@Rotor13
What kit are you working with ?
It always helps to "ream out" the brass tube ends into which you are pressing the components. ie. slightly enlarge the ends into which you press things.
You can do this with sandpaper wrapped around a wood dowel.
If you overdo that operation, you may have to glue in your components or use Loctite, but that is no harm.
Measures like these are a bit more work but well worth it to avoid cracking the pen barrels.
This cracking problem is most common for small diameter pens such as the slimlines.