Threads showing where?. It looks tight in the photo, and you don't describe a problem.
If the headset doesn't stay adjusted, remove the adapter and locknut entirely, and compare the height of the steerer to the depth in the locknut up to the stop at the top. If all is right, the locknut would be deeper than what's sticking out so it can't bottom against the top of the steerer.
If the steerer is too long, it might be because this headset has a lower stack height than the original, or that someone along the line removed a spacer, or maybe a reflector bracket or brake cable hanger. No big deal, add a spacer so that the steerer extends only about 7-8mm (less than depth of locknut) and get on with your life.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.