I looked up the thread where I remembered this was discussed:
https://www.penturners.org/threads/anyone-try-the-rollster-pen-kit.147784/
I'm not sure if anyone has really come up with a good solution. I believe the problem is due to the plastic insert not being pressed in far enough. However, there is a very small sweet spot, if it's pressed in too far then it won't snap on and there's no way to pull it back out (that I'm aware of). I just used a hammer and a center punch, but maybe an arbor press or something like that would be more controlled. I do really like the kit for my own personal use - fits in my front pocket nice and is comfortable in my hand, it's a really nice size for a rollerball. Yours looks really nice, I hope it doesn't develop the crooked cap syndrome
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It's about the worst possible time of year to visit Houston, but you're close enough that you know what it's all about. The heat has been oppressive the last couple weeks. Looks like we'll have a cool front down into the low 80s middle of next week, which will hopefully be nice. We are on the SE side, towards Galveston, if it wasn't for all this COVID stuff I'd say way should meet up for a bite or a drink. Enjoy your time with your son! All of our family is up in Minnesota, we have really been missing seeing them this year, times like these really show you what is important.