After enough use, your Troy-Bilt TB425CS trimmer inevitably will run out of string. With this model, which is one of the more reliable trimmers on the market, there are two ways to replenish string: using a prewound spool or winding new line along the inner reel. Installing a prewound spool usually takes only a couple of minutes but typically is more expensive than attaching new line. Rewinding with line you can purchase from your hardware store usually takes five minutes or less.
Measure out 20 feet of line and cut this in half. Grasp the outer spool and rotate the inner reel in a counterclockwise manner so that arrows are aligned on the inner reel and outer spool. Remove any old line from the line loading and locking holes.
Insert an end of one section of string into one of the eyelets on the outer spool. Keep pushing the line until it slides into the inner reel's line-loading hole. Loop no more than 1/2 inch of the line into the locking hole; then pull the line until it is snug.
Use the same procedure as in steps 1 and 2 to secure the second line. Use a counterclockwise motion to wind the inner reel. Stop when 4 inches of line emerges from both sides of the outer spool.
Unscrew the bump knob, pulling it off the spring and foam seal if you choose to install a prewound spool instead of winding on new line. Take out the inner reel and any remaining scraps of line.
Take the two ends of line on the prewound spool and insert them through the outer-spool eyelets. Secure the new inner reel, with the arrow side facing up, onto the outer spool. Reattach the bump knob, spring and foam seal to the inner reel.
Rotate the bump knob in a clockwise manner until it's snug. Free the lines from the inner reel's holding slots by pulling both lines.