The two collars go between the flanged end of the exhaust pipe and the other flange that you slide up onto the exhaust studs, for bolting it up in place. I have found that the best way to install them is to set them in place around the pipe, wrap a piece of 3/4" masking tape one time around, to hold them together. Insert the pipe into the exhaust port, slide the collars up, and then slide the flange up to hold the collars and pipe in place while you thread the nuts onto the studs. It also helps to loosely bolt the muffler to the frame so you can use both hands at the other end.
