Spark plug construction-
Sealing to the cylinder head：
Some spark plugs have a tapered seat that uses no washer.
The torque for installing these plugs is supposed to be lower than a washer-sealed plug.
Spark plugs with tapered seats should never be installed in vehicles with heads requiring washers, and vice versa.
Otherwise, a poor seal or incorrect reach would result because of the threads not properly seating in the heads.