As I did my research on Engine Rebuilding, I came across a article that I could not have written any better aa1car

Back in the 1970s and 1980s, the average life cycle of an engine was about five to seven years. After 60,000 to 80,000 miles of everyday driving, most engines would develop an oil consumption problem and begin to experience other signs of wear (loss of compression, loss of power, increased emissions, lower oil pressure, internal noise, etc.). Carburetors were partly to blame for the wear because rich fuel mixtures wash the lubricating oil off the cylinder walls and dilute the oil in the crankcase. These older engines were also built much "looser" (wider tolerances) than most of today's engines, which also increased blowby. Consequently, the rings, bearings and valve guides all experienced accelerated wear.

Today, the situation is much different. The average service life of a 1990s vintage engine is about 10 to 12 years! Fuel injection has all but eliminated the fuel wash down problem, and much tighter tolerances have greatly reduced blowby and oil dilution in the crankcase. So fewer engines are being rebuilt today as a result.

Improvements in engine technology have extended engine life and reduced the need for engine service. Even so, the current "technology trough" will eventually pass and the numbers of engines being replaced and rebuilt will once again rise. The number of five- to 10-year-old light trucks on the road, for example, has jumped from 18 million in 1985 to nearly 60 million today. Many of these will need engine work before long.

REPAIR OPTIONS

When an engine needs major repairs, you are faced with an important choice: you can replace the engine with a new, remanufactured or used engine, or you can repair or rebuild the original engine.

Replacing an engine with a brand new one is usually too expensive for many people's budgets, so the choices come down to a remanufactured engine (or short block), a used engine (and the risks that go with it), or overhauling or repairing the engine yourself. A used engine is a temporary fix at best, and only buys the current owner a little more time. Sooner or later, most used engines experience problems of their own and have to be replaced or rebuilt, too.

Remanufactured engines are a popular option these days because they are readily available at competitive prices, which has caused a decline in the number of engines being custom rebuilt ("repowered") by repair facilities and machine shops. A quality remanufactured engine can provide good value for the investment, and most come with a 90-day to one-year warranty. Even so, there are still valid reasons for doing your own engine work.