Perkins 4108 Thermostat Replacement

I have a Perkins 4-108 Diesel Engine on my 35’ sloop and I want to test and/or replace the thermostat. There is a Perkins cast metal Surge Tank mounted on the top front of the engine running from left to right, with a radiator-type fill cap mounted on top and with only 1 hose going out of the right back of the Tank into the Exhaust Manifold.

The Thermostat is apparently located under the surge tank. No problem in getting the Surge Tank off since it is held in place by a threaded rod coming up through its top capped with a nut. But when I begin to lift the tank slightly, engine coolant begins to seep out between the bottom of the Surge Tank and the casting underneath. I can and will remove all the engine anti-freeze coolant before I continue any further but my real question is:

1. Is it possible that the metal bottom Surge tank is acting, by itself, as a gasket on the top of the thermostat? This seems unlikely since in any typical engine I’m familiar with, any thermostat is sealed in its housing with a gasket and screwed on flange of some sort.

2. Does the Thermostat sit under this Surge Tank thus requiring its removal to test and/or replace the thermostat?

3. Is the Thermostat mounted vertically under this tank and does it have a gasket and screwed down flange covering it?

Appreciate any info and help you can provide. Thanks

Delphi@iwlink.com