How to pull off the trimmer bottom off the my craftsman 31cc 17 inch gas powered weedwacker with the model #316.798220 which is probably abouy 18 years old?

Jason, April 2010

The bump knob assembly has a hidden bolt that fastens it to the shaft. Hold the spool and use large channel lock pliers to turn the bump knob counterclockwise (when looking at bump knob from bottom). This will allow bump knob and inner spool removal. Replace line and then refasten spool and bump knob assembly.

applesales, July 2010