O-ring under the radiator cap housing

[NHMax]

Mine appears to have sprung a leak

Was it always not round?
A normal round o-ring work?

 

[MaxMidnight]

Are referring to #47 on the parts fisch?

 

[NHMax]

Are referring to #47 on the parts fisch?

No #45

 

[one2dmax]

The plastic caps on the t-stat housings do get distorted and even a new oring only may not resolve it. If that is where your leak is coming from. Another common leak area is the drain assembly.