So i got a 18mm joint and a 18mm bowl and they don't fit together? i'm new to this stuff and i'm completely confused on the concept.... Incase of my Self Stupitity here's the link's to the product's Bowl: http://www.ebay.com/itm/271364024495?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 Downstem: http://www.ebay.com/itm/131050954554?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 Please someone tell me what sizes connect