I'm having an unbelievably tough time getting my 1970 440 thermostat housing to seal I've installed 2 different aluminum housings (one brand new repop from Frank Badalsen) a different pump housing using several different sealers (high-tac , rtv , silicone) I'm almost ready to try "J B Weld" or just arc weld it After 30+ years of working on these cars , I'm stumped! ANY help from our great Mopar forum will be much appreciated THANK YOU!

I'm having the same problem on my 69 GTX. Even tried an original off a 69 block and still have leaks.

One or both of your parts are possibly not flat ?

I have a new mopar cast iron outlet if you need one. Mike

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yes and ???

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Maybe it will work better than the aluminum one. Is that ok with you? Just checking.

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oops , I miised the IF YOU NEED one , that's I assumed this was ... a for sale post ...

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So far what help have you offered?

John it is a shame I had to force you into helping out. I know it is easier for you just to throw out the smart a.. comments.

I had the same problem. I started with the MP chrome neck - mistake. It warps very eaisly. I got a steel water neck, sanded it flat on a belt sander and installed with the hardening type permatex as insurance. Problem solved.

The iron, heavy aluminum and steel water necks seem to work fine if all parts are flat. One key: don't overtorque. It can warp the flange and cause a leak.

I have a 67 coronet R/T convert that has the same problem.I tried different factory housings and steel replacements and it still leaks.I checked all surfaces with a straight edge,tried different gasketing material still leaked.What I was thinking of doing was having one of my aluminum necks machined to accept a rubber o ring sealed thermostat like most of the newer cars use.

I'm still having problems with my Valiant. RT66Jim (member here) asked if maybe I'm having a problem with the radiator cap.

My problem is just sitting there it will seal. For 2 weeks with the cap on. I'll fire it up for the first time after putting in the thermostat housing back on and let it fun...no leak. Come out the next day and it's leaking. Once it starts leaking nothing that I do will stop it unless I take it back off. Take it off put another gasket, maybe another thermostat housing and it will do it again.

Well, after Jim said something about the radiator cap....after the first run of the motor getting up to operating temp....I'll let the motor cool down crack the cap and it hasn't leaked since.

There is still a problem.....for me. I shouldn't have to do that. But it's building too much pressure after I run for awhile and the cap isn't able to get rid of it I guess.

Anyways I'm saying this for maybe food for thought.........

Good luck, let us know some of the things that your try please?

Just my $0.02:

Flat. Smooth. Clean. Has to be these.

Flat...the opposing surfaces are in contact at every point (or reasonably so). Excludes: warped, curvy, wavy, distorted by excessive torquing of fasteners, etc. Cast iron resists all of these better than aluminum or steel plate. Check in many directions with a true straight edge and 0.001" "feeler" gauge. Or against a surface plate. Or using machinists layout blue. Or even maybe using an extremely thin film of light oil.

Smooth...no nicks, scratches, gouges, pitting, etc. that connect the coolant with outside. The pressure of the hot coolant is constant when the engine is running. Even a tiny channel will eventually give the coolant a way to get through. Gaskets, sealers, etc can only do so much if the parts don't fit or aren't smooth and clean. Yes; epoxy it on and it probably won't leak, although if they are mismatched, even epoxy might give up after a while. Things that don't fit will vibrate at different frequencies and in different modes when clamped together.

Clean. If its smooth it should be clean.

oops forgot one: if its cracked somewhere, it may not be leaking at the joint. tiny crack can seep and be real hard to find...

Sorry for the long...hope it helps...good luck, I know it can be frustrating.

I too had that problem. I don't have any fancy tools, so I purchased some thin cork gasket material from NAPA. Made a thermostat housing gasket. Put a little Permatex on it. Never a leak since.

Can a cast iron housing be mild

Make sure both housings are flat first and then if not make them flat before any thing else
There always reasons they leak, finding and fixing them is the hard part

Just an FYI: we have had issues with the one sided self stick gaskets that a lot of parts stores are selling. They start leaking after install. Another source is to make sure the hose surface on the housing and ID are both clean. I have had more than one hose leak make me think it was the gasket. Lastly a good idea to retighten the clamps (worm gear) every so often until they take a "set" .

Like many I have tried all the tricks trying to get a stock housing (both used and new) to seal including having a machine shop check and mill both surfaces.

Some attempts lasted longer than others but they all eventually leaked.

Thankfully the car is not a stock type vehicle so I spend a little extra $$ and bought this billet aluminum O ringed housing and have never had a leak issue since (4 years now)

I've had the housing off a few times and it has been leak free every time even with the same O ring and never use any sealant.

Somebody should make a stock looking T stat housing with an O ring.

https://www.summitracing.com/parts/bsp-90820

must be flat. clean with brake parts cleaner . apply this --> https://www.permatex.com/products/g...kers/permatex-optimum-grey-gasket-maker/ lightly tighten the bolts when still wet , wait 30 minutes to tighten it down. you could probably lift the engine without the bolts in it.