Long block is a term given to a nearly complete engine assembly. The major components of the engine are pre-assembled and ready to install and have original accessories added. If an engine has been internally damaged or has simply become worn over time, an owner may wish to replace the engine rather than the entire vehicle.
Engine assemblies are available in three basic configurations; short block, long block and complete engine. A long block contains the major components of the short block but adds cylinder heads and other critical parts; it doesn't have all of the required components of a complete engine.
If an engine fails from a major parts breakage or simply wears out from normal usage, an owner must repair or overhaul. Repairs of major damage and overhauls take considerable time and expense.
Short blocks are replaced when only the lower part experiences damage. The existing engine's cylinder heads and remaining major components are transferred to the new short block. Long block assemblies exchange all of the above. Original accessories are re-installed.
Many reputable vendors provide engine assemblies in differing stages of completion. Engine suppliers specialize in engine building and reconditioning, and offer warranties that assure a cost effective, high quality product.
The terms long block or short block have nothing to do with physical dimensions. They are industry terms to describe the state of completion of an assembly.
