A replacement automobile engine is an engine or a major part of one that is sold alone, without the 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 poor condition or broken, or to install a more powerful or more fuel efficient 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 engines" 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.

The four most common types of replacement engines are:

A short block is an engine sub-assembly comprising the portion of the cylinder block below the head gasket but above the oil pan, which usually includes the assembled engine block, crankshaft, connecting rods, and pistons with piston rings properly installed. An in-block cam engine short block includes the camshaft, timing gear, and any balance shafts. Overhead cam engines don't include those parts. Short block engines became popular after World War II, when mass production led to great consistency between individual engines; before then, most engines were hand-built and had idiosyncratic variations. Short blocks became less popular after the 1970s when overhead camshaft (OHC) engines became the norm, as the rational unit of replacement was the long block, which includes the head, camshaft and valve gear.

A short block is the preferred replacement component for a worn-out engine that requires major servicing beyond the capabilities of a local repair garage, when instead a machine shop may be needed. The short block represents the major wear items of an engine: piston rings, and potentially a rebore of the cylinder bores or replacement liners, together with reground bearings on the crankshaft. Although replacing the rings or bearing shells was at one time considered typical garage work, the need for a boring or crank grinding machine now exceed the capabilities of a standard automotive repair garage. A short block includes the preassembled set of major parts needed that generally exceed the capability of the garage, in one item.

The third item sometimes requiring machining, the re-cutting of valve seats in the cylinder head, was less frequently needed. Grinding of valves to fit was once a regular garage task, as was light re-cutting with hand tools, when cast iron seats were common. Once steel seat inserts came into use, either as a result of the switch to unleaded petrol in the 1970s or fitted into high-performance aluminium heads, machining of heads and the replacement of seats became equally commonly required. Aluminium cylinder heads could also be damaged by warping after overheating, often requiring machining to re-flatten them.

A short block has advantages over dismantling the engine and sending the crankshaft and other related automotive parts away for rework. It is usually quicker to obtain, requiring only a single shipment, rather than having to ship parts to and from the machine shop and the interim time spent at the shop to re-machine those parts. The short block would also have been built in a workshop that was hopefully cleaner and more organised for the specialism of engine building.