Help Please!!!! engine wont run

I looked closely at the timing chain and everything checked out. It still looks new and the marks were lined up so it didnt jump a tooth. both keys are fine for the gears and chain isnt too loose so I don't think thats the issue, I wish it was. what is normal for chain tension? anyone have a measurement for slack in chain? mine has about 3/8" or so, I think that's fine