Automobile engine replacement
An automobile engine replacement is an engine or a major part of one that is sold individually without any other parts required to make a functional car (for example a drivetrain). These engines are produced either as aftermarket parts or as reproductions of an engine that has gone out of production.
Replacement engines are used to replace classic car engines that are in bad condition/broken, or to install a more powerful engine in a vehicle. Replacement engines are often used to make old cars more reliable for daily driving. Classic car hobbyists may also install reproductions of a rare powerplant in a classic car (this is most often seen in Mopar muscle cars that have the 426 Hemi installed into them).
Aftermarket engines are used in many forms of motorsport. Some late model racecar series use "crate motors" many of which are made by independent firms. This ensures that drivers all have similarly powered racecars. Legends and Allison Legacy Series cars also use sealed crate motors.
Replacement blocks 
New castings of some engines are sometimes produced by independent companies. These blocks commonly replace rare or popular designs for aftermarket rebuilding, especially when the original is no longer produced. They are sometimes available in aluminum instead of original iron, or in stronger alloys. Often they imitate the larger available displacements that were produced in small numbers or allow for displacements never available.
- short block - everything between the cylinder head and the oil pan (excluding those items).
- long block - a short block, with mounted and gasketed cylinder head, valves and camshaft.
- crate engine - often more than a long block, including intake manifold, and carburetor or fuel injection system, and perhaps an alternator.