I present my pen refill adapter, for use in pens that take the Pilot G2 type gel refill. It allows such pens to use the Pilot Hi-Tec-C Coleto gel refills instead (not included).
The Pilot G2 refill is around 110mm long and 6.2mm in diameter. The Pilot Coleto refill is around 93mm long, 3.1mm in diameter and has the Pilot serial number LHKRF-10CX (where X is either 3, 4 or 5, which represent the tip size).
I appreciate that the Coleto refill is generally a less practical refill, compared to the Pilot G2, in terms of ink volume and manufacturer diversity, considering that the popular Euro Rollerball refill is close enough in size to often be interchangeable with the Pilot G2 refill. However, the Coleto gel refill is quite highly regarded for it's pleasant writing characteristics and the very fine writing tips it offers (0.3, 0.4 & 0.5mm), so this adapter exists to hopefully provide flexibility, between your favourite pens and refills.
Other refills that have a 3-3.1mm tube diameter should also be suitable for use in this adapter, such as: The Zebra Surari EK, Pentel Sliccies XBGRN and Uni Signo UMR-109. However, these refills are usually longer at 98-99mm long, so some trimming of the plastic tube with a craft knife would be required, to make them the same length as the Pilot Coleto.
The adapter needs to be printed in the polished white or coloured plastics, as the smooth polished finish is functionally desirable. The Coleto refill is held in place, by the adapter, using friction. The holes around the side of the adapter are present to help with the removal of unbonded plastic powder from the core, when being polished. However, some powder may still need to be removed, with a pin or thin metal rod, before inserting the refill.
Due to the very varied nature of pen design, I cannot guarantee that this adapter will work with any specific pen, although I've made every effort to make it as universal as possible. The neck of the Coleto refill is wider than the Pilot G2 (3.1mm vs 2.5mm), so this may be an issue in some pen bodies.
I plan to make as many refill types compatible with as many host pens as possible, so If you have any suggestions on refill adapters, please let me know and I can hopefully make it available, depending on the relative sizes involved. I have now created dimensioned drawings of over 80 standard and obscure ballpoint, gel and rollerball refill types, so I like to think that I have a fair idea of what is and isn't possible.