Hey all:
I got a frustrating problem that won't go away.
I changed the antifreeze last week and took off the water outlet/thermostat housing to
add coolant to get most of the air out. In the process to replace the housing I got a gasket (Fel-Pro 35271) and cleaned off the housing mating surfaces and installed gasket and tightened. Started getting seepage right away. So waited till it cooled down and slathered some Permatex clear RTV silicone adhesive sealant (p/n 66B) around where the 2 parts join and that slowed it down somewhat.
This time I used the Fel-Pro gasket and applied the RTV sealant to the top side of the gasket and tightened it up. Still a little bit of seepage.
Then I got 2 Fel-Pro gaskets, applied RTV silicone on the top gasket and in between the gaskets. Tightened it up. Still seepage.
Now I went and got a ROL gasket (P/N WO8357-001) and applied Permatex Water Pump & Thermostat Housing RTV silicone Gasket (comes in a small 1/2 ounce tube) and applied that to both sides of the gasket and tightened it up. Small seepage again.
I'm at my wits end. What am I doing wrong?
I checked the top movable part of the housing (top) and checked it for straight and it is.
I checked the bottom surface (water pump side) and that is straight.
There are no gashes on the surfaces, although the top housing looks a tad on the rough side but nothing serious.
I always wait at least 24 to 48 hours for curing of the RTVs before operating the truck.
I always clean both surfaces before applying sealant and gasket.
Any ideas will be appreciated.

Hey all:
I got a frustrating problem that won't go away.
I changed the antifreeze last week and took off the water outlet/thermostat housing to
add coolant to get most of the air out. In the process to replace the housing I got a gasket (Fel-Pro 35271) and cleaned off the housing mating surfaces and installed gasket and tightened. Started getting seepage right away. So waited till it cooled down and slathered some Permatex clear RTV silicone adhesive sealant (p/n 66B) around where the 2 parts join and that slowed it down somewhat.
This time I used the Fel-Pro gasket and applied the RTV sealant to the top side of the gasket and tightened it up. Still a little bit of seepage.
Then I got 2 Fel-Pro gaskets, applied RTV silicone on the top gasket and in between the gaskets. Tightened it up. Still seepage.
Now I went and got a ROL gasket (P/N WO8357-001) and applied Permatex Water Pump & Thermostat Housing RTV silicone Gasket (comes in a small 1/2 ounce tube) and applied that to both sides of the gasket and tightened it up. Small seepage again.
I'm at my wits end. What am I doing wrong?
I checked the top movable part of the housing (top) and checked it for straight and it is.
I checked the bottom surface (water pump side) and that is straight.
There are no gashes on the surfaces, although the top housing looks a tad on the rough side but nothing serious.
I always wait at least 24 to 48 hours for curing of the RTVs before operating the truck.
I always clean both surfaces before applying sealant and gasket.
Any ideas will be appreciated.

Are you talking about the thermostat housing or the coolant crossover? With the thermostat housing I always take a piece of sticky back sand paper and stick it to a machined surface, then I use this to redress the gasket surface of the housing. The two bolt flange on these seems to alway deform the housing. I always use 100% dap silicon on all engines that don't have a oxygen sensor. YOU CAN'T USE IT ON ENGINES WITH A OXYGEN SENSOR. All of the automotive silicons have the silicon removed since it will screw up the oxygen sensors. unfortunately this also makes them not work very well. I used to build DT466 motors for unlimited class tractor pullers, they use compression rings on the sleeves and silicon for the head gasket. DAP brand was the one that would hold the longest.
The key to sealing any gasket that has the bolts on the outside edges is to not use too thick of a gasket and to not over tighten the bolts. As the bolts tighten down to far they compress the gasket at the outside and cause the center of the surface to arc up. That is the reason for the head bolt tightening patern, it always starts in the center. I hope this helps.

If your t-stat housing is like mine, at least one of the bolt ends is exposed at the bottom, I had to run a tap through mine to let the bolts thread all the way down and clamp the gasket. The threads were corroded a little, and the bolt would get tight but not clamp the gasket well enough.

put a straight edge on the block/crossover and see if it is warped badly,do the same with the t-stat cover.And also check the small recess where the t-stat sits and make sure it is cleaned out. Other than that I would get a new cover.

At this point I'm going to try Radrick's suggestion with the DAP 100% silicone.
It that don't work then I'm going with zman17's idea and get a new cover. Those suckers are expensive. Last time I checked they went for $70 ea.

I'll not quite sure what you mean, but a will ask a silly question.

Is the thermostat gasket mounted on the thermostat before install?

I was confussed when you said "used two gaskets".

The Tstat should go in dry, no sealants. Even with a little corrosion in the housing, it should still seal unless its not assembled right.

If you take apart, can you post a picture?

Hey jmiller:
I meant the thermostat housing or what's otherwise called the water outlet (where the top radiator hose goes to.
The thermostat is installed first and then the gasket goes on.
There is supposed to be only one gasket used, but in trying to solve the seepage problem
I used 2 gasket instead of 1.
The only place where I use the sealant is on the gasket surface not the thermostat.
Sorry I can't post any pictures. Camera is out of order, looking for another one.
I've tried varying the torque on the bolts to keep from leaking to no success.
I don't know what the bolts are supposed to be torqued to.
I looked in the LMC catalog and they want almost $100 for a new housing cover.

Your torque should be 31 FT/LBS.

Your torque should be 31 FT/LBS.

This is an important point. A lot of leaks are caused by tightening too much.

Rob ;)

yep. Tighter is not always better.

This is an important point. A lot of leaks are caused by tightening too much.

Rob ;)

x2 That was my point exactly. If you use silicon you should still use the gasket and just spread a very thin coat of the silicon over it. You have to be very carefully not to end up with excess pushing out and interfering with the t-stat. My main point was that you shouldn't over tighten the bolts. the backup plan is to use your finger to spread a VERY thin layer of silicon on both sides of the gasket. Make it as thin as you can.

Hey Radrick:
That's what I'm going to try today. Keeping my fingers crossed.
Thanks jbsaxman for the torque figure.

Success!
I think the 100% silicone sealant on both sides of the gasket and the correct torque
figure (31 ft./lbs) for the 2 bolts on the housing did the job.
Thanks to Radrick and jbsaxman.