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Production 400ci GM blocks feature two holes between adjacent cylinders to vent steam between the large 4.125-inch bores. When building a street-driven 400, it's essential to drill matching holes (arrows) in your new heads. The hole closest to the intake side must be drilled at a 45-degree angle.
The Impersonator

 Production 400ci GM blocks feature two holes between adjacent cylinders to vent steam between the large 4.125-inch bores. When building a street-driven 400, it's essential to drill matching holes (arrows) in your new heads. The hole closest to the intake side must be drilled at a 45-degree angle.  View Related Article

 

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