In-head thermostat gasket

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rridge
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In-head thermostat gasket

Post by rridge » Sat Jