A short block is an engine sub-assembly comprising the portion of the cylinder block below the head gasket but above the oil pan. An in-block cam engine includes the camshaft, timing gear, and any balance shafts. Overhead cam engines don't include those parts.
A short block engine is a replacement component for use when a worn-out engine requires major servicing, usually beyond the capabilities of a local repair garage, and instead a machine shop is needed. The short block represents the major wear items of such 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 exceeded this. A short block represented the set of major parts needed, those beyond the garage capabilities, 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 a regular garage task, as was light re-cutting with hand tools, usually into a cast iron seat. Only once steel seat inserts came into use, either for unleaded petrol in the 1970s or fitted into aluminium heads, did machining of heads and the replacement of seats become equally commonly required. Aluminium cylinder heads could also be damaged by warping after overheating, often requiring machining to re-flatten them.
A short block would have advantages over dismantling the engine and sending the crankshaft etc. away for rework. It would be quicker to obtain, requiring only a single shipping, rather than shipping twice and the time for machining. The short block would also have been built in a workshop hopefully cleaner and more organised for the specialism of engine building.
Short blocks were OHV engines. Sidevalves were pre-eminent before the short block appeared as a common item, and they also offered little saving by omitting the (simple) head. When overhead camshaft (OHC) engines became the norm, the rational unit of replacement became the long block, together with the cylinder head, as the camshaft drive could also be built up in the factory.
Short blocks were a common item from the 1950s to the 1970s, but disappeared after this. They appeared post-war when mass production of consistent engine models became widespread, rather than hand-building of varying engines. OHC engines after this were instead serviced by swapping long blocks, with their head in place.
Mechanics purchase a shortblock as a quicker way to rebuild an engine—avoiding the work and time of rebuilding and assembling the shortblock components. Companies that provide short blocks may also offer performance improving engine work. Machine shop work can increase performance by boring out the engine to increase cylinder diameter (which increases internal volume), balancing rotating assemblies (such as the crankshaft), installing a higher performance camshaft, etc. Bored-out engines require larger pistons and new piston rings. This kind of work can be done by amateurs ("shade tree mechanics") or by professional machine shops and engine rebuilders.
A short block is considered destroyed when it either warps or cracks, often due to overheating.