What is long block engine and short block engine？

• Short block - everything between the cylinder head and the oil pan (excluding those items)

• Long block - a short block, with mounted and gasket cylinder head, valves, and camshaft

  
  

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A long block is an engine sub-assembly that consists of the assembled short block, crankshaft, cylinder head, camshaft (usually), and valve train. A long block does not include fuel system, electrical, intake, and exhaust components, as well as other components. A long block may include the timing cover, oil pan, and valve covers.

A long block engine replacement typically requires swapping out parts from the original engine to the long block. These parts include the oil pan, timing cover, valve covers, intake manifold, emission-control parts, carburetor or fuel injection system, the exhaust manifold(s), alternator, starter, power steering pump (if any), and air conditioner compressor (if any).

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. They appeared post-war when mass production of consistent engine models became widespread, rather than hand-building of varying engines. While they were common from the 1950s to the 1970s, but disappeared after this, as when overhead camshaft (OHC) engines became the norm, the rational unit of replacement became the long block.

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.

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