There is no extention on mine. There is a special fitting that comes with the muffler that screws into the head. It traps a flange plate the mates with the muffler body. There is a red silicon Oring the goes on the adaptor and it seals the joint when the two screws are tight. It sounds like either they sent you the wrong muffler, or you are missing a part. If you can, post a photo of what you have and I can tell if it is right or not. The only other 4 stroke pits I have fits my O.S 1.20, but it uses a different arnagement at the head.

Actually, take a look at the YS 91 muffler. http://www3.towerhobbies.com/cgi-bin...?&I=LXFH03&P=7 The difference between the ony for the O.S/mag/TT 91 is that the flange shown on the photo at 3:00 has two holes vs one. The black fitting with the red Oring is shown.

Don