I was wondering if there was actually a point at which just replacing your crank bearings would not fix anything (if the crankshaft was actually damaged).

I've got knocking coming from my motor. Local mech. says I'm about to spin a bearing. Does this mean that I can replace just bearings right now? Are the bearings what is causing the knocking?

 

You can't really tell what you are going to need without tearing it down first.