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The perfect pen barrel for the Uni-ball Signo UM-151 (DX) gel pen refill.
The perfect pen barrel for the Uni-ball Signo UM-151 (DX) gel pen refill.
994 backers pledged $88,322 to help bring this project to life.

Home stretch + Refill details

Posted by Brian Conti (Creator)

Hello all... Brian here with the weekly update.   

A big thanks to all for the continued support! The Spoke Pen project is in the home stretch of funding... finishing up on Saturday. We're excited to be over 920 backers with maybe a chance to get to 1000 with a strong finish!

Schedule Note

The bulk of this update is to provide more details on refills, but before that, I always like to touch on the item of most interest... schedule. As we've noted before, the great start allowed for us to get things in motion already. The final adjustments were tested out, and we have part orders in place and are on the schedules of the production machine shops we're using. We feel good about the dates we've targeted and of course we are going to try and beat them! After funding concludes we'll continue to keep everyone posted regularly on our progress.

the wicked cool Spoke Pen Joker edition
the wicked cool Spoke Pen Joker edition


The refill discussion can be a confusing topic because there are so many different manufacturers/styles/types/versions.  We'll do a quick overview here, then include a 'deeper dive' section for those who want to see more details.

  • Yes (refills that fit): Uni-ball Signo DX, Uni-ball Signo 207/307, Uni-ball RT1, Pentel Energel, Zebra Sarasa Clip, Uni-ball SXR-# refills (ballpoint)
  • No (refills that don't fit): Pilot G2, Pilot Juice, Pilot Hi-Tec-C, Pentel Slicci, Pentel Hybrid Technica, Parker-style, Uni-Ball Signo UM-100 (refill too long).

As our project tagline notes, we set out to make 'Spoke Pen, the perfect pen barrel for the Uni-ball Signo UM-151 (DX) gel pen refill.' My partner on this project, Brad, the Pen Addict himself, gives this refill a top rating and his highest recommendation for gel refills.

Note that pens and their corresponding refills typically have different designations.
Note that pens and their corresponding refills typically have different designations.

We're clearly big fans of the UMR-1-# refill and all Spoke Pens will ship with a packaged 0.5mm black refill. During assembly we will install a test refill and to make sure everything fits up correctly and do a test scribble... but to ship we'll remove the test refill and include a brand new sealed refill for you to install. The pen simply unscrews and you place the refill inside.

As we've designed for the UMR-1-# refill, we've not exhaustively tested all other other refills for compatibility, but as listed above many refills will fit properly in Spoke Pen, including these very common options...

In the gel pen realm, these two very popular refills are compatible with Spoke Pen:

1) Uni-ball 207 and 307 retractable Pen refills fit Spoke Pen. The refills are available in a wide range of sizes and colors and are designated UMR-8#, with UMR-85 being 0.5mm etc...

Uni-ball 207/307 pens and their refills are widely available and fit Spoke Pen
Uni-ball 207/307 pens and their refills are widely available and fit Spoke Pen

2) Pentel Energel RTX Pen refills fit Spoke Pen. These refills are also widely available in a wide range of colors and sizes. Enegel refills designated LR#, with # indicating size LR7 being 0.7mm, etc. are compatible with Spoke Pen.

Pentel LR energel refills are widely available and fit Spoke Pen
Pentel LR energel refills are widely available and fit Spoke Pen

For those in need of a ballpoint solution, the Uni-Ball SXR-# Jetstream series refills are compatible with Spoke Pen (once more the # indicates size, SXR-5 being .5mm, SXR-10 being 1mm, etc.). These SXR series Jetstream refills are widely available in colors and sizes, and are highly rated as well.

