An engine stand is a tool commonly used to repair large heavy gasoline or diesel engines. It uses a heavy cantilevered support structure to hold the engine in midair so that the mechanic has access to any exposed surface of the engine.
While small single-piston engines can commonly be laid on a table for repair, a large engine is normally meant to be supported from its engine mounts or from the flywheel transmission case mounts, and fragile components such as oil pans and valve covers would be crushed if the large engine were placed on a flat surface.
Engine stands are typically mounted on large casters so than an engine can be moved around the shop to different test and repair stations, and the engine can often be rotated in midair to provide easier access to underside surfaces of the engine.
The engine stand is commonly used in combination with the engine crane to remove or install an engine in a vehicle, break in that engine, and perform repairs.
(Source: http://en.wikipedia.org/wiki/Engine_stand & http://www.mikestoolsusa.net/ac-delco/acdelco-2000lbengstand.asp)
