The Difference Between Rebuilding and Overhauling an Automobile Engine


There is a vast difference in rebuilding an engine and overhauling it.

When rebuilding an engine it is removed from the vehicle, disassembled, and the motor block is checked for the following:

  • cracks or broken components
  • excessive cylinder wear
  • crankshaft wear
  • condition of connecting rods
  • condition of camshaft, and
  • pistons checked for size and condition of piston ring grooves.

The cylinder head or heads should be taken to an automotive machine shop to have the valves and valve seats properly refaced and any other problems addressed. If the motor has extremely high mileage or has had over-heating problems, then most likely the motor will have to be bored out, the crankshaft re-ground, the connecting rods possibly re-sized and the cam bearings replaced. If the motor needs to be bored, the old pistons will have to be pressed off the connecting rods and the new pistons pressed on. The motor block needs to be cleaned thoroughly and it must be kept clean during the assembly process. Keeping everything as clean as possible during rebuilding will have a positive effect on the life of the motor after it is assembled. Once the block preparation has been done, then the master parts kit, with the proper piston and ring sizes, and rod and main bearing sizes, can be ordered for assembly of the motor. It is always best to replace the old camshaft and lifters with a new camshaft and a set of new lifters.

An overhauled engine is one that has all the accessories removed from the engine including the cylinder head, or heads, and the oil pan. The engine block is not normally removed from the vehicle. New piston rings and rod bearings are installed, with the proper sizes to correspond to the size of the piston and rod journal sizes. It is always a good idea to install a new oil pump also. New gaskets should be used when re-assembling the engine. The assembling of the accessories, when done right, includes cylinder head preparation by a good automotive machine shop. Quality machine work on the cylinder heads assures quality performance from those heads. Most engine re-builders from the automotive machine shops will tell you that re-building of cylinder heads is one of the most critical, if not the most critical operation that can be performed in the re-building of automotive engines. If these guidelines are followed, the overall performance of your completed engine rebuilds will be far superior.

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