I took the lower unit off to replace the water pump impellor and have had a heck of a time getting it back on. Everytime I try to reassemble it I can get it to within 1/8" of mating and no closer. I worked 2 1/2 hours on it takeing it apart and putting it back together,but get no closer. I am afraid to tighten it and break a housing,any idea what could be stopping it from getting any closer? The four screws don't seem to be fouling,try turning the prop,shifting in gear and out and looked it the bottom of the unit to see if anything could have fell in even turned it upside down and shook,nothing.Any ideas? Thanks, Ben