1979 Ford 351 Windsor- Sealing the Thermostat Housing
 

Last fall, I replaced the thermostat housing and thermostat. I used a traditional gasket and RTV for where the housing bolts to the manifold. Shortly after, I discovered a very small leak. I ran it like that all summer, knowing I would fix it this winter. This past weekend, I took apart the thermostat housing and installed a new housing, using only a gasket and Edlebrock gasket glue/ sealer. Needless to say, it did not pan out. When I filled the cooing system, coolant was running out of the bottom of the thermostat housing. Does anyone have any tricks or tips to sealing the thermostat housing? Should I use just RTV, RTV & gasket, or something different? Also, do the thermostat housing bolts have a torque spec or just tighten them down? Thanks for your help.

Are you sure this is a 351W? Unless your 1979 is an Econoline, the 351W would have been swapped in, as it was not available in F150/350 & Bronco until 1981.

NumberDummy- I am sure it is a 1979 351 Windsor. It actually sits in a 1971 Ford F100 Shortbed but as you pointed out, it came out of an Econoline.

When it comes to stuff like that it always seems to come down to preparation. Whenever I get impatient or half-ass things it's always Leak City. Clean all the old gasket material away with a scraper, and any sealant. Acetone or naptha, carb cleaner etc, on a clean rag will remove any oil or grease on the surfaces. Clean clean clean.

A good RTV ("Right Stuff" or Permatex Copper) will sub or replace for gasket material, but everything needs to be clean and flat. Torque for 1/2" bolts will be about 35 ft/lbs. While supposedly it can be fired up right away or within a couple hours I try to wait 24 hours before filling with coolant to let it cure fully.

Is the stat cocked off to one side keeping the housing from sealing. Many a housings have been broken that way.

The two things in my experience that cause more than a trickle are forgetting to take the sticker off of the gasket and as GLR points out, that the t-stat moves ever so slightly out of its recess and doesn't let the housing sit square on the engine.

I always contact cement the T-stat housing gasket surface and the T stat gasket side that mates to the housing.
Let the contact cement tack up, install the t stat in to the housing and put the gasket on the housing. Make sure both the housing and intake are cleaned of all old gasket material and wipe down with some lacquer thinner or brake clean before using the contact cement.

Once the gasket is glued to the housing the T-stat will stay in place and no worries about it being misaligned when installed. Then install it on to the intake, use no sealant on the intake side of the gasket. I've never had a leak doing it this way.

If the intake surface is rough or pitted form corrosion use a skim and I mean a skim not a bead of RTV sealer on the intake side of the gasket.

With no sealer on the intake side clean up prep next time you need to swap it out is snap. The housing can be cleaned up quick on the wire wheel.

You should avoid using RTV on cooling system gaskets any way, and if you do use it, do so very sparingly. I can't tell you how many cooling systems I have seen loaded with chunks of RTV plugging up rads and heater cores.

As already mentioned, be sure the housing is not distorted. Place it on a flat surface and see if it touches throughout the mating surface. If not, either "mill" it or replace it.

Update- Fixed!
 

I finally got the housing sealed properly on the third try. In the end, I super-glued the thermostat in place inside the housing and used a good thick bead of RTV, no gasket. Thanks for all the suggestions.