The serial number on a 4.3L Vortec engine isn't located in one single, easily accessible spot like a VIN. It's cast into the engine block, but the exact location varies slightly depending on the year and specific casting of the block. You'll need to look carefully along the passenger side (right side when sitting in the vehicle) of the engine block, often near the bellhousing (where the transmission connects) or lower on the block itself. It might be partially obscured by components.
It's often a series of numbers and possibly letters. Don't confuse it with other casting numbers which are also present on the block. The serial number will be distinct from these and likely in a slightly different font. If you have trouble locating it, providing the year of the vehicle will help narrow down the search.
