@jds26
Just a (significant) correction:
The Manhattan pen kit tube diameter is 27/64", according to the Rockler instruction sheet for that pen kit.
This is an important fact because it means that your tubed barrel could be used on the Berea Sierra pen kit, on the PSI Gatsby pen kit, and on other kits with same barrel length and brass tube diameter. There are actually quite a few other kits that use the same brass tube diameter and brass tube length. .
But, having said that, the bigger problem might be that your particular brass tube could turn out to be a loose fit on all of those alternative kits.
As
@jttheclockman said, your best bet could very well be to apply glue onto the appropriate hardware parts where the loose fit exist.
Be very sure that you know what you are doing before you apply the glue.
For one thing:
You have to be aware that your finished pen may eventually require a refill replacement.
You must not glue parts together if the design of the pen requires them to be separated eventually for refill replacement.
Be sure that you bear this in mind before gluing things together.
The instruction sheet that I am referring to is here:
I think that the "tip end fit" must in fact remain somewhat loose so that the pen barrel can be twisted to extend and retract the refill.
The "upper end fit" may very well be the problem. . At that end, the the top of the refill transmission should press, rather tightly, into the cap of the pen.
It would be that end where the Locktite ( jttheclockman's suggestion) or glue would be required . . . NOT THE LOWER, or TIP END.
But be careful not to get glue on the threads of the transmission.
I would advise you to carefully check that out, and carefully compare your pen with the detailed assembly diagram in the instruction sheet.