Buying a used engine may seem like a strange concept, but sometimes there is nothing a mechanic can do short of replacing the whole engine unit of a car. Purchasing an engine that is from a similar model, whether earlier or later, can reduce overall repair costs for the customer, and can reduce time for the mechanics.
Many people do not even run into the issue of having to pay for a used engine for their car, because it takes quite a bit of abuse in order to push an engine to the point where it needs to replaced. Repeated overheating, blown spark plugs, fuel injectors, head gasket, and in the case of an automatic transmission a buster carburetor is no small amount of work to be done. Not to mention the starter and alternator.
While all of the problems listed above are separate, if a car is repeatedly left without oil, left to overheat, and is not used engines for sale regularly maintained all of these issues can pile up. This is true for both automatic and manual transmissions alike — and if a car burns oil excessively, there is little to be done than get a used engine.
All of the components of an engine are separate in most cases, but when they are all damaged or in need of repair at a time then it may just be easier — and cheaper — to invest in a new engine altogether. This is a resort that is usually called upon for older cars, as many of the manufacturers either do not make the parts needed anymore, or just excessive wear and tear has gotten to it. Newer car models usually do not go through an engine transfer because they do not need it unless they are not taken care of in the least.
Some people change out their engines by choice or more power. This is not uncommon, especially among older cars. It is possible to exchange the engine unit among many older models of the same make within the same few year span, depending on how much the car manufacturer changed the schematics up within a period. For example, the Honda Civic 1991, 1992, and 1993 models all have similar internals, with the 1991 and the 1992 being comparable in power, and the 1993 having markedly more horsepower. Changing them between one another is possible because they are similar, though each has a couple of its own features to keep in mind. It is also possible to switch some Honda Accord engines within that year bracket with Civic engines.