Martinsr, this is a good tip for a topic that anyone working on old cars has to deal with eventually.
I have made it something of a specialty to successfully remove rusty fasteners. Vehicles in this part of the country are exposed to years of road salt, acid rain, and high humidity air and mechanics here have plenty of chances to practice if they want to be skilled at bolt removal. There are a *lot* of good tricks but knowing how rusted bolts get stuck is important. Since this is a "Basics of basics" thread maybe it would be good to explain it.
When iron turns to rust it expands. Bolts left rusting for years can actually be larger than they started until the rust is removed. If a bolt and nut rust together the rust on the bolt will try to swell and split the nut while the rust in the nut will try to compress and grip the bolt. That fastener which was nice and loose years ago ends up being locked tight in a press fit. In order to get the bolt to spin the press fit must be broken be removing pressure.
It can be important to know what types of metal you're working with. MARTINSR's tip is very good for many mild steel fasteners. Heating and cooling parts with a torch can cause several side effects. Some of the rust may burn off. Some of the rust may fall off and be blown away by the force from the torch flame, like air blowing dust. As the fastener and / or parts around it are changing dimension the bond between rust molecules can be broken. With a heat and fast quench on mild steel it's possible for the fastener's dimensions to actually be changed slightly after the part cools. With short screws and small nuts this simple heat/cool trick is often good enough to get the job done. Another version of this works great with fasteners in cast iron, especially pipe thread fasteners. Heat the fastener to red, walk away until it's nice and cool, then easily remove the offending part. It's important not to heat the cast, only the fastener.
Not all applications can be heated. And heat doesn't always work alone. In many cases it's important to take a different approach or to add a second step: Remove as much rust out of the threads as possible. Rust is iron oxide. The molecules are abrasive and they are harder than most steel and cast. Trying to unscrew very rusty and dry fasteners causes the rust to scar the threads. What's worse is the pressure of the press fit combined with the abrasiveness of the rust creates heat at the area where the bolt or nut is turning. It's actually possible to create enough heat to weld the threads together! This is where a penetrating oil can really help. While some penetrating oils can actually break or loosen the bonds between rust molecules, the best use of penetrating oil is to provide a lubricant under the abrasive rust molecules and to flush rust away from the fastener. Don't be afraid to douse a severely rusted fastener with penetrant during removal. Use as much as needed. And combined with the oil, it's common to have to work fasteners back and forth several times during removal. Spray, turn spray, turn, back and forth. This helps flush out rust and it helps push penetrating oil back through the threads to push oil out the other side. In many cases a few repititions are enough to allow the fastener to move easily.
In some cases you may not have heat, or the part may not be heatable. Repeated vibration can do wonders to break up rust. I have developed a technique using my air hammer and a mushroom bit so I can gently tap away at a rusted fastener to break up rust. Then I will use air and penetrating oil to blow garbage out of the threads. This can be followed by an attempt to move the fastener or more air gentle hammering. Sometimes turning an impact wrench down low can work. And I've even been known to tap, tap, tap away with a hammer on the head of a bolt when it's critical to remove a fastener without damaging the surrounding components.
It also helps to develop a feel for what you're removing. Soft steel brake fittings in calipers or wheel cylinders are a great example of this. I spent years learning how to have more success than failures with these parts. Torching them often isn't an option. The trick is to get them to move ever so slightly then to work them back and forth using penetrant to flush the rust out. But you really have to develop a feel for how much pressure to use and how far the fittings want to turn before they try to sieze up again. I've found that a longer ratchet or wrench allows much more control over the torque applied. With a short wrench you usually crank 'em up pretty hard and there's no turning back if the fastener starts to break.
Ultimately it may be necessary to drill out a bolt or stud and use an easy-out. This is where taking time to prepare pays dividends. Make sure you use a center punch to start the drill correctly. Use a sharp drill and make a pilot hole. Check to ensure the drill is drilling the hole straight through the bolt to start. It may be necessary to "walk" the hole to get it centered. Once the hole is drilled it's time for the easy out. I prefer fluted, drive-in types. I have one of these at work and one at home.
The screw-in, tapered type don't seem to work as well. First, they try to expand the bolt into the hole as you tighten them. The goal is to reduce pressure! Second, they only grab the fastener at the outermost edge. This can allows them to and oblong the hole you worked so hard to drill. Third, driving the straight ones with a hammer in helps break up the rust.
Love to hear more tips. As much as I know it's always good to hear more!
