@MikeMiranda You need to Dremel out the inside of the housing to accommodate the vial, then add a threaded cap and a spacer when loading the cartridge. This is the original NovoPen modification that allows you to use Humapen-style cartridges.

You’ll still get around 8% overdosing, though, because of the screw’s throw distance. It has to travel farther to accommodate Penfill cartridges, which is what causes the discrepancy.

@Stevepep If you have a moment, it would be helpful to know which version you have, which batch it came from, and what led up to the break.

You're right that the walls are thin. In my experience, most breaks happen when the pen is very cold and sideways pressure is applied to the holder, such as when pulling the cap off at an angle. The connection point acts as a lever, amplifying the force and concentrating it on the connector.

The other situation I've seen is when people push too hard, either because the liquid is too viscous or because they're trying to push past the last dose. In both cases, the failure happens for the same reason: too much force being transferred to the connector. For what it's worth, I've seen the original version break in exactly the same way.

That said, I've never seen this type of break actually damage a pen. If you're unable to use yours afterward, it's likely that the broken ring is still lodged in the housing and simply needs to be removed.

I always do my best to help anyone who has an issue, but I need people to let me know when something happens. Getting these details also helps me determine whether there's a potential batch issue that needs investigating.

@guymarkparker as the creator of the NP Max, I’d suggest not trying to print one. The latest version that is even printable on FDM is 129b which has issues with clutch slipping on some carts and is a pain to print correctly. The testing team had to do a ton of work to get them to print right as the tolerances are very tight, and they still didn’t work with some needles and carts.

The current version is 146 and starting around 139 there was a full redesign to address all these out of spec carts.