When you work with brake, fuel and vacuum steel lines and fittings, you need the right wrenches! The quickest way to round the corners on tight or frozen flare nuts is with an open ended wrench.
The recommended tool for flare nuts is, not surprisingly, a "flare nut wrench". Even with a flare nut wrench, there are times when frozen brake line nuts or old fuel line flare nuts will simply not want to come loose. The hex corners begin to round, the sign of real trouble ahead!
I have high quality Bonney flares wrenches, a backup set of Craftsman flare nut wrenches and several chain wrenches—including a small, Wheeler Manufacturing chain wrench from the 1960s. That Wheeler chain wrench has saved my hide more than once, and it is probably my most versatile tubing work tool:
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In a pinch, if the chain links are short enough, you can grip a softening flare nut. A chain wrench can also grip the base of a collapsed oil or fuel filter! My small chain wrench will grip a rusted brake flare nut securely, and that's small. It will also grip a collapsed oil filter canister near its solid base.
There is also this Vise Grips model: http://www.amazon.co...ps chain wrench. It's useful and contemporary, although its chain link size will only drop to 5/8" diameter—okay for hardware but not good enough for smaller brake tubing nuts and most fuel fittings.
The side benefit of the Wheeler chain wrench is that when used properly, it does not leave a mark on the steel fitting nut! I've used these wrenches on high-end restoration work where the OEM hardware must appear original. You simply cannot achieve this with higher torque settings and a flare nut wrench. Even the most expensive flare nut wrenches will, by design, spread under load.
A tip and caution: Wrap the hardware with a layer of shop towel before gripping with the chain wrench. This will help protect the surface (although it will shred the towel!). Modern chain link wrenches with a toothed, flat leverage point are very rough on hardware and will leave marks if not used with extreme caution...See my Wheeler Manufacturing wrench's design (above), much easier on corners and flats of the nuts and other hardware!
So the next time you're at a garage sale, estate auction or used tool source, keep your eye open for a Wheeler Manufacturing chain wrench and other specialty tools "from the day". They truly do not make smaller chain wrenches like they used to...
Trust this helps your tubing flare nut and stuck oil filter situations!