External Clues (Less Reliable):
* Casting Numbers: This is the most reliable method, but requires you to find the casting numbers on the block itself. These numbers are usually found on the front passenger side of the block, near the bellhousing, or on the passenger side towards the rear. You'll need a casting number chart specific to Chevy small blocks to interpret these numbers and determine the displacement. There is no single number to look for, as there are many different casting numbers for both engine sizes.
* Intake Manifold Bolt Pattern: While both use a similar pattern, some aftermarket intakes are specific to the 400 due to its larger bore. This isn't a guaranteed method, especially with modified engines.
* Valve Covers: While not a definitive indicator, some valve covers were designed specifically for the 400 due to its larger size. However, aftermarket valve covers are common, so this isn't reliable.
* Oil Filter Adapter: The oil filter adapter mounting location might slightly differ depending on the year and casting. This is subtle and unreliable without direct comparison.
Internal Clues (Requires Engine Disassembly):
* Bore and Stroke: The most definitive method requires measuring the bore (diameter of the cylinders) and the stroke (distance the piston travels). A 350 has a smaller bore and stroke than a 400. This requires taking the engine apart, so it is not a practical method in most cases.
In short: The casting numbers are your best bet for identification. Unless you're comfortable taking apart the engine, relying on external visual clues alone is unreliable and prone to error. Use an online Chevy small-block casting number database to cross-reference the numbers you find.
