what goes into totally rebuilding an engine?

I was wondering what goes into "rebuilding" an engine. Once you rebuild an enigne it's like new right?, so i was wondering what new parts need to replace the old ones to make the engine like new, and is it worth it to rebuild or just buy another engine?

rebuilding an engine makes it to new or better than new spec. when rebuilding a motor, you'd be replacing everything or upgrading things along the way. you'd typically change all seals, bearings, gaskets, water pump, belts including timing, probably pistons, con rods, oil pump, etc etc etc.

if you go a rebuild route, your gonna need a machine shop to rehone the block for the new pistons, etc. also if you rebuild you can add aftermarket high perf stuff too, instead of just going with factory.

complete rebuild = 0 miles.