Knock Sensor
 

If the knock sensor is installed in the front rotor housing, what goes in the same spot on the rear rotor housing. I can't recall what I pulled out of it.

It's just left open. The hole doesn't lead anywhere that can leak or anything.

Thanks Andre, I was 90% sure that was the case but wanted to confirm.

Mazda did it like that just to make production easier, the rotor housings are fundamentally the same save for some EGR passages. You can mix and match them - build a motor with 2 front housings, etc.

But, yes, it is just a threaded boss, it won't leak anything and that's the way it is stock.

Dale