About Replacement String
Weed eater replacement string (line or cord) for non-commercial models comes in diameters from 0.065 to 0.095 inches. Check the owner's manual for the correct size for your model; using a size larger than recommended will wear out the motor or engine sooner. A number of popular brands sell pre-loaded spools, making replacement easy but relatively expensive. Alternatively, you can reload the spool by hand using line bought in bulk packages (approximately 300 or 900 feet long) much cheaper by the foot.
The string is the most important thing to maintain on your weed eater. When the string gets low you can remove the lower part of the head (the part that the string comes out of) by pushing in slightly and turning the head counterclockwise. Cut the new string no shorter than 6 inches or the weed eater will not advance the string properly when in use.
Thread the string back though the guides, making sure to follow the arrows on the guide. If the string is not put through the guides the right way, the head may pop off during use. Next, replace the bottom of the head by pushing and turning clockwise. You can totally skip this step by buying a string replacement cartridge. Though it is much more expensive, it saves you the trouble of having to restring your weed eater yourself.