The heads of older bolts can become rounded after much use. Some of these are the result of abuse, such as hitting a stuck bolt with a hammer. In most instances, rounded bolt heads are the result of using the wrong sized wrench or using a standard size wrench on a metric bolt. Here are some proven methods for removing a bolt with a rounded head..
Soak bolt in penetrating oil to facilitate removal. Apply oil to all sides and tap the bolt with handle of screwdriver, using vibrations to help the oil penetrate. Repeat the application several times.
Allow the penetrating oil to soak in for 5 minutes before placing the closest fitting open-ended wrench over the bolt head. This wrench will be slightly larger than the bolt head. Insert the blade of flat screwdriver in the gap created between the wrench face and the remaining flat side of the bolt head.
Using the screwdriver as a wedge in the open-ended wrench face, turn the wrench in counter-clockwise direction to loosen the bolt.
Apply penetrating oil as in Step 1 in the previous section.
File one side of the rounded head until flat, using a metal file.
File the opposite side of rounded head until flat using the metal file. You will now have two parallel flat sides on opposite sides of the bolt head.
Use the adjustable (crescent) wrench tightened over the two flat surfaces to grasp and turn the bolt in a counter-clockwise direction, removing it.
The use of penetrating oil will reduce the amount of force needed to remove the bolt. This in turn will reduce the need to reconstruct the bolt head surface.
When using a flat head screwdriver as a wedge, be very careful to maintain control as, under pressure, the screwdriver could be ejected and cause injury.