I present my pen refill adapter, for use in pens that take the Mont Blanc Ballpoint refill. It allows such pens to use the Uni SXR-80 refills instead (not included).
The Mont Blanc BP refill is around 98mm long and 6.0mm 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.
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.