"ITEM IS TWO-WAY AIR FLOW THAT COMPENSATES FOR AIR EXPANSION AND CONTRACTION. MATERIAL: ALL METAL PARTS ALUMINUM, INTERNAL SLEEVE IS SELF-EXTINGUISHING LEXAN (POLYCARBONATE RESIN PLASTIC). WALL THICKNESS: 4 INCHES STANDARD. DRILLED FOR NUMBER 8 (4MM) SCREWS. ROUND COVER STANDARD."