2.0 LITER GAS. Front. Knock sensor bolt. A threaded rod with a drive head (such as hex, 12 point, torx, etc.) on one end used to secure the Brake Vacuum Pump to it's mounting point. Ignition Knock (Detonation) Sensor Bolt. HEXAGON HE. Ignition (Detonation). Power Brake Booster Vacuum.