OP- E-mail me the MFG code and part number of the items.

i.e, the stat should have a CTH and then a part number, the gasket will either have the same CTH mfg code or GSK.

You'll be fine with the gasket. I've used both the GSK and the CTH, the CTH is half the cost of the GSK and IMHO works fine. Be sure to "clock" it so the "jiggle" pin on the stat is free and not touching the gasket. Last, when you install the stat the "pin" needs to be straight up - 12 o'clock.

 

And after another look at the pic's....the gasket on the old one is 180 off. That "hump" or "cutout" on the face should be towards the "pin"....but as you can tell - it's been working and flowing fine, it is not a deal breaker.

Having the gasket "clocked" right and the "pin" at 12 o'clock is all about the bleeding of the air from the system with ease.

 

Yes I suppose I'd consider my location temperate seacoast climate (SE Alaska), relatively mild, not too much in the way of extreme cold, never sub-zero. Coldest snaps in the dead wintertime are in the low teens for maybe a few weeks in a row. With the 180F stat that's been in there, I suppose I appreciated the extra uumph then. Hope I don't miss it too much with the 170F stat. Thanks OhOh.

 

Thanks poorman212. For the record I feel compelled to mention that I recall it was an auto repair shop "professional" who did the installation of that stat and gasket some years back. I'd think he should've known better. But I guess, as you mentioned, it seemed to be working/flowing fine as far as I could tell, since then. Although I do know several times I've seem to have struggled with trying to bleed all the air out of the system, despite following procedure.

 

The little vent (rattle pin) is brass-color in your photos. It's about 11-o-clock position in the first photo.

Seems like a Honda dealer T-stat has the pin located closer to the edge, requiring the cut-out in the gasket.

All in all, the requirements are that the little rattle-pin vent has to be free to move and located to the top. The T-stat housing has a little recess because the factory T-stat vent is closer to the edge of the T-stat. And the little bumps in the factory gasket are simply to "help" the mechanic to install it in the proper orientation.

The factory T-stat opening is a bit larger, and the bar across the middle goes out closer to the edge. So the 2 wide cutouts in the gasket at 2-oclock & 8-oclock are needed for that bar.

If I would guess, I think you have an aftermarket parts-store thermostat with a factory gasket.

 

Okay, sincere thanks everyone for all the helpful info/input here. I'm not near as clueless as before (about this anyway... )