Well here is my easy version on how to bore something out.... this works for manifolds or spacers... you'll need to take off the manifold to do it as you don't want anything to go into your engine.

Today I decided to bore out my throttle body spacer since it was smaller than my new HR Throttle body. So first thing I did was put my parts together and open the throttle butterfly



You can see it isn't smooth and flush. So I then stuck a pencil inside and traced around the spacer. Here you can see the light marks.


Now out to the garage. I'm going to use this grinding bit


and this polishing stone



Then clamp my spacer into the vice



and go to town!




I then used some sand paper by hand to just smooth it all out and take off any burrs. I didn't put the whole thing back together because I'm going to instal my supercharger tonight. But it is now smooth and will flow better!