This is the factory Subaru oil gallery plug located on the top of the motor for all EJ motors.
This plug is sometimes swapped out for aftermarket gauge installations or accessories. There are two on the EJ motor; you can see them if you look direcly down on either side of the throttle body. The plug comes with thread lock compound pre-installed.
About 25mm/1" in diameter. Sold individually.
Fits all EJ motor Subaru cars 1998-2021.