Hole Diameter: 2-1/2 inch, Bolt Spacing: 3.78 inch, Bolt Hole: 3/8 inch 2-bolt hole. Multi-Layered Steel (MLS) header collector gaskets are designed for extreme cylinder pressure produced by high horsepower-high compression turbocharged, supercharged & nitrous oxide engines. Manufactured from three stainless steel layers, resists corrosion, will not burn through or push out & maintains maximum fastener torque retention. Stress formed inner and outer layer embossments act like springs that rebound under exhaust flange clamp load. Applications: Dynomax, Flowtech, Hedman, Hooker Headers, etc..