Professional™ Engine Coolant Thermostat Housing Gasket (12G29) by ACDelco®. Material: Paper. Bolt Hole Diameter: 11.13 mm. Hole: With 2 Mounting Holes. Designed utilizing the latest technology, this product by ACDelco features premium quality and will perform better than advertised. Perfect for your vehicle and lifestyle, it is manufactured to meet or exceed stringent industry standards.