A short block is an engine block assembled with the crankshaft, connecting rods, bearings, pistons, and rings installed at a minimum. Some short blocks may include additional parts such as a camshaft with timing set installed as well as an oil pump and pan. However, a short block will not include the cylinder heads or anything assembled above the engine block deck.

On the other hand, a long block is a more complete version of a short block. A long block includes all of the parts of the short block package with the addition of a cylinder head, camshaft, and valve-train. Some long blocks also include the valve covers, oil pan, and timing cover. Long blocks are usually used when diagnostics indicate that a vehicle needs an engine overhaul.

The main difference between a short block and a long block is the number of parts included and assembly completed. Short blocks are generally cheaper than long blocks, but they are more likely to malfunction and require more maintenance. Long blocks are generally more reliable and offer better performance than short blocks.