Uni-Ball SXR Jetsream Ballpoint refills are compatible with Spoke Pen
Uni-Ball SXR Jetsream Ballpoint refills are compatible with Spoke Pen

Refills, Deeper Dive

For pens, there really are no exact refill 'standards'. Most pen refills are specific to the actual manufacturers... but many have similarities that allow interchangeability. With all of the machined pens that have been launched on Kickstarter, why isn't the UMR-1-# refill used more often? The UMR-1-# refill has some unique aspects to it, and in most cases it simply does not properly fit in other pens. Here's a detailed look at the UMR-1-# refill:

The UMR-1-# refill has a somewhat unique/large flange feature (A) located close to the tip end that prohibits it from fitting into many pens. The Spoke pen internal cavity is designed to accommodate this geometry.

The tip end diameter (B) is ~2.3mm for the UMR-1-#. The Spoke Pen is designed to closely fit this diameter for a solid writing experience.   

Uni-ball and Pentel refills are typically sized at ~2.3mm, so the refills for the popular Pentel Energel and Uni 207/307 pens fit properly in Spoke Pen. We get asked a lot about Pilot G2 refill compatibility... the tip end diameter for the G2 is ~2.6mm... if we made the Spoke Pen larger to accept the larger G2, we would introduce 'tip wiggle' for the Uni-ball refills and compromise the writing experience. As such the G2 refill simply doesn't 'stick out' of the Spoke Pen cone and is not compatible. We may make a G2 specific cone version for Spoke Pen in the future, but that's not planned for this project.

On the other hand, refills like the Cross ballpoint have a tip end diameter of ~2mm... while they technically'fit' the Spoke Pen, the reduced tip diameter makes it very wiggly, so it's not appropriate to use.

Compatible refills also have a collar feature (D) spaced back by a distance (C) from the tip end. Together, these parameters assure the writing end protrudes a proper distance from the pen cone.

The ink reservoir diameter (E) varies widely on many refills. Spoke Pen is designed to accept up to a 6.5mm diameter refill, and the internal spring is configured to properly engage with all diameters.

In addition to the unique geometry (A), the overall length (F) of the UMR-1-# refill often poses fit challenges to other pens. It is substantially longer than the typical gel pen refill.

Spoke Pen is designed to accommodate the length difference between typical refills and the UMR-1-#. The custom internal spring is configured to provide an appropriate amount of preload force when compressed by either length refill.

To recap, there are numerous options for refills, but your Spoke Pen will come with a Uni-Ball UMR-1-05 black refill.  It's the preferred 'engine' of the Spoke Pen and we hope you like it as much as we do!

As always, Brad and I are super appreciative of your awesome support!

Best/Brian & Brad

Andrew, Harvey Simmons, and 28 more people like this update.


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    1. Brian Conti 14-time creator on

      @Coby, that will work just fine. Thanks for backing!

    2. Coby Stilp on

      My wife really likes her Uni-Ball Signo UM-153. Your post indicates that the refill will be too long to work in the spoke pen. What If I cut the top of the refill down to match the length of a UMR-1-#?

    3. Jay on

      Tip wiggle (or lack thereof) is the main reason I’m backing this project so thanks for confirming it won’t be there!

    4. Missing avatar

      Andrew Young on

      Thanks for the detailed update! This more technical explanation is exactly what I was curious about. Sounds like your spring mechanism is the key to handling different lengths without mod, and I'm happy the UMR-1-# refills have the same tip diameter as the 207/307 and Energel, so I won't get the tip wiggle I get with the usual G2-compatible pens. Great job! Eagerly anticipating delivery!

    5. losabio on

      That's awesome about the refills showing up still sealed. I might throw in a .38mm Blue and just keep the .5mm Black to refill a black Signo DX when needed. Or if I get a Joker maybe I can throw in a .38mm Lavender Black...

    6. Chris Hokanson on

      Excellent, thorough update. Love the refill explainer - super helpful. Like @pkshiu said, this is great for future reference - would certainly be worth a PenAddict post.

    7. Wolf Wolfinger on

      Thanks s lot, you've really done your homework!

      Great job, can't wait to get mine.

      Kind regards

    8. pkshiu on

      This is probably the best write up of refill sizes and fits ever! I am saving this update for future reference.