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I present my pen refill adapter, for use in pens that take the Euro Rollerball type refill. It allows such pens to use the Zebra JK Sarara gel refills instead (not included). The Euro Rollerball refill is around 111mm long and 6.3mm in diameter. The Zebra JK refill is around 96mm long, 3.5mm in diameter.
The Euro Rollerball refill doesn't have a well-defined standard size and different manufacturers make their refills in slightly different lengths. This refill has been designed to be adjustable, using an M3 x 8mm long socket screw (not included), inserted into the adapter's tail.
This adapter exists to hopefully provide flexibility, between your favourite pens and refills.
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 Zebra JK refill is wider than the Euro Rollerball (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.