What Sealant to use on Thermostat Housing?

Platonic Solid

Founding Member
May 29, 2002
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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?
 
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... 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.
 
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.
 
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. :nice:
 
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.