Air Vent Grease Fittings are used on differentials, transmissions, and gear boxes. Vent fittings allow air to escape and enter as the internal pressure increases or decreases. The cap on Air Vent Grease Fittings prevent water or dirt from entering. Vent fittings must be installed above normal level of lubricant to allow air to escape when pressure rises. Air Vent Grease Fittings are also referred to as Air Vent Zerk Fittings, Air Vent Lubrication Fittings, Air Vent Grease Zerks, or Atmospheric Vents.