I am one of the rare people that like the look of the ugly stick known as the Rotring Core. I love the windows in the barrel. However, they are in absolutely the wrong place to tell you anything about the ink level in a converter or international short cartridge. Upon looking at the barrel closely, it occurred to me that there isn't just a little plastic over the holes for the windows. There is an entire clear plastic sleeve inserted into the barrel. I put some water in, and it didn't leak, so I started thinking about how to make this pen an eye dropper (ED) pen.
I know there are many threads on multiple forums about making a pen an ED pen. So what I am posting here is nothing new. In general, you take a pen that has a sealed barrel and no internal metal parts in the barrel apply some silicon grease to the threads and are good to go. However, that route will not work with the Rotring Core.
Unfortunately, the designers placed holes past the threads. Greasing the threads just won't work. Also, you see that annoying orange thing, The Core is supposed to use this gripper to pull the cartridge back some when you are low on ink to allow air into the cartridge and continue the ink flow. In my experience, it was just a way for the cartridge and converter to come loose and leak into the barrel. So, that part just had to go anyway. Simply sqeeze in on the tabs and it can be pulled out.
Ah there we go, much better. Annoying orange flange thing is gone..
I used my calipers to measure the pen and came up with a measurement for the O-Ring. I decided to purchase an O-ring that has an ID (Inner diameter) of 10 mm, and is 2 mm thick. Unfortunately, I couldn't find this size in the local stores where I stopped, so placed an order through an e-bay vendor. 2 weeks later it arrived. (price was about $3 with shipping for 50 O-rings)
Here we see the O-ring slid onto the section. It slid on fairly easily without much resistance. I realized at this point that I may need a smaller size. I took the O-ring back off and added some silicon grease to help the O-ring seal between the section and the barrel. Notice that the holes are going to be right under the o-ring.
As you can see, you do not want to over tighten the barrel. As the barrel becomes tighter, the O-ring changes shape and mushrooms out of the gap. This will not make a good seal nor will the cap fit properly this way. I definitely need to order a smaller o-ring.
Here is a completed pen, with the barrel not completely tight. When it is tightened as shown, the O-ring is flush with the barrel and it does not interfere with the cap. However, I am not happy about the barrel not being as tight as I would like.
Smaller o-rings have been ordered 9.5 mm ID, but I will not receive them for 2 weeks or more (price was about $3 with shipping for 12 O-rings). So in the meanwhile, I will be testing for spontaneous leaks, explosions, spurts, splurts, burps, leaks, etc... with this pen. I will let you know what I discover in case there are any other closet Core owners that would like to give it a try.