What Sealant to use on Thermostat Housing?

Platonic Solid · May 19, 2009

I think I messed up on the 351C. I put high temp silicone on both sides of the thermostat-housing gasket and bolted it down (kinda messy, looks like crud). A few hours later (long before the silicone had cured) I added antifreeze. Now I see coolant oozing out right under the bolt heads and the engine hasn't even been started. The threaded holes for the bolts go all the way through to the coolant passage so I'm assuming I should have put some RTV on the bolt threads and I should have waited the full 24 hours curing time before adding coolant.

Looks like I'll have to drain the fluid again, clean up this mess and try again. Should I be using something other than RTV? Maybe something less messy?

iskwezm · May 20, 2009

dont drain it completly,just lower the level under the manifold. When using silicone, less is better.Too much will not flatten out and probably squeeze out and not seal. I use the water neck with a o-ring and light coat of rtv

StadEMS3 · May 20, 2009

I found this stuff at Pep Boys, worked great-

Permatex 22071 Water Pump & Thermostat Housing RTV Silicone

Platonic Solid · May 20, 2009

iskwezm said:
... I use the water neck with a o-ring and light coat of rtv

An O-ring would have been nice, but that still (I assume) leaves you putting RTV on the screw threads, right? Or are the: "screw holes going through to the water passage" an exclusive 351C feature.

Thanks for the Permatex 22071 tip StadEMS3. I will use that.

2+2GT · May 20, 2009

I prefer Permatex non-hardening sealer. It's lacquer based, dries in minutes, and seals coolant or water well.

rbohm · May 20, 2009

i use very a very thin coat of black rtv on both sides of the gasket and bolt the part in place. usually by the time i am ready to add coolant the rtv has set enough to be water tight.

2nd Mustang · May 20, 2009

When it comes to water or oil connections, I use the gasket or O-ring if the case with a very light coat of gasket DRESSING. Yes, there are silicones that are meant to be used in place of a gasket but using the wrong silicone will cause minor slippage which causes the leaks, with or without gaskets.

Gasket dressing are just meant to keep the gasket in place during assembly and make it easier to remove item later.

blown65 · May 20, 2009

I use that thick black sealant. Forgot the name but it really doesn't dry. I know GM uses it on their stuff, but it works great. PITA to get out of the tube, but you don't need very much of it.

Platonic Solid · May 20, 2009

After looking at the Permatex website, I think their High Temperature Anaerobic Flange Sealant may be the best choice for use as a gasket dressing and bolt thread sealant.

Edit: Maybe the Permatex Ultra Rubber Gasket Sealant & Dressing would be a better choice since it doesn't sound like quite such a pain to remove at a later date.

My initial silicone assembly would have been fine were it not for the retarded 351C design that has the threaded mounting holes in the block going through to the coolant passage (which is why it leaks out between the bolt head and the top of the thermostat housing). This would have leaked even if I had waited the 24hrs for the silicone to cure. I assume, since no one has commented on this aspect, that most other engines don't have this design deficiency. Now the real pain will be getting the threaded bolt holes in the block clean enough to provide an adequate bonding surface for the sealant that I apparently must apply to the bolt threads.

afast93stang · May 20, 2009

I use something called The Right Stuff. Its made by Permatex and comes in a Cheese-Wiz style can.

What Sealant to use on Thermostat Housing?

Similar threads

Privacy & Transparency

Privacy & Transparency