jjoos, who's extension are you using, is it the sullivan or Thndertiger. Also I would not attempt to cut part of the tip off the glow plug, as I dont think that is your problem. I think your problem is where the clip is hitting the base of the engine where the glow plug sits. I bet if you try loosening the glow plug slightly it will go right on, but that is just a test (dont run it loose). That would tell me, if you need to remove any material, it should be removed from the bottom of the glow plug clip base, a dremel tool with a drum sander would probably do the trick. Daz...