So this happened!
The old rad cap all but disintegrated and gave us a real start yesterday... we've found the exact same cap that was on there, at Canadian Tire, which leads me to believe that it isn't the right one for the job. You can also see that the threads on the neck aren't all there anymore. So... what's the course, for both the cap and the neck? I've seen a replacement part# for them both but
Westerbeke seems to have stopped making them a long while ago.