Cannondale FSA 'Si" Expander Plug and Compatibility

Hi there! My 2017 SuperSix Evo has a the stock expander plug in place, with FSA and ‘System Integration’ branding. I like the look of the S-Works aluminum stem (it’s shiny) and it ships with its own expander plug. My question is, can I run either expander plug or will there be compatibility issues?


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The official line from Cannondale is going to be to use their expander plug, and that it “should only be completed by a professional bike mechanic.”

That being said, in my experience, Cannondale’s expander plugs are so sketchy. You can visibly see the top stem bolt clamp further than the bottom bolt. For my Cannondale Topstone and my friend’s 2020 supersix, I replaced both plugs with the specialized plug, which is far more secure.

The only compatibility issue you may run into is that the specialized plug extends ~1.5mm above the steerer tube. Depending on your setup, you may need another 2.5mm spacer, or shave off ~2mm of steerer)

I would recommend using the specialized plug, but you are smart, you can use your best judgment. Worst case, you can just swap everything back.


Can’t thank you enough for the detailed breakdown, much appreciated! Fortunately I have a service booked at a top local shop upcoming. The S-works stem will be in the bag of goodies (unless anybody has a 12 degree alternative :slight_smile:

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There’s no reason it technically won’t be compatible - but to echo the comments above I’d look to use something else. This isn’t an area to save weight, function is much more important. I’ve used the Si plugs on a few builds and moved away from them every time - they just aren’t secure enough. FWIW I’ve spoken to a few bike shops about this and they aren’t big fans of them either.


As a Cannondale dealer - just make sure you are careful no to use spacers above the stem - Cannondale will void you warranty. (It’s #1 on the instructions but I’ve seen it missed)

Sorry to hi-jack this guy’s thread a bit but always wondered what was the point/benefit being marketed of these expanders anyways?

I had a poorly made Super Six Evo with one of these things and it didn’t seem to do anything but prevent me from using a custom top cap with a picture of my dog on it. I have had a number of frames since with normal expanders and I’ve yet to die or lose a bike race as a result of them.

Currently running 2x5mm spacers. Steerer trimming is going to happen.

No worries. My 2017 SuperSix Evo has been incredibly faithful since day one, nothing to indicate any poor quality (swapped most components out twice now in upgrades tho). My takeaway from all these points is probably just to stick with my existing 3T ARX II Team stem, cut the excess steerer and call it a day.