SRAM 8-bolt chainring interface CAD

General Discussion
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A while back I reverse-engineered the SRAM 8-bolt spline for their current generation cranks.
The 3D printed test piece I made has a way better fit than SRAM’s own chainrings.

Regardless, before I go ahead with ordering a custom 8-bolt 104 BCD spider for someone to use a Gates belt sprocket with SRAM’s T-type cranks, I figured I’d share it here and also invite others to upload their reverse-engineered interfaces for a sanity check.

Here’s mine.
SRAM_8-Bolt.STEP (245.2 KB)
SRAM_8-Bolt.DXF (44.2 KB)

[Edit: Updated with new STEP file and DXF of a tested and well-fitting interface as seen here. 25 Nov 2025]

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I’ll take a look when I get a minute. I’ll admit I didn’t spend much time and went pretty quick with my “reverse-engineered” interface.

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SRAM 8 Bolt Chainring Interface.step (79.3 KB)

I decided I didn’t know how to express the differences. But this is the one I have. All I did was 3D print a test, so I don’t know if yours or mine is better.

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Did either of you end up getting a spider machined out of these designs and how do they fit?

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Found my 3D printed test-piece and the fit wasn’t as good as I remembered it to be (I got it mixed up with another project). Still as good as SRAM, but I’d prefer a tighter fit.

So I ended up going back to the beginning. Purchased some radius gauges and reverse-engineered the interface from scratch again.

I’ve got some spiders on order and should have them within a few weeks.
I’ll report back with a report on how well they fit (or don’t fit). :+1:

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Received the 104BCD spiders I needed to reverse-engineer the 8-bolt interface for.
Made this spider to work with a SRAM T-Type crank to provide a 55mm chainline/beltline suitable for use with Rohloff hubs (their standard QR model has a 55mm belt line as standard).

I’m pretty happy with the fit.
It’s quite disturbing that the standard SRAM fit is so sloppy.

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Do you know about these ?

I use the Shimano version to run a Gates beltdrive sprocket (104BCD) for a Rohloff on an XT crank. They maybe already sell what you need?

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Had not seen those.
But the variant we need is one they don’t make. 8-bolt / 104BCD.
Looks like it wasn’t a waste of time making my own. :slightly_smiling_face:

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FYI all.
I’ve updated the original post with the STEP file and DXF of the 8-bolt spline interface and bolt pattern used for the spider in the video in my previuos reply.

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Appreciate the work! can i used the files for my design and maybe make a physical part from it? thanks

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Absolutely. Go for it!

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