A special profile rubber sealing o-ring for the 4BD1 thermostat. Should be replaced when fitting a new thermostat (8944723311). These do go hard with age and can cause a coolant leak from the thermostat housing. Genuine Isuzu.*** Specific Brand Part *** This product is not specific to one brand. If the product photo shows or the description lists a specific brand, we may provide you with the same or similar part of a different brand. For more information, please see our Terms and Conditions.