I have a friend who rebuilt the engine in his car and it only took him a day or so to do a very good job. I know it's supposed to improve the engine, but how? Is it really worth all the effort? I'm guessing the engine needs to be cleaned every few years. [inside and out]

 

if you totaly rebuild an engine it makes it like new again. you replace all the seals and bore it out. i doubt he did a total rebuild in a day or two. maybe he just cleaned it out and replaced some worn things.