The rounding of a stubborn bolt head happens all the time. It's frustrating -- the harder you try, the worse it gets. The smooth, rounded surface of the head resists any efforts to get a grip with an ordinary wrench. Take it easy. Simple tools can usually remove it.
Cause and Effect
Rounded-off bolt heads are usually the result of an attempt to remove a bolt with the wrong wrench. Adjustable wrenches are the typical culprit. Another example is using a metric wrench for an American bolt, or vice-versa. The miss-matched wrench rounds the corners.
Start with a few simple solutions. If they don't work, there are other options.
Tap On It
Bolts can sometimes become stuck when heat causes them to expand. Tap the head of the bolt lightly with a hammer to break it loose.
Rust, corrosion or cross-threading can result in a stuck bolt. Spray the bolt liberally with penetrating oil. Allow it to soak in for at least 10 minutes -- overnight if possible.
Get a Grip
Locking pliers, or vice grips, are one of the first choices for removing stuck bolts. Lock the pliers to the rounded head as tightly as you can. Twist in a counterclockwise motion, 1/4 turn at a time, to back it out. Release the locking mechanism, and repeat as needed.
Channel-lock pliers have more leverage than locking pliers. They don't lock, but the added length of the handles make up for it. Grasp the head of the bolt with the pliers. Squeeze the handles together tightly with both hands and push counterclockwise.
Use a file to flatten opposing sides of the bolt if the previous steps don't work. Repeat twisting it counterclockwise with locking pliers or channel locks, using the flattened sides to grip the bolt.
If the file won't flatten the sides, switch to an oscillating tool with a small cutting wheel, and grind flat spots on the head. Use an adjustable wrench, standard wrench or pliers to remove the bolt.
If the grinding and twisting don't do the trick, an impact wrench might have more influence. Begin with an oscillating tool to cut a groove across the top of the rounded bolt head. Place a flat, screwdriver tip on the impact wrench. Push down on the impact wrench, and pull the trigger to back out the bolt.
If All Else Fails
If the bolt head resists any efforts to budge it, other tools are available to remove it.
Locking sockets have a setscrew. Place the socket on the rounded head -- if there's enough left -- and tighten the setscrew to grip the head. Use the socket normally to remove the bolt.
Bolt splitters also look like a socket, but have a sharp point that cuts into the rounded head. Tap the sharp point into the head with a hammer and use the socket normally.
Stud Extractors and Oscillating Cutters
Use an oscillating tool to cut the bolt head off as flush as possible. Stud extractors , also known as easy-outs, work by drilling into the exposed stud of the bolt. Follow the instructions for drill bit size, and drill a hole in the stud. Twist the extractor counterclockwise into the hole with a wrench. As the extractor goes deeper, it grips and backs out what's remaining of the bolt.