest setting or the spring takes a set and the reading drifts.
When it is already rounded
Once the corners are gone, no correctly sized tool will grip. What works is a tool that bites into what is left: a gripping socket with spiral flutes, a nut splitter that cracks the nut off the thread, or vice grips if there is enough left to hold.
Cutting is the last resort and often the fastest one. A nut splitter, a grinder or a hacksaw destroys the nut and leaves the thread usable, which is a better outcome than three hours spent on a fastener that is worth pennies.
- Gripping sockets
- Spiral flutes bite into a rounded head. They mark the fastener, which is already the case.
- Nut splitter
- A hardened chisel driven through the nut wall. Destroys the nut, saves the thread and the stud.
- Weld or braze a nut on
- Where a head has sheared. Adds heat too, which helps on its own.
- Know when to cut
- A nut costs pennies. Three hours does not. Cut it and move on.
Frequently asked questions
- What is the difference between a wrench and a spanner?
- Mostly geography. British usage calls the fixed sized tool a spanner and reserves wrench for adjustable and specialised tools. American usage calls all of them wrenches. The tools are the same, so a 13mm spanner and a 13mm wrench are one item.
- Why do open ended spanners round nuts?
- Because they only touch two flats of the hexagon. All the force goes through those two faces and the jaws want to spread, so on a tight fastener the tool slips and takes the corners with it. A ring spanner or a six point socket surrounds all six instead.
- Should I use a six point or twelve point socket?
- Six point for anything tight, seized or already marked, because it bears on the flats rather than near the corners. Twelve point fits in twice as many positions, which is genuinely useful in a tight space, and it is more likely to round a stubborn fastener.
- How do you loosen a seized nut?
- In this order: penetrating oil left for hours, heat applied to the nut so it expands away from the bolt, sharp shock from a hammer or an impact driver, then leverage with a breaker bar and a six point socket. Trying to loosen it slightly tighter first often breaks the bond.
- Can I use an adjustable spanner instead of the right size?
- For light work, yes, and it grips two flats with a jaw that has play in it, which is why it rounds fasteners more readily than anything else in the box. Always pull toward the moveable jaw so the load closes it rather than opening it.
- What size socket set should I buy?
- A 3/8 inch drive set covering 8 to 19mm handles most jobs on a car or around a house. Add 1/4 inch drive for small fasteners in tight places and 1/2 inch for wheel nuts and suspension. Buying by drive size rather than piece count is the useful way to choose.
- Do I really need a torque wrench?
- For wheels, engine work, alloy castings and anything structural, yes. A bolt clamps by stretching elastically, and past its correct torque it yields and loses that clamp while feeling tighter than ever. Aluminium threads in particular strip well below what feels tight.
- How do I remove a nut with rounded corners?
- A gripping socket with spiral flutes bites into what is left of the head. If there is nothing to grip, a nut splitter cracks the nut off the thread and leaves the stud usable. Cutting it off with a grinder or hacksaw is a legitimate answer rather than a failure.
