This is an outline of the common practice of Reconditioning an engine. The process is pretty much the same regardless of what type of engine is being reconditioned. Buy used engines.
The entire process starts with a core unit, the core units are either acquired from running vehicles or purchased second hand. These core units are normally stripped down to their bare form to begin with. All ancillaries are removed so you are left with a basic head, block and sump assembly. From here the core units are then stripped further down to their individual components, cylinder heads are removed, sumps are removed to expose the crankshaft, the connecting rods, pistons and crankshaft are then removed leaving a bare block.
This engine block is then sent for cleaning and inspection. Normally engine blocks are cleaned using a hot wash system to remove any grease or oil from the exterior of the engine and also clean any residue from the inside of the oil or cooling galleries. Once the engine block is clean, it is thoroughly inspected for any cracks or damage. If the block passes through the first stage of quality control and is deemed as being rebuilt able the unit then goes over to the machining department.
In the machining department, the engine block goes through a different type of inspection. This inspection tells the machinists how much work is required to bring the unit to acceptable tolerances. Different things are checked such as block war page, internal cylinder wall dimensions and ovalness. Once the inspection is done the machinist then goes about different processes to bring the engine block up to standards. Sometimes this can mean machining the cylinder walls and installing oversize pistons and ring sets or the machinist might elect to machine the block to a suitable size in order to insert a cylinder sleeve into it. The benefit of the cylinder sleeve is that it can be machined to a factory standard size and finish so stock pistons and rings can be used. The main lines of the engine are then machined so all bearings line up properly and all shafts run true. Then the engine block deck is machined flat to ensure a good seal with a new gasket later during assembly.
All the while that the block is being machined the other components from the original engine are being thoroughly inspected. Original pistons are broken down from the connecting rods, connecting rods are checked for both straightness and squareness. Piston rings are removed and ring landings cleaned and inspected for damage, the crankshaft is inspected for cranks and wear, if it is within tolerances it is polished, if it is out of tolerance then it is replaced. The cylinder head is disassembled and cleaned thoroughly visibly inspecting for any cracks or abnormal problem areas. All valves inside the cylinder head are checked to make sure they are straight. Valve seats are machined to ensure a good seal once the valves are lapped into place once assembled. Cylinder head surfaces that are within tolerances are then machined flat to create a positive seal with the new head gasket during assembly.
Once all of the machine work is completed on the various components they are then moved to the assembly station where the rebuild can begin. The bare block is turned upside down and the piston/connecting rod assemblies are installed in the cylinder bores. The crankshaft is then inserted and new bearings are fitted to both the main bearing caps and the connecting rods. Everything is torque to the manufacturer’s specifications. The engine is then turned right side up and the rebuilt cylinder head is installed with a brand new cylinder head gasket and torque to manufacturer’s specifications. A timing belt or chain is fitted on the engine and the crankshaft is rotated to ensure everything has proper clearance inside the engine block.