It says compatible with Energel and Pilot G2, so it would be the same length as a rollerball, but the fit depends more on the shoulder of the cartridge. I've found that Pilot G2s will fit in a Jr pen, but even though it's the same length, it doesn't extend fully from the tip of the nib. That being said, test it in a few kits and let us know how it works. I wouldn't trust them in pens I sell until I went through a couple to see that they are consistently reliable.