The situation with the domes is like this;
The center section and domes are made with the correct sized groves and correct sized o-rings.
However, due to the size, profile, material properties and design used
some domes can expand. They dont all expand, they dont all expand to same extent (which is why different domes/o-ring combos fit on different units). The instability of the dome is inherent in the material and the fact it is a larger thin walled machined part (rather than a (far more expensive to make in small batches) molded fiber reinforded part)
The domes that expand can see their diameter increasing which can if left unfixed eventually cause loss of sealing with the original o-ring when it reaches a certain size.
Solution:
A simple solution is to fit a bigger o-ring. This works for some (depending on degree of expansion and if it stops increasing) .
Theres some speculation that the pressure of the o-ring over prolonged time will speed up or enhance the deformation (which is why I dont store my unit with o-rings installed)
However with just changing the o-ring the dome is still 'free' to continue to expand if its nature is such. In many cases the dome doesnt but in some it continues to distort so that even the large o-ring eventualy loses its seal. To gaurantee no issues ever with expansion (of the original design domes) we ideally need a way to fix the outer diameter of the dome to prevent any or all expansion.
Better Solution :
The stainless steel band clamps provided by the factory fix the dome outer dimension and prevents change in dome diameter -
it can even be used (within limits) to 'pull back' an expanded dome to correct size*
Fit the band, fit the original (or larger) o-ring, adjust the band to give the correct seal and forget about it. The best thing with the band is you can adjust it from time to time to make sure seal is always perfect. Fit it and forget it.
With the band, deforming/leaking domes is a non issue. Best/future solution:
I believe (please correct me if im wrong) that the factory has been using a new design of dome for a while with thicker rim to help prevent distortion issues?
I never got around to it but a nice neat solution would be to make up a stainless steel ring the height of the dome ring heat it up and press it over a chilled dome rim rather like the way they used to put rims on barrels. Does the same thing as the stainless bands but looks nicer
This should be an easy thing for factory to add to new domes (with circular outer rims)
*to ascertain the correct size what I do is just tighten the band so that I have to push the dome on and off with a reasonable amount of difficulty. Mine is adjusted so I can only just suck the dome down with a negative and blow it off with a +ve .