I just measured the diameters on the water pump necks: Actual measurements, using digital calipers: top neck lip: 1.85" top neck behind lip: 1.75" bottom neck lip: 1.76" bottom neck behind lip: 1.65" (42mm? or 1 5/8"?) Not real sure what the right move is for what size lower hose to run. Those with running cars - what lower radiator hose are ya'll using? I ordered a 1 3/4 to 1 1/2 adapter last week. It's not here yet. I'll report back when it gets in.