I have an aluminum coolant tank that came without a cap. [one of the cheap ones off Amazon] The OEM plastic cap leaked [a very tiny bit] and I figured out the top of the tank where the gasket touches had a slight defect. Using dye and a flat plate with 600 grit, got that flat.
I don't know if my cap is a little worn out but as it has only partial threads it doesn't screw on real tight and there's still a tiny leak.

Before I try a new plastic cap does anyone know if a fully threaded billet cap with a relief is available?

I tried the usual suspects but didn't see one unless it was with the complete tank, and with those I couldn't tell if the cap had the pressure relief.
A link would be terrific.
Thanks,
robj

 

I didn't find any when I was swapping out my stock tank. That's why I ended up going with the Canton tank that used a universal cap.

 

So I tried the OEM cap off of a friends car and it worked, no leaks but it seemed like I had to screw it on pretty darn tight, [which my have started the issue with my original cap, ie the treads looking a little funky and sort of stripped.] I'm wondering now if the top sealing surface of the tank isn't exactly perpendicular to the threads

The new cap came and I removed the gasket from my old cap and added it to the new cap ending up with 2 gaskets. It sealed fine and it didn't seem that I had to tighten the cap as much.

I'm going to call it fixed.
robj