Here's why:
* Bore and Stroke: The 403 has a slightly larger bore and a shorter stroke than the 400. This means the pistons, connecting rods, and crankshaft are different.
* Cylinder Heads: The 403 cylinder heads are different from the 400 heads. They have different combustion chamber designs, valve sizes, and bolt patterns.
* Intake and Exhaust Manifolds: The 403 has a different intake manifold design than the 400, and the exhaust manifolds may also be incompatible.
* Cam Shaft: The 403 camshaft has different specifications, including lobe profile and duration, compared to the 400 camshaft.
However, there are some parts that are compatible:
* Some accessories: The alternator, power steering pump, water pump, and other accessories may be compatible between the 403 and 400 engines.
* Some internal parts: While the pistons, rods, and crankshaft are different, some internal parts like oil pump, timing chain, and lifters may be compatible depending on the specific model and year of the 400 engine.
Important note: Before swapping any engine parts, it is crucial to consult a repair manual for both the 403 and 400 engines to confirm compatibility. Mixing parts from different engines can lead to performance issues and even damage.
