What is a short block / long block?

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Quick answer:

Short block = the engine block without the head (block casting, pistons, rods, crank, etc.)
Long block = the complete engine, including the head (engine from valve cover to oil pan)

A short block and/or long block are NOT substitutes for what most companies sell as a "complete changeover package". These changeover packages typically include the transmission, ECU, and other accessories you'll need to make the engine run in your car. If you buy just the long block, you'd better have the transmission/wiring/ECU for your application- and if you buy just a short block, make sure you have the proper head as well.