This a is a Manganese Silicon Bronze Valve Guide Set (8) for the Triumph TR2, TR3 and TR4 to CT21470 with 3/8 inch Exhaust Valve. Why choose a Manganese Silicon Bronze Valve guide? The Bronze has the amazing ability to self lubricate plus the Bronze provides superior heat transfer and resists seizing compared to standard Cast Iron Valve Guides. Rebuild your Cylinder Head once with High Quality Manganese Bronze Valve Guide.