How often do these three parts needs to be replaced? I'm currently doing a rebuilt on a b16a2 with a new bottom and head. I'm salvaging these three parts from my old and problematic b16a2. So normally when you do a rebuild do you replaced these parts base on the amount of high mileage alone or what?

 

If they look good, run them. I don't consider them really a wear or maintenance item. No need to replace them normally during a regular rebuild in my opinion.