Too bad that the forum software only allows me to like a post oncePut some antiseize on the threads of all the bolts when you go back together. That will make the removal process way easier the next time.
This issue with the non-original T-stat diameter being smaller can be a real pain if you try to reuse the old O-ring on it. They get a bit stretched out with age and are even larger diameter than a new one, and they get firm making things worse. It can even cause it to not seal correctly. I agree with Dan, get a new O-ring along with new gaskets. A new O-ring will still fit loosely on a non-original T-stat (i.e. any of the Stants or similar commonly available replacements), but it will be better than the old one and should go back together without leaking. That being said, if you are OCD like me it may still bother you how these T-stats fit. The correct diameter replacement is available, but is isn't nearly as inexpensive as the others. The non-original T-stat is the same as early VW's, and can be found extremely cheap in any temp desired from any parts source. Also, some time ago I bought a new O-ring from RockAuto, listed for the X1/9, but it was not correct. Therefore it might be best to get the O-ring from a source that knows the difference.Typical replacement stats like the aforementioned Stant are ever so slightly smaller in flange OD as compared to the OEM unit, so they will seem to "swim" in the o-ring, but most people don't seem to notice or mind.
Does this cause any "dissimilar metal" issues?Generally speaking for any/all engines I prefer to replace ALL coolant system related hardware (bolts, washers, loc-washers, studs, etc) with stainless steel. In addition to the use of anti-seize.
It is an odd arrangement. Especially since it uses a good "grommet" rubber seal around the T-stat, but still requires a old fashion paper gasket as you say.What's nuts is that the ring seat is flush