Your not going to get an 18mm nib into a 17mm hole, although you could probably make a smaller one, like a 16mm stay and look good in a slightly larger hole. The nib will "flare" our quite a bit after it gets wet the first time. You should measure your hole size after the old bristles are removed. This can be done by cutting them off as close as possible with scissors, then using a drill press to drill out the old nib.