Generac 0C2981F Valve Cover (Rocker Cover) No Fill Cap For 990/992/999CC Air Cooled Engines. For rear cylinder that doesn't use the oil fill cap.
For the cover with the oil fill see here:
For the gaskets for this valve cover, see here: Generac 0C2979 Valve Cover Gasket For 990/992/999CC Engines
Generac replacement parts will also usually fit the following Generac branded generators: Honeywell, Guardian, Siemens, Carrier, Watchdog, Centurion, Eaton & Bryant.
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