This is the replacement aftermarket tension idler wheel for the John Deere 50D mini excavator serial number 275000 and up. This is the large pull wheel on the opposite end of the undercarriage from the sprocket. If your serial number is 274999 or lower, you will need a part number 9237507. If you are unsure, please call us to verify measurements. This idler comes with a protective guard to keep debris from passing through the idler wheel and causing extensive damage to the rubber track.