Stihl trimmers have interchangeable cutter heads that allow you to switch back and forth from stringed cutters to blade cutters. Grass and weeds yield easily to string, but thick brush calls for a blade. Stihl trimmers are designed to convert easily to blade cutters for heavier, string-breaking trim jobs. The process of switching from string to blade cutting is relatively straightforward.
Pull the cap off the spark plug to keep the trimmer from accidentally starting. Place the Stihl trimmer upside down so you can easily access the black cutter head assembly.
Push the locking pin into the side of the cutter shaft, and rotate the cutter head until the locking pin locks in place.
Remove the knurled plastic bump-down button from the cutter head assembly by turning it clockwise.
Pull the string spool off the shaft. Remove the nut that holds the spool cover onto the shaft with a wrench, and then pull the cover off the shaft.
Place the blade shroud onto the trimmer shaft, then place the blade onto the trimmer shaft. Secure the blade and the shroud firmly onto the trimmer shaft with the nut you previously removed. Remove the locking pin from the cutter shaft.