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A short block is an automobile engine sub-assembly. The assembly, commonly referred to as a short block, typically includes the engine block and the rotating assembly, which consists of the crankshaft, pistons and connecting rods, and the cam shaft, complete with timing set. The cam shaft is only included on short block assemblies that do not have over-head cams. New bearings and piston rings are also included with the assembly.
The typical scenario that involves replacing an automobile's engine with a short block assembly is when there has been significant damage to the engine block or rotating assembly, and time does not permit rebuilding the original engine. The cost of machining an engine block can be very expensive. Piston damage nearly always involves replacing the original pieces with oversized pistons. This requires boring the cylinders over size and honing the bored cylinders to create a surface that the new piston rings can seal against.
Many components from the original engine are swapped onto a short block to complete the engine assembly. Items such as the cylinder heads are typically reconditioned and bolted onto the block using new head gaskets. To complete the engine's top side, the intake and fuel delivery system is also swapped over. The exhaust manifolds and all of the engine's accessories such as water pump, alternator and power steering pump are also placed on the new engine assembly. The oil pan and engine mounts finish off the engine's bottom end. A new oil pump is nearly always installed in any new engine assembly prior to installing the oil pan.
Many companies offer complete block assemblies that have been built using reconditioned engine blocks. These re-manufactured assemblies include reconditioned crankshafts and connecting rods, but will often be built with new pistons. In the event that a mechanic determines a replacement or rebuilt engine is the best course of action to address a customer's needs, the new assembly is often a cost-effective option in completing the repair. Once the decision to use a new assembly has been made, the old engine is stripped of all needed parts and sent to a rebuilder as a rebuildable core.
Due to the quality of replacement parts, a re-manufactured short block is often equal to or superior to the factory engine found in most automobiles. In the case of a rare or collectible vehicle, the engine block should not be changed in order to retain the vehicle's value. When this is the case, the original engine can nearly always be rebuilt using quality parts.