I present my pen refill adapter, for use in pens that take the Caran d'Ache Goliath type ballpoint refill. It allows such pens to use the Uni SXR-80 refills instead (not included).
The Caran d'Ache Goliath refill is around 96mm long and 5.9mm in diameter. The Uni SXR-80 refill is around 88mm long and 3.1mm in diameter.
Other suitably lengthed refills are the Tombow: BR-CL, BR-CLE and BR-CS2.
This adapter exists to hopefully provide flexibility, between your favourite pens and refills.
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, Uni Signo UMR-109 and Pilot Hi-Tec C Coleto. However, these refills are usually longer at 98-99mm long, except the Pilot Coleto, which is 93mm long, so some trimming of the plastic tube with a craft knife would be required, to make them the same length as the Uni SXR-80 (88mm).
The adapter needs to be printed in the polished white or coloured plastics, as the smooth polished finish is functionally desirable. The 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 Uni SXR-80 refill is wider than the Caran d'Ache Goliath (3.1mm vs 2.6mm), so this may be an issue in some pen bodies.
This adapter and the Pilot Coleto adapter will not work with the Caran d'Ache 849 pens, while the D1 adapter should work correctly. The issue with the Coleto and SXR adapters is with these replacement refills' plastic body tube being too wide, close to the refill's metal tip and it not being able to protrude enough out of the 849 pen, to allow the click mechanism to cycle. The D1 is long and thin, just like the neck of the Goliath, so does work.
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.