In my building of an intake manifold it would be necessary for me to make quite a weird cut on a mandrel bend... I pretty much need a donut cut in half. Any ideas on how this could be set up? Do they make donuts that have the seperation 90* to the standard donut?

((( <--- The middle line if looking at a donut half from the top. The reason this is necessary is i am having a tube curve down to a flat piece at the end of the manifold. The flat piece will be 4" wide and the plenum will have a 4" tube cut in half with some flat plate to space it from the manifold.

 

are you trying to cut straight down the middle of the "pipe"? if so, i my self would use a cut off wheel, about the only thing to cut a curved surface that i have.