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 1990's 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" may eventually pass and the numbers of engines being replaced and rebuilt may 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 eventually need major engine repairs or a replacement engine.
When an engine needs major repairs, you're faced with an important choice: You can replace the engine with a new engine, remanufactured engine or used engine, or you can attempt to rebuild the old engine yourself.
Replacing an engine with a brand new one is usually too expensive for most budgets, especially if the engine costs more than an older car is worth. 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.
Remanufactured engines are a popular option these days because they're 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.
One reason is that rebuilding an engine typically costs less than replacing it with a new or reman engine. Assuming the original engine is rebuildable (wear is not excessive and there is no serious damage), and the amount of machine work required to restore it is minimal, it may cost half as much to rebuild it as to replace it. Most of the savings (profit) comes from the labor put into tearing down the engine and then reassembling it after any necessary machine work has been done. A automotive machine shop can do the remachining (as needed), and you can put the parts together yourself (assuming you have he know-how and tools).
The tools required to rebuild an engine are minimal: normal hand tools, some feeler gauges, a torque wrench, and a ring expander and ring compressor.
If the cylinders are worn, they need to be bored or honed to accept oversize pistons and rings. If the cylinders are still within service limits, a glaze breaker can be used to restore the crosshatch in the bores.
Valve and seat refacing requires special equipment, so the valve job should be performed by a machine shop. If the valves and seats are still good, but the valve guides are worn, the guides can be reamed out, and bronze guide liners installed. Or the guides can be reamed out to accept new valves with oversized valve stems.
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