I love my traveller vise but I like having a longer stem to tie on. I ordered a 9" stem from Renzetti, but I can't get the top of the old stem off. Has anybody done this before? I took of the arm with the jaws and the handle, but the coller where the knob that tightens the arm will not come off. Iam afraikd of destroying it if I put too much pressure on it. Any ideas??