Had too cool of a thermostat (160) pulled it and replaced with a 180 and now the housing is leaking. Used new gasket, thin film of thermostat housing sealer, torqued bolts to spec and it's leaking. What did I do wrong and how can I fix?

what engine?

What housing?

340 stock

Is it the right stat.There are 2 one large one small.Is it leaking or seeping.Big difference.The surfaces must be real clean.I use a new gasket and a thin film of brown Permatex.Never had a leak and I did a bunch at home and the shop.Rocky

It's the large one and it's just like the one that came out except it's a 180 instead of a 160. Was really clean and dry. It's seeping out the back middle of the housing trickling down the passenger side of intake and accumulating in the front right bolt crevice. I'm going to pull it back off and use indian head shellac this time.

Check the housing.They can crack pretty easy at times.That and Ive seen whee the stat shifts a bit and keeps it from sealing.Rocky

I would run the housing over a flat file...they can tend to warp, etc.

I've had more trouble with leaking stat housings than anything else I've ever dealt with. Get a new WIDE gasket. Wouldn't hurt to drawfile the housing and the flange it sets on (put a rag inside to catch the shavings). with it installed you should be able to reach in the neck & grab the "arch" & move it a bit which tells you the stat is in the recess tho I dont see that as an issue here but I do check that. On one I had to use "the right stuff" ON a gasket to get it to seal & that is a gasket maker itself in a can, just a real light coat to get a good seal. Still ain't figured out why that piece is so problematic as you'd think it'd be the exact opposite considering its nature. I think using or cutting a new wide gasket is the key & some cement of course

Run a draw file across the housing to see if it's flat as well. If you need a replacement, stay away from chromed ones, they don't last.

Like many others I fought that issue too many times. I went to a Billet Specialties housing with an O-ring. They have a nice thick base to prevent warping. I don't use any sealer and I haven't had any leaks. I've had three of them on three different race cars. If you don't have to have a stock looking housing, these are great.

terrysautolab.com

That's a good housing.. I've used the O ring style and have found that over time even those will weep from expansion/contraction..Now if I use that style I add a little RVT to help the sealing duties ...

Put a NOS factory housing or the cast iron one from NAPA and forget all the fancy stuff.

I used the NAPA cast iron one & the width of the flange was thinner than I woulda liked but it did seal with a very thin/even coat of "the right stuff" gasket maker on a cut (wider than OE) piece of gasket material (+ drawfiling both). the nipple has no raised hose barb so I ground one as this is a high psi app & its thick enough there to easily do so. EDIT I think the narrow flanges and slight warpage is the problem cause. If one of those chrome ones has a wide flange (& the barb) I'd like to try one & sand off the chrome/get it flat

Appreciate all the input and suggestions. Thing is, it sealed before not sure why it wouldn't again if I can get it done right. I've reinstalled the oem one and used Indian head shellac. We'll see. I do wonder how long I should let it dry?

FWIW. My 69 GTX 440 stat housing leaked when I first got it. The housing bolts were too long and bottoming out before they tightened.

Yes,also make sure that the bolts holes are cleaned out and not full of old sealer, corrosion etc. that would keep the bolts from going down tight.

[/quote] That's a good housing.. I've used the O ring style and have found that over time even those will weep from expansion/contraction..Now if I use that style I add a little RVT to help the sealing duties ... [/quote]

I could see that happening but mine are on race cars and they don't get the amount of running time that a street car would have. I know my first one has to be about 8-10 years old and is still holding. I can see where the o-ring could get hard and brittle. They sell replacement o-rings. Have you ever tried that?

Did you check to make sure it was flat or did you just ignore that suggestion?

They do warp over time and you can tighten the bolts all you want, if it's cupped it will not seal.

Why did it seal before ?? thicker gasket, more sealer that was allowed to fully cure before coolant being added again ?

They do get warped and/or pitted. Also, make sure the t-stat fits in the recesses of the thermostat housing (not too wide). Chrome housings have a tendency to leak too - sand the chrome off on the gasket sealing surface.