The NAPA THM 155 thermostat fits the 261 bypass housing as reported earlier, but I had the same question that Mike B raises above about how the bypass is closed off when the thermostat opens so that all coolant can flow to the radiator.

The thermostat fits, but looks different than the slotted ones that both Deve and Tim have shown.

The THM 155 comes in three pieces as shown in this picture--a gasket, a thermostat and a base.

http://s844.photobucket.com/user/ma...thermostat/IMG_1373_zpsyfobn6ez.jpg.html

One of the pieces looks like a 'regular' 235 themostat and even measures 2 1/8" as seen in this picture.

http://s844.photobucket.com/user/ma...thermostat/IMG_1372_zpsdfeb2kio.jpg.html

The brass colored base has raised ears on both sides that the spring loaded thermostat fits into. The ears create slots on both sides that allow coolant to flow through the bypass when the thermostat is closed. The thermostat fits tightly between the ears. Here are a couple pictures that show the ears and the slots.

http://s844.photobucket.com/user/ma...thermostat/IMG_1374_zpslm0bpv5b.jpg.html

http://s844.photobucket.com/user/ma...thermostat/IMG_1375_zps5f1ea8yu.jpg.html

I did the boiling water test. When the thermostat opens, the slots remain open so some coolant will go directly to the water pump through the bypass and not to the radiator. Contrast this Tim's thermostat which looks like it has a sleeve that will close off the slots so that all coolant will go to the radiator when open. It will be interesting to hear when he confirms this.

Even though the THM 155 fits and allows for the bypass when the thermostat is closed, it appears to have a design flaw because it does not close off the flow to the bypass when the thermostat is open.

Matt


Last edited by Maybellene; 03/16/2016 4:06 PM.