A replacement automobile engine is an engine or a major part of one that is sold individually without any other parts required to make a functional car . These engines are produced either as aftermarket parts or as reproductions of an engine that has gone out of production.

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.