The small block/big block is a strict definition, just the size that classifies it doesn't. Basically, the small/big block are names for engine families. Ford, for example, never had a small/big block but just various engine families (Windsor, Cleveland (335 series), FE, FT, 385 series, Modular, etc.) but those engine can be classified into big/small block by bore spacing (small block being everything but the FE, FT, and 385 series). Chevy on the other hand only had 2 engine families which were small and big block.
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