Expansion tank leakage

[Strife]

I did some fairly major work by my standards (water pump, thermostat,belts, hoses, timing stuff), and I noticed that my temp (86 560SL) is about 5 degrees C higher than it used to be - 85 vs the former 80. I think it's possible that this is tolerance of the thermostat itself (like, +/- 5-10%?). In the meantime, I noticed that my overflow tank was leaking around the neck and making noise. I got a replacement cap recently because the old one was probably 19 years old. When I put the new one on, I noticed that the metal flange around the expansion tank neck was loose. Thinking that any leak is not good, and seeing no practical way to fix the neck, I bought a new OEM MB expansion tank, and installed it. Guess what? The new one leaks around the neck too - but not nearly as much - just a few drops and a little hiss). It is not loose.

Is this normal?

Do you think the cap could be defective (not releiving pressure?). I find this hard to beleive because you would think that a cap would be faulty in the opposite way.