Belt number 587686701. I always recommend purchasing from your local repair shop. Take your model and serial numbers to them, have them double check the part number and get it for you. You might pay a bit more, but if it would be wrong you can take it back. Also gives you a place to get help if you need it in the future. Replacing the belt is not hard, remove deck, install belt, reinstall deck. Less than an hour. I recommend removing the deck now and check the blades and pulleys. You may need to sharpen or replace the blades and can get the deck a good cleaning plus check over before reinstalling.
