i just recently bought a new radiator for my 2000 civic si, the hose outlet on the new one issmaller than the old one, it is the factory replacement, what could be the problem

 

well you could have the wrong hose in the first place. it could be an "ebay" item that is a "oem" replacement but just a little different. i wold say just to clamp the hose on there as long as the radiator fits and most likely it will be fine