I have a cannondale premium carbon fork with full carbon steerer that uses a compression plug inside of the carbon steerer instead of a starnut. What torque should this plug be at?

Only enough to remove all of the slack from the headset bearings. Once the stem pinch bolts are set the compression plug bolt serves only to hold the cap on. The stem pinch bolts should have 5 nm at most.

Assemble with carbon assembly compound. It's minimum necessary torque to preload the headset without pulling the plug out.

I read through winwoods instructions (first one's that came up on google) thinking i'd get a straight answer.

https://www.winwoodbike.com/pdf/Winwood_fork_inst.pdf

"Using a 5mm hex key, turn the expander bolt clockwise until the expander ring tightens firmly against the inner diameter of the steerer tube. DO
NOT OVERTIGHTEN."

Just use a Tri-Allen wrench and turn pretty hard, don't put all you've got into it. If you end up not setting it tight enough the only bad thing that could happen is that it'll slip while setting headset preload. And if it does slip, you'll know and you can just tighten the plug slightly more. So if you have to err on one side, err on the side of 'not tight enough'.

For the type of internal plug where the expander function is independent of the headset-preload function, if there's no spec given by the manufacturer, then I grease the threads and wedging bits, put carbon assembly compound on the exterior where it'll be touching the steer tube, and use about 45-60 inch-pounds depending on how it feels.

As others have said, The plug only has to be tight enough not to pull up as the headset is adjusted. The torque required depends on the design of the plug itself, and how well it bites and holds inside the steerer. Grit paste helps lower the required torque but is rarely needed.

If you're unsure, approach the needed torque by degrees from the low side. Tighten it until you feel it'll hold & try to adjust the headset. If it holds, all is good, if it slips, loosen, tap it back down, and tighten a bit more. Repeat until you're done, knowing you probably could go tighter but don't need to.

Well, part of the expander's job is to support the steer tube against the compressive force of the stem, too. It's more than just an anchor point for the top-cap bolt.



Example: Specialized, with a suggested torque of 80 in-lbs. It would anchor a top-cap bolt at far less than half that torque, but the goal is to keep the steer tube happy. In Specialized's case, the plug also has a top lip that prevents the stem from compressing the open end of the steer tube, which is smart since I've seen carbon steer tubes split from the top down without support.

So in the big picture, if the manufacturer doesn't provide clear guidance, I suggest getting the expander snug enough to counteract some of the stem's compressive force.

And why do we care if the plug is going to be squished by the stem, even if that were to happen? Those bolts aren't tightened until the headset is preloaded anyways.

mechBgon is talking about damaging the steerer, not the plug/expander.

Most carbon steering tubes (except Alpha-Q whihc uses glued-in support tubes) are thick enough that the compression plug can be removed entirely and still function just fine. As as example, LOOK changed the headset on most models to a type that has a threaded adjustment ring on the the head tube. No top cap or compression plug is needed. They didn't add one to prevent cracking the steering tube.