How big can you bore a small block 400?

A small block 400 engine can typically be bored up to about 4.155 inches, depending on the specific block and its condition. However, the maximum bore size may vary based on the manufacturer and any previous machine work. It's important to consult with a professional engine builder to ensure the block can handle the increased bore without compromising structural integrity. Always consider the potential for overheating and other performance issues when increasing bore size.