I present my pen refill adapter, for use in pens that take the Schnieder Express 740 ballpoint refill. It allows such pens to use the Mini/Multi D1 refills instead (not included).
The Schnieder Express 740 refill is around 107mm long and 5.0mm in diameter. The D1 refill is around 67mm long and 2.3mm in diameter.
I appreciate that the D1 refill is generally a less practical refill, compared to the Schnieder refill, in terms of ink volume. However, since the Schnieder refill is a propriatary refill (although it is similar to the Papermate Lubriglide G1 refill), the D1 refills come in many more varieties, like the Pilot Hi-Tec-C Slims or Zebra JSB, which are not available in the Schnieder refill size, so this adapter exists to hopefully provide flexibility, between your favourite pens and refills.
The Papermate Lubriglide G1 refill is similar to the Schnieder Express 740 refill in overall length and width, however the G1's tip is only 1.8mm in diameter, so any pen designed for this refill won't be suitable for the 2.3mm diameter of the D1 refill.
The adapter needs to be printed in the polished white or coloured plastics, as the smooth polished finish is functionally desirable. The D1 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.