The stock brass radiator cap has a hole in the side for a overflow hose, unsure if they normal had the barb installed but mine did not or it had been broke off at some point. This left a 1/4" hole right on the plate, any pressure that would release from the tank would allow water right in if present. The seal on the cap was also questionable. Beyond that the hold with the cap removed was way to small to inspect or fill.
So I found a replacement cap from these guys, I wanted this one as it was the biggest cap I could find at 2.75" diameter:
https://www.meziere.com/Products/Chassi ... -Bung.aspx
But I messed up my cart and ordered this one instead, slightly smaller at 2.25" but still really nicely made:
https://www.meziere.com/Products/Chassi ... -Bung.aspx
I really considered running a sealed tank given that over half the tank volume is air but got the following for a vent. It's plastic but has a filter in it that lets air in and out but not moisture. It's also covered so shouldn't get junk into the top of it, bummer is it's a off ball metric thread.
https://www.mcmaster.com/5715N16/
So got a flanged metric thread nut for a bung: https://www.mcmaster.com/90374A221/
The old cap is brazed into the cover plate but got that removed, cut some new holes, and welded the bungs in. I did machine a slight shoulder on the main cap to center it in place.
Painted with cap and vent installed, ready to install once I get the tank cleaned out.
