Snug away but also use the teflon tape on the threads in the usual manner. Be sure to use the correct mm size. You neither want to raise any burrs nor leave any metal shavings inside the valve. They make a base for multiples of this valve that will make more sense to use if you are going to line up multiple solenoids. I found them once, but we skipped pneumatics last year, so my notes are in another book . . . . . somewhere.
