Long block is an automotive term describing an engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 sub-assembly consisting of the following:

• Short block
  Short block
  Short block is an automotive term describing an engine sub-assembly.A short block is the portion of the engine block below the head gasket but above the oil pan. An in-block cam engine will also include the camshaft and timing gear. The overhead cam style of engine will not include the...
  
  . The long block is always a superset of the corresponding short block.
• Cylinder head
  Cylinder head
  In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
  
  . A complete new or remanufactured cylinder head assembly is installed with a new head gasket
  Gasket
  thumb|sright|250px|Some seals and gaskets1. [[o-ring]]2. fiber [[Washer |washer]]3. paper gaskets4. [[cylinder head]] [[head gasket|gasket]]...
  
   and torqued to specification. Complete cylinder heads include new or reconditioned valves
  Poppet valve
  A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
  
  , valve springs, valve guides (if applicable), valve seats (if applicable), valve locks, and valve keepers. The bare head is free from cracks, warps, or holes.
• If the engine is a flathead
  Flathead engine
  A flathead engine is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine...
  
  , then the valve train is installed if the short block did not already have it.

On most engines, the camshaft

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

 will be installed. On many engines with timing gears, the camshaft will be timed.

A long block engine replacement will typically require swapping out parts from the original engine to the long block. These parts include the intake manifold, carburetor

Carburetor

A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....

 or fuel injection

Fuel injection

Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....

 system, the exhaust manifold(s)

Exhaust manifold

In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...

, alternator, power steering

Power steering

Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...

 pump (if any), and air conditioner compressor

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

(if any).