is it on a tri bike? if yes, then it's probably coated with a nice layer of sports drink, sweat, spit, snot, water, and anything else that's been near your front end. try letting some degreaser soak it (i.e. spray it on and let it sink in). then spray with water maybe, then repeat. you can also use lube like triflow or something to place into specific parts to loosen.

Yep, it's caked with gunk on the inside like you say. I've actually kept the outside very clean, but I stored my bike for a bunch of months during marathon training, and now it's frozen.

I have tried (but not extensively) degreaser and lube, may need to keep applying. May also try boiling water to remove any stuck water soluble stuff.

Proper penetrating spray will help. One thing you can do is try and tighten it a touch, this can often break the screw free. Make sure you're using a good tool, well seated of course.

I wouldn't use just water myself (you've had degreaser in there so should be clean), but if it does the job, then great. I'd try hot air next if the above doesn't work.

---

Developing aero, fit and other fun stuff at Red is Faster