New Member
Jul 26, 2009
I thought I would share a recent experience with the lowly thermostat housing. I wanted a new one and these are easy to acquire aftermarket for most any 289/302/351W from the 60s to the 90s. After installing with the paper gasket and some Form-a-Gasket No. 2, it promptly blew out the gasket near the bypass hose connection. After a few clues from other forums, I concluded there might be a mismatch in OEM vs aftermarket housings.

It was not the fact that I had an aftermarket intake manifold (Edelbrock Performer 289). See attached pic from tracing the mounting surfaces and superimposing - really no difference. Some argue that Edelbrock did not cast a good replica of stock, but this doesn't seem to be the case.
Water Outlet FoMoCo vs Edelbrock.jpg

So let's study the housing itself. I learned, after opening boxes at NPD, O'Reilly, NAPA, Autozone, Pep Boys, Advance Auto, CarQuest, they are all identical (a common part number out there is 84831). They may be branded Four Seasons, Murray, Dorman, NAPA, etc. but they are the same casting manufacturer. Who knows where these are sourced and then rebranded. Note that this unit has a threaded port for temp or vacuum switch (other aftermarket housings without the port do not seem to be a problem).

I got my hands on a FoMoCo C9OZ-8592-A housing (ported also). Note that there are a ton of others that are practically the same from C3OZ-8592-X to C8OZ-8592-X, except the older ones appear to be cast iron and later ones are cast aluminum. I traced the configuration of the thermostat opening, the mounting holes, and the bypass hose opening. The aftermarket units have bypass hose openings about 5/32 outboard of the FoMoCo unit. The mating surface on the intake manifold tracks with the dashed line so the aftermarket housing has that much less contact area to seal against cooling system pressure. See attached pic (the dashed line is FoMoCo) .
Aftermarket vs FoMoCo C9OZ-8592-A.jpg
So lesson with these is find NOS instead of aftermarket for this application. Configurations ARE different!
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Adding a couple pics of a FoMoCo housing and the common aftermarket housing...


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    C9OZ-8592-A Water Outlet a.JPG
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  • Aftermarket housing.jpg
    Aftermarket housing.jpg
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I have used the chrome after market ones with out any problems .Paper gasket and silicone and no leaks .If you tighten one side down all the way and then tighten the other side down it will leak ,they must be tightened on both sides the same time or they will warp .And do not over tighten .