[AuburnCXxxx]

Making this post to leave what I found that works. I had difficulty searching this up, so this is here to make it easier for others.

I went to my local auto parts store and bought a cap meant for a 1989 Honda Accord, and it was the perfect fit for my CX650C's Radiator. $8 to replace.

 

[bahn88]

Thanks for posting....Theres plenty of alternatives for the cap...its basically a car part for 13psi...for USA based members stant thermostats are an option vs Oem..
Stant #29958

 

[Randall-in-Mpls]

Used on virtually every Honda passenger car in the 70s and 80s.
Thanks to Honda for using off-the-shelf parts in this cooling system.

 

[Sidecar Bob]

Not just Hondas but the majority of cars and light trucks until they started making them with small rad necks (some time in the lat '90s IIRC)