Engine internal components move in relaxation to each during engine operation. This creates friction, though lubrication has been introduced, it doesn't altogether stop friction, it only reduces it to acceptable levels. This means with time the internal parts are going to wear beyond their tolerance limits. If the internal components wear get beyond the tolerance limits the operation of the engine is heavily compromised. The engine starts exhibiting negative symptoms like, loss of power, high oil consumption, overheating, excessive smoking, etc. Such symptoms render the operation of the engine very uneconomic. To address the situation all worn parts require to be replaced with new parts that will get the desired operating conditions hence the need for an engine overhaul.
Others information:
1. Most engines over time wear out due to heat and normal wear and tear. Some deteriorate quicker from being driven too hard, not enough lubrication (engine oil low or none). Also lack of basic maintenance can wear an engine quicker.
An Engine overhaul would include several things. Most overhauls include new pistons, new piston rings. New Main Bearings, all new valves and springs. The cylinder and cylinder head would probably be re-bored to ensure no leaks or compression loss. All new gaskets for cylinder head(s), intake manifold, exhaust gaskets.
2. ‘Way back when the useful life of a car was usually seen as being limited to 100,000 miles, that delineation was kind of odd when you think about the amount of metal that was put into a car’s body and chassis back then, but the really important components all tended to wear out about that time. At the heart of the car’s engine was the interface between the pistons and the piston walls, the rings. That’s why the single most telling diagnostic that could be done on pre-electronics cars was a compression test. It’s also why so many snake-oil patent-medicine elixirs were sold at auto-parts stores that were “guaranteed to restore compression.” And some of them could, too…. for the time it took to run a compression test.
But also deep in the engine were things like the babbitt bearings and a few other consumables that could only be reached by taking the engine out and then completely apart. Almost every one of these items could have their lives extended by simply changing the oil regularly and often. Oil changes couldn’t grant immortality, any more than eating healthy does for a human, but they both could make the difference between dying at 100 versus dying at 65.
All therefore I like the classic VW Beetle. They were built with the same 100K lifespan, but as VW phased out the classic, a cottage industry cropped up and - as far as I know is still alive - to supply engine swaps. Most shops and supply houses that specialize in Vee Dubs offer a range of rebuilt air-cooled engines from the standard putt-putt that came installed when the car was new to a variety of over-cranked, bored-out, super-charged monsters that need a shoe-horn to fit inside the engine compartment (but only if the hood is left off) and might just put out so much torque the drive axles will break.
