REMOVE that one massive bolt that attaches the transfer box, where the hidden bolt is under. - Without thinking it thru, how I would drill a 17mm hole, I just started with what I had. First - VERY IMPORTANT - mark the spot, where you want to drill. Mark twice, drill once and not mark once and drill twice. That way I positioned the first little hole with my "pilot drill" right bang centre, followed by 10mm and 12.5mm drill bits.