Get the Factory Service manual for your year engine. That will have all of the specs needed or checking the valves and seats.
The biggest thing you're looking for is that the seat and the valve face have flat surfaces where they seal. If the seat is worn to a point or there is a groove in the valve then the seat need to be recut and the valve replaced.
Lapping is generally only necessary when replacing a valve. If the valves all have nice flat sealing surfaces then I would not bother with replacing any. You will need to set the lash clearance after reinstalling the valves, and then you'll need to check it again after running the engine for a few weeks.
VALVE AJUSTMENT EJ22 MOTOR HEADS OFF HELP
in 1990 to Present Legacy, Impreza, Outback, Forester, Baja, WRX&WrxSTI, SVX
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