How to Replace the Gearbox on a Troy-Bilt Trimmer

The gearbox assembly attaches to the end of the trimmer shaft and the trimmer head attaches to it. Inside, it has a couple of gears that alter the shaft angle and set the trimmer head speed. Over time, the gears can wear and fail, and the bearings can fail and seize. Either of these failures can also lead to a cracked housing.

Let's get started.

REMOVING THE GEARBOX   1. Removing the cutting head.

a. Disconnect the trimmer attachment from the power unit.

b. Rotate the cutting head until the opening on the flange aligns with the notch in the gear head.

c. Insert a small hex wrench into the opening to prevent the spindle from rotating.

d. Unscrew and remove the cutting head from the spindle. (The spindle is reverse-threaded.)

  2. Remove the gearbox.

a. Remove the anti-rotation screw from the gearbox.

b. Remove the two clamp screws from the gearbox.

c. Remove the gearbox from the lower boom assembly.

INSTALLING THE NEW GEARBOX

  3. Install the gearbox.

a. Install the new gearbox onto the lower boom assembly.

b. Rotate the spindle (as necessary) to align the lower drive shaft with the squared slot in the gearbox.

c. Install and tighten the anti-rotation screw.

d. Install and tighten the two clamping screws to secure the gearbox.

REASSEMBLING THE UNIT

  4. Reinstall the cutting head.

a. Thread the cutting head onto the spindle.

b. Lock the spindle in place (using a hex wrench).

c. Tighten the cutting head (until hand-tight).

  5. Reinstall the trimmer attachment.

a. Connect and secure the trimmer attachment to the power unit.