Spark plug

A spark plug (5; 5') may comprise a shell (20; 120) surrounding a lower part of an insulator (15; 115). The shell (20; 120) may be provided with a tool engaging portion (25; 125), a seal face (30; 130) configured to contact a corresponding seal of a cylinder head (200) of an internal combustion engine, a cylinder engaging threaded portion (40; 140) having a thread diameter (D) configured to screw the spark plug (5; 5') in a threaded bore (210) of the cylinder head (200), and an intermediate portion (35; 135) extending between the shoulder (30; 130) and the cylinder engaging threaded portion (40; 140). The intermediate portion (35; 135) may have a length (L 1 ), wherein: 0.20