@Avitt, noticed the same thing with the gen tubes. This gap really annoyed me. As i looked closer why this gap occured i noticed that the tube is not threaded all the way to the end. What i did was that i just forced the tube with the help of a pair of pliers. I was worried that this might do some damage to the tube or the case, but it all worked out fine and now the tube sits as it should.
I got some locktite Lock'n seal (thread lock and sealant) at the same time as the T8. Just to be on the safe side i will use a drop of it.
Thanks for the advice