Why is one of the intermediate housings bigger than the other?

 

One has 2 small primary ports. the other has 1 small primary and a larger secondary port.

 

the large one you see near the front has a stationary
gear in it and mazda made it bigger to hold the gear.
as well as the fact that there are tension bolts that
go into this housing from both sides, the room
was needed for thread depth.

matt

 

And to keep it simple, Mazda went with a thickness of 80mm, just like the rotor housings.

So to get an idea of the length of a 20B, just measure a 13B and add 160mm to it. That's sorta what I did.