That is the old stock with the old rubber gasket so yup! Cowpie!

 

I searched here Search Chevrolet HHR Hatchback Parts

12t103d turns up no results, makes me think it's probably the older design.
131-158 turns up the thermo. But I couldn't find any pics online with the viewing angle that you took, to see if that inner gasket was there.

Now when I searched the ACDelco parts catalog ACDelco Parts , it says the 131-158 is GM original equipment, and the 12t103d is ACDelco Professional.

When I googled your 461-180, all I could seem to find was MotoRad.

 

I Googled 19137281, no picture


Thermostat Asm Eng Cool W Gskt - 19137281 - Cap Stats - GM Parts - by Genuine GM - Thermostat Assembly

 

Live and learn.

From the ACDelco website ( About Us: ACDelco Auto Parts, Auto Repair & History ) "Our GM Original Equipment auto parts are the original parts for GM vehicles and meet or exceed GM OE specifications. ACDelco Professional aftermarket auto parts and products cover most GM vehicles as well as many other makes and models. ACDelco Advantage products offer fit, form and function for most makes and models at a more affordable price. For some auto parts, we also offer a Specialty line." [By implication, their 'Professional' line does not meet the OE specification. In other words, they fit but they're %#$@!]

 

This is my new thermostat. As you can see it's the 131-158 new style.
Look close it still has a small rubber o-ring.
$31.89 from rock auto. GM P/N 12622410
I'll give it a try and see what happens.

 

At NAPA anything with "pro" on it means the same as "contractor" at Home Depot; Cheaper stuff so you can make more profit. A devious marketing ploy.

 

I got a 131-158 off the bay for $27 (shipping included) coming tomorrow. I lost $6 sending back the "pro" piece.



It would have been easier to rig up some louvers in front of the radiator, moved by a control cable mounted in the dash. (Depending on your age, you may remember when mechanical chokes were operated in this manner.)

 

Which way does it sit. I forgot to look when I took mine out.

 

jtfrog!!! Please stop posting the same question in multiple places!!!!

this thread closed.