Not sure if that is the correct name for this part? But the thermostat is pulled out on certain aftermaket turbos that are only oil cooled and a lot of people will braze the hole shut,But if you want to keep the thermostat you can just plug it and save some room this way
this is a 30mm 1.5 mm pitch and this plug came with a new washer for the price of 9.80 Cdn
I just welded a 3/8" collar on for later use