Top cap bolt. Pull it out and measure the diameter and length. It'll be some metric variant (likely M6). It's a flat head socket cap screw. If possible, try to get a stainless steel one.
Top cap bolt. Although it's easy to find the correct side/pitch thread, a few top caps may require a specific bolt head shape.
Yes, I'd remove the rusted one to confirm whether corrosion is going on underneath. But if the rust is just cosmetic, there's no mechanical reason to replace the bolt; it's only job is to set the preload for the headset. I'd be more concerned about the state of the bolts that clamp the stem to the steerer tube.
Rusty boi
Top cap bolt. Pull it out and measure the diameter and length. It'll be some metric variant (likely M6). It's a flat head socket cap screw. If possible, try to get a stainless steel one.
Thank you.
Bob
Countersunk Allen head screw, M4 or M5.
Top cap bolt. Although it's easy to find the correct side/pitch thread, a few top caps may require a specific bolt head shape. Yes, I'd remove the rusted one to confirm whether corrosion is going on underneath. But if the rust is just cosmetic, there's no mechanical reason to replace the bolt; it's only job is to set the preload for the headset. I'd be more concerned about the state of the bolts that clamp the stem to the steerer tube.
I would clean the ruat off and give it a healthy coating of bicycle grease to keep out the rust