The LT166 John Deere lawnmower comes standard with a 48-inch mower deck. A common misunderstanding is that a belt adjustment must be made because the belt stretches; this is untrue. The belts on John Deere lawnmowers are corded. The symptom is that the belt will start to wear thin, causing the mower blades to start to slip. In this case, you will need to adjust the mower belt. This procedure will take 10 minutes to complete.
Park the lawnmower on a hard, level surface. Set the parking brake and remove the key from the ignition. Pull the PTO lever on the right side of the dash to the OFF position.
Push the mower deck lift lever on the right side of the fender deck to the lowest position. Locate the mower engagement rod on top of the deck, on the right side. Pull the engagement rod forward to align the pin on the rod with the hole it fits into. Remove the cottar pin and washer from the top of the pin that is going through the hole. If the pin does not line up with the hole, loosen the locknut at the end of the engagement rod. Turn the nut counterclockwise with the 13mm wrench until the pin and hole align, and then remove the washer and cottar pin.
Pull the engagement rod out of the hole. Turn the 13mm nut clockwise one to two full turns to tighten. Install the mower engagement rod into the hole, slide the washer onto the pin and slide the cottar pin back into the hole.