I recently received a pack of PenBBS spare parts, which includes (among other things) standalone feeds, housing for nib units, and plenty of O-rings in different sizes. I'm happy to let you know that after pairing a Nemosine #6 EF nib (from a Fission pen, which is a model I've come to dislike and regret buying) with a new feed and forcefully jamming them into a nib housing from the pack, the replacement nib unit fits the PenBBS 308 perfectly (with almost no headroom when the cap is screwed on), in spite of the Nemosine nib being marginally longer than the PenBBS nib.
That's a much, much better idea that trying to pull the factory nib out of the pen's original nib housing unit, given how fragile the fins on the PenBBS feeds are. One of the five standalone feeds I received already has a small number of fins that are misshapen and fractured. The replacement nib housing is rudimentary and does not have any 'keying' for the nib body, and it took a lot of force to push the nib as far in as the original nib unit (which is clear, if slightly blue in colour) shows where the factory nib is sitting. Even after that, the nib wrote terribly wetly (and sorta defeated the purpose of replacing the nib on the pen with a Nemosine #6 EF, which I have tested and know is capable of putting down very narrow lines), and it took some more pushing on the nib to finally get it to behave. I have no doubt that it'll now be nigh impossible to remove the nib from the replacement nib unit without permanently wrecking the feed, housing or both, but since I have no intention of ever returning the nib into a Nemosine Fission pen, it isn't all that much of a worry.
I just wish Nemosine had offered its remaining stock of Singularity pens (with Stub nibs in different widths) earlier in its closeout sales, so that I needn't have purchased my three Fission pens just to get those #6 nibs. Their awful painted brass pen bodies with the glossy metal sections and most uncomfortable step-down can now go into the trash, even though the factory PenBBS nib units with F nibs won't (as much as I don't find them fine enough for my tastes).
Thanks for this informative post. The penbbs feeds really work well for me. They are distinctly better than jinhao feeds for example. Next time I get a penbbs order I will throw in some spare parts too. Did you apply an o-ring to a singularity to make it more securely eye-dropperable?