I agree with Doc, I too have used these type tanks for decades during my long stint on the dark side. I still use the today and you do not need to crank the screw down as hard as you might think. I just does not have to leak, and later if it starts to leak screw it down until it stops but again avoid the desire to screw it down until nothing will move. Sometimes they split because they are just defective, but in most cases it is do to over tighening. One last thing, I always take some light sand paper and remove the flashing on inside of the stopper hole.
Andy