Allen head screws pose a problem when you don't have the right screwdriver. The head is made with a six sided hole in the center. The Allen wrench fits down into it. Allen head wrenches usually come in sets, many times they're attached to a ring to make it easy to keep track of them. Even decent Allen wrench sets are inexpensive compared to other tools. Read on to learn more.
Try to find a standard screwdriver that fits snugly between two of the points of the screw. It should fit with no wiggle room. If it's a perfect fit, you can often turn the Allen head screw with that. Don't force it if it isn't an exact match. Just try the next step.
Get your hacksaw. Put a small toothed blade on the saw. Cut slowly with the very end of the saw. You don't want to cut too deeply and mar the surface around the screw--or even break the screw itself. Make your cut according to the screw head thickness.
Use a standard screwdriver that fits the groove you just made snugly. Turn the screw slowly being careful as the screw head is not as strong as it was before it was cut. Your screw should back out nicely without ruining the surface it's screwed into. If you need to replace the screw later, use another screw with a standard or Phillips head.