I have a '98 Accord with a hairline crack in the radiator cap, which is plastic. Any thoughts on a cheap repair?
new radiator caps can be had for abour $10-$14.
If you’re talking about the actual radiator cap (pictures of which showed metal ones) then you must replace it. If you’re talking about the cap of the coolant reservoir, it’s not critical to repair or replace, if it stays in place OK.
Replace it. It’s not worth the effort to repair it. Plus a repair most likely will not last.