It cant fall out, because the SP spacer with the yellow line, its hole is to small for the honey comb to fit into, and the HC is just long enough so that the 02 sensor hold it in place. It is just a little loose inside the bottom SP spacer. If you try to force it into place, it will brake and crumble. I wore a dust mask to grind the HC, because it makes a lot of dust. If you brake some of it, no big deal, because there is a lot inside of each cat. I would guess that I ground the HC to just under 1/2 inch around, where it would just fit inside the bottom SP spacer. Then I cut it to length. I put the hacksaw in a bench vice, and drug the HC in one direction to cut it just long enough so that the 02 sensor would not quite touch it. I then put the HC inside the undrilled SP spacer, and installed the 02 sensor with the drilled out SP spacer on it. I could shack the unit and hear the HC shacking around inside just a little bit. The HC can not be so tight that anything will smash it, because it is brittle, and you will cruse it, then it will not get any exhaust flow. Once you grind and cut the HC you will see what I mean. After it fits rite in the SP spacer, blow off the dust with an air hose. Keep in mind that the exhaust has to flow through the little holes in the honey comb. I gained 1.5 MPG and a little more WOT power after this mod. The OE cats are restrictive. LOL it makes up for the 1 MPG I lost with the shorty headers and true dules, but I do have a lead foot.