10.5 compression on pump gas

In general the colder the plug, the shorter the nose of the plug. The nose of the plug is the part of the insulator that is freestanding inside of the plug. To put it in NGK-speak, a BKR7E-11 is colder than a BKR5E-11. The underline is to emphasize the number that denotes heat range of the plug. It is not part of the part number.