Dp-sm out drive won't spin when I put props on.

[mklearl]

Dp-sm out drive. I installed it today without issue as per the manual. Prop shafts both spin freely when in neutral. I put the front prop on and both shafts spin easily in neutral. When I put the rear prop on they will spin normally. When I tighten the rear nut to just snug the shafts seize up and won't spin. I'm thinking this is something stupid but I can't for the life of me figure this out. Anyone?
Thanks in advance!

 

[mklearl]

Well I answered my own question. I have new prop nuts. The old one goes on further. The gasket in the nut seems to need to be worked on. I put quite a bit of pressure on the new nut but I don't want to force it. Is this common? Would I be OK using the old one? It is the larger forward nut that is the problem. Thanks