I believe the gasket is metal. Its thin. Not sure on an exact measurement.
The part is available on ebay and many other online part sellers.
You may be able to take the tensioner to the auto parts store and find a metal drain plug gasket that fits.
The part is available on ebay and many other online part sellers.
You may be able to take the tensioner to the auto parts store and find a metal drain plug gasket that fits.