I purchased a new sail for the Dart 18 from Whirlwind. The grommet at the top of the sail is thicker than the opening on the halyard ring. The halyard ring looks like this, exception mine has a spring loaded bar that flips down to lock: 
I tried simply bending the hook open enough to accept the new sail, but then the bar doesn't clear far enough to close. Any thoughts on a solution?
I have the same halyard ring as in your photo on my Dart 20 and I bought a whilwind mainsail in December 2020 and it fits perfectly. I've never seen a halyard ring like the one you described. Maybe get a halyard ring like the one you have pictured? Or do away with the spring loaded bar. What purpose does the spring loaded bar serve?
Re-posting after logging in
I've had this problem in the past. I hammered the grommet to be a bit flatter and allow the shackle 🙂
@boatbore - I think he means that instead of a screw pin, the shackle has a pin with a clever end that self-locks. I've had them on other boats.
If filing down isn’t an option, consider having the grommet replaced with one that fits the halyard ring. A sailmaker or a marine shop drift hunters can assist with this.
Does anyone have an extra halyard ring they'd be willing to sell? I'd like to help the new owner out.
Just get a steel ring the appropriate diameter and a twist shackle. You can tie the line portion of the halyard directly onto the the steel ring. I did it that way for years with my Nacra with no problem. The boat came with a halyard ring like the one pictured but I discovered that a smaller ring made it easier to release the ring from the hook.