spd6

Hey Folks,
I am installing (or trying to) a new impeller in my 96 Challenger and it is not working. I am putting a 14.5/22.5 skat trak on and the pump and all went together fine. While on the bench the impeller turned (hard) in the pump but it seemed fine, I installed a new wear ring and thrust bearings at the same time. I reinstalled the pump, venturi, and reverse bucket and let it set for 24 hours for all the sealants and locktite to cure. Dropped it in the water (I know I should have tried it on the trailer...) and it would not turn over. Checked the battery and it was fine. Took the intake grate off and the spark plugs out and tried to turn the drive saft by hand, no joy. Took the pump off the back of the boat and it turned fine. Cleaned it off again, put it back on, being very careful with alignment and it turned by turning the drift shaft until I began to really snug the nuts up to 26 ft. lbs, then it became so hard that the starter would turn it over but only without the spark plugs. Any ideas why it works until I snug the pump back down? Impeller not all the way on the shaft? There is a gap between the aft end of the impeller and the pump of 1/16 to 1/8 on an inch.

Sorry this is so long and thanks for your time to look at it...
Stephen