Which penetrating fluid frees a seized bolt?
Which penetrating fluid frees a seized bolt?
A dedicated penetrant or a 50/50 acetone and automatic transmission fluid mix, plus time, heat cycles and shock — and anti-seize on reassembly.
A seized fastener is usually corrosion in the thread acting as a wedge, and steady increasing torque simply loads it until the bolt shears — leaving a far worse job. The successful approach applies energy in ways other than twisting: penetrant given time to creep in, heat and cooling cycles that break the rust bond by differential expansion, and sharp shock loads that crack the corrosion. Comparative tests, including the well-known Machinist's Workshop magazine ones, put a 50/50 mixture of acetone and automatic transmission fluid at or near the top of the field, comfortably ahead of general water-displacing sprays. Once free, the fastener should go back with anti-seize: copper-based for general use, nickel-based for stainless and high temperatures, where galling is the failure mode rather than rust.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Dedicated penetrating fluid | Low viscosity and formulated to creep into the thread by capillary action, with additives that attack rust. | Needs repeated applications over hours, not one squirt and a spanner. |
| Acetone and ATF mix | Independent testing has repeatedly found this home mixture to perform as well as, or better than, commercial penetrants. | Has to be mixed and shaken each time because it separates, and acetone attacks paint and many plastics. |
| Heat | Expanding and contracting the outer part breaks the rust bond mechanically; heat cycles do more than any fluid. | Fire risk near fuel, brake fluid and undersealing, and it destroys the temper of hardened fasteners. |
| Impact and shock | Sharp blows on the head or an impact driver break the bond rather than twisting the fastener until it shears. | Can deform the head or crack a casting. |
Technique that changes the outcome
- Wire-brush the exposed thread first so penetrant can reach it
- Apply penetrant repeatedly over hours, tapping the head between applications
- Heat the surrounding part, not the bolt, and let it cool before trying again
- Use a six-point socket or a ring spanner, never an open-ended one, and prefer an impact driver
- Apply anti-seize on reassembly, and reduce the torque figure accordingly
What goes wrong
- Steadily increasing torque until something gives
- Heat near fuel lines, brake components, plastics or underseal
- Anti-seize on a fastener with a specified dry torque without adjusting the figure — lubricated threads reach the same preload at 20-30% less torque
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other lubricant questions
| For | Use |
|---|---|
| A bike chain | Wet lube for wet conditions, dry lube for dry and dusty ones, wax if you want the cleanest and lowest-friction option — and not a general penetrant. |
| A lock | A dry lubricant — powdered graphite or a PTFE spray. Oil and general penetrating sprays gum locks up and are the classic way to ruin one. |
| A squeaky door hinge | A drop of light machine oil or a smear of white lithium grease on the pin — and lift the pin out to clean it, which is usually the actual fix. |
| A squeaky floor | Talc, graphite or powdered soapstone worked into the joint as a stopgap — the real fix is screwing the board down to the joist. |
| A sticky drawer | Paraffin wax or beeswax on wooden runners, dry PTFE on metal ones — and never oil on wood. |
| Rubber seals | Silicone grease on most rubbers, and a PTFE or fluorosilicone product on silicone rubber itself — petroleum products swell and destroy natural rubber and EPDM. |
| A garage door | White lithium grease or a garage-door lubricant on rollers, hinges, springs and bearings — and not on the track, which should be clean and dry. |
Common questions
- Which penetrating fluid frees a seized bolt?
- A dedicated penetrant or a 50/50 acetone and automatic transmission fluid mix, plus time, heat cycles and shock — and anti-seize on reassembly.
- What should I avoid?
- Steadily increasing torque until something gives Heat near fuel lines, brake components, plastics or underseal Anti-seize on a fastener with a specified dry torque without adjusting the figure — lubricated threads reach the same preload at 20-30% less torque
Related pages
- Which lubricant should I use on a bike chain? Wet lube for wet conditions, dry lube for dry and dusty ones, wax if you want the cleanest and lowest-friction option — and not a general penetrant.
- Which lubricant should I use on a lock? A dry lubricant — powdered graphite or a PTFE spray. Oil and general penetrating sprays gum locks up and are the classic way to ruin one.
- Which lubricant stops a squeaky door hinge? A drop of light machine oil or a smear of white lithium grease on the pin — and lift the pin out to clean it, which is usually the actual fix.
- What should I put on a squeaky floorboard? Talc, graphite or powdered soapstone worked into the joint as a stopgap — the real fix is screwing the board down to the joist.
- What should I put on a sticky drawer? Paraffin wax or beeswax on wooden runners, dry PTFE on metal ones — and never oil on wood.
- Which lubricant is safe on rubber seals? Silicone grease on most rubbers, and a PTFE or fluorosilicone product on silicone rubber itself — petroleum products swell and destroy natural rubber and EPDM.
- Which lubricant should I use on a garage door? White lithium grease or a garage-door lubricant on rollers, hinges, springs and bearings — and not on the track, which should be clean and dry.
Sources
- Typical range compiled from manufacturer specifications — At One Place