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The lower radiator hose enters the engine at a housing with two bolts. After you've drained the radiator, remove those two bolts and there is the t-stat. Get a Subaru part as parts store varieties are often incorrect. Also get the gasket. The t-stat will have a small floating pin in the flange above the stopper. This pin has to be located above the stopper when you put the t-stat in. Clean the lip on the housing that the gasket sits in, install the t-stat (confirm that the t-sta is installed in the direction of flow, it is marked) and bolt it back in with the two bolts. Re-connect the radiator hose and re-fill the system with coolant.

Todd