Generac 0C8644 Belt Tensioner Arm QP-75D.
For entire replacement tensioner kit with pulley, arm, and hardware, see here:
Generac 0C86440SRV Belt Tensioner Kit With Arm & Pulley/Hardware QP-75D
Generac replacement parts will also usually fit the following Generac branded generators: Honeywell, Guardian, Siemens, Carrier, Watchdog, Centurion, Eaton & Bryant.
If you need Generac generator troubleshooting support, please visit our interactive forum