The reason you can't use engine hours after say year fifteen is that time also takes it's toll. The same in cars. Gaskets deteriate, metal corrodes, and even the insulation on the internal wireing goes bad with time. Your parts including both the rubber and metal ones have now been soaking in a slightly toxic envionment for a long time. Plus the short uses it's most likely to have expeirenced is harder on an engine then long hours are.
It's the same in cars. A 1973 ford truck with only 45,000 miles will have as many or more problems then a 2003 ford truck with 145,000 miles. In the end if you think your engine, transmission, and the other parts that are attched to it are in really good shape then rebuild it. But there is an old adage that goes " You can put a rebuilt transmission on an old engine, no problem, but a rebuilt engine on an old transmission will blow with in two years."
Same goes for rebuilding the top of an engine without rebuilding the bottom. Your basicly going to put a new power plant onto a worn transmission, as well as impellor pump and altenator. This engine's going to turn faster and give more tourqe then the old tranmission can handle. The bearing in the altenator are older and have fatigued both from they're use and from corrossion and the leaching out of carbon and iron that make up the steel.
Thats why its not recomended. Plus for a couple of bucks more you get to advertise a new drivetrain when you go to sell it. A big advantage because most people are afraid of getting stuck with a blown tranny or engine. Add in the fact that the new stuff comes with a longer warrenty on more peices then the short warrenty that you'd get on a rebuild. And you can see why after twenty years no one really goes by the hours any more.
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