How to Remove Rusted Allen Bolts

Hex keys and bolts, often referred to by the trade names Allen in the United States and Inbus in Europe, are a very common fastening system. Used in everything from self-assembled particle board furniture to car parts, hex bolts enjoy widespread usage due to their resistance to stripping. However, the bolts can be very tricky to remove when they have rusted through. Learning how to remove rusted Allen bolts is a matter of assembling a few tools and exercising a lot of patience.

Steps

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Before attempting to loosen the bolt, remove as much rust as possible. The less rust there is on the bolt, the less likely you are to snap off or strip the head when loosening it. Use a brush with stiff wire bristles to scrub rust from the threads and head of the bolt, the nut, and the surrounding metal.
Penetrating oil is one of the best tools available for loosening rusty bolts, though its proper application requires patience. Apply the oil to as much of the bolt as you can access, and then let it sit for a long time. If you have several days to spare, apply the oil a few times daily for a few days. At the very least, try to give the oil a few hours to penetrate the rust. If you don't have penetrating oil, apply another type of lubricant.
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After brushing and lubricating the bolt, use a hex key to try to loosen it. It may help to turn the bolt in both directions to begin with. Using a ratchet-style hex key with a larger handle will also improve your chances of loosening the bolt. If the head of the bolt is large enough, you can try loosening it by twisting the entire head with a wrench.
If the hex bolt cannot be loosened using the above techniques, you can try applying heat. Heating the nut and the surrounding material can cause it to expand slightly, giving the bolt a little more room to turn. Apply heat using a hand held propane torch, being careful to heat the bolt itself as little as possible. If you are working near plastic or other materials that will melt or burn, be careful to watch for smoke.
If none of the above methods work for removing the bolt, you can break the bolt off. This is best done with an impact gun, but can be done by hitting the bolt with a hammer or sledgehammer. After breaking the bolt off, you may have to drill the remaining piece out.