The Ryobi 790R is a combination trimmer and brush cutter. Unlike most trimmers, the 790R does not use a bump feed button on the bottom of the trimmer spool. When cutting through tall weeds or overgrown areas, it is necessary to use the brush cutting blade. Installing a Ryobi 790R brush blade involves locking the shaft of the trimmer so the head does not move. Before installing the blade, locate all the parts. The blade comes with a 5/8-inch nut to attach it to the trimmer. Replacement blades along with the necessary hardware are available at home improvement centers.
Turn off the Ryobi 790R and unplug the spark plug wire. The spark plug is on the rear of the engine. Pull the end of the plug wire off the spark plug with your fingers.
Place the trimmer flat on the ground right side up. Remove the three screws securing the cutting shield to the back of the shield mount with a Phillips-head screwdriver. Pull the shield off the mount.
Insert the locking rod that comes with the Ryobi into the hole on the side of the shield mount. If you do not have the locking rod, insert a nail into the hole. Rotate the trimmer spool until the rod or nail inserts into the trimmer head bushing hole. This locks the head so it does not spin while removing the spool.
Grasp the trimmer spool with one hand while holding the locking rod or nail. Turn the spool clockwise and unscrew it from the trimmer spindle. Lift off the blade retainer washer from the spindle. The blade retainer washer is a concave washer under the trimmer spool.
Put the cutting attachment on the trimmer spindle. The wording on the blade should be pointing away from the trimmer. Place the blade retainer washer over the spindle.
Thread the blade retaining nut counterclockwise onto the spindle shaft until it is hand tight. Holding the locking rod or nail in one hand, tighten the nut with a 5/8-inch socket wrench.
Remove the locking rod or nail. Push the spark plug wire over the spark plug.