How to Loosen a Rusted or Seized Bolt
The short answer
Clean the exposed thread, soak it in penetrant, give it time, tap the head, then apply leverage with a six-point socket — not an open-ended spanner. In that order. Most seized bolts come loose somewhere in the middle of that list, and most snapped bolts are snapped because someone skipped straight to the end of it.
The spray matters less than the waiting. Ten minutes of soak is worth more than ten more kilos on the end of the bar.
Step 1 — Clean the threads first
Penetrant travels down the thread by capillary action. If the exposed thread is packed with rust scale and dirt, nothing gets in. Thirty seconds with a wire brush on the exposed part of the bolt roughly doubles how well the next step works, and almost nobody does it.
Step 2 — Soak it properly
Spray the penetrant where the bolt meets the surface, not on the head. You want it running down the thread.
Either works. This is the one job a multi-purpose spray is genuinely built for — it is a penetrant and water displacer first, which is exactly what a seized thread needs.
Now wait. Ten minutes minimum. For something badly rusted, spray it, walk away, come back in an hour and spray it again. An overnight soak on a really stubborn one is not excessive. People lose bolts because they sprayed and immediately pulled.
Step 3 — Shock it
Tap the head of the bolt squarely with a hammer, two or three firm knocks. The vibration breaks the rust bond and helps the penetrant wick further down. Re-spray after tapping.
On a nut, tap the flats on opposite sides rather than the end. Do not hit it hard enough to deform the head — you want shock, not damage.
Step 4 — Leverage, with the right tool
This is where most bolts get rounded off.
- Use a six-point socket, not a twelve-point, and never an open-ended spanner. Six points grip the flats of the head; an open-ended spanner grips two faces and spreads them. On a corroded head, the spanner will round it almost every time.
- Push the socket fully home before you load it.
- Pull smoothly and steadily. Sudden jerks snap bolts; steady increasing load lets the thread release.
- A longer bar beats more effort. Use a breaker bar before you use your body weight on a short ratchet — and never stand on a ratchet handle, you will destroy the mechanism.
- Try tightening slightly first. A quarter-turn tighter often breaks the corrosion bond, and then it will come loose.
Browse our sockets and wrenches — and if you do this often, an impact wrench is the real answer. It delivers hundreds of sharp rotational hammer blows a minute, which is exactly the shock-plus-torque combination a seized fastener responds to, without putting bending force on the head.
Step 5 — Heat, carefully
If it still will not move, heat expands the surrounding metal and breaks the bond. A heat gun on the nut or the surrounding casting — not the bolt itself — for a minute or two is often enough.
Before you heat anything, read this. Never apply heat near fuel lines, fuel tanks, brake fluid, hydraulic lines, rubber seals, painted surfaces you care about, or anywhere you have just sprayed penetrant — many of these sprays are flammable until they flash off. Let the solvent evaporate first, and keep an extinguisher within reach if you are working on a vehicle.
If the head is already rounded
- Bolt extractor socket. A reverse-spiral socket that bites harder as you turn. The right answer for a rounded head.
- Hammer on an undersized six-point socket. Crude, effective, usually sacrifices the socket.
- Cut a slot across the head with an angle grinder and use a large flat screwdriver.
- Grind or drill it out, then re-tap the thread. Last resort, and the point at which calling someone is cheaper than carrying on.
The two mistakes
- No soak time. Penetrant is not a solvent that works on contact. It needs to travel.
- An open-ended spanner on a corroded head. You will round it, and then the job is three times harder.
Afterwards
Once it is out, clean the thread and put it back with a smear of grease or anti-seize. The reason it seized is Singapore humidity meeting bare steel, and it will do it again. A multi-purpose spray is not enough on its own here — it is a penetrant, not a long-term barrier. White lithium grease ($15.00) on anything you will need to undo again outdoors is the difference between a two-minute job next time and this one.
Not sure which spray is which? WD-40 or Rexco: which spray you actually need sets out penetrant, cleaner and lubricant side by side. Browse the WD-40 range and Rexco range, both in stock.
General guidance only. Read the manufacturer's label and safety data before use, and take particular care with heat and flammable solvents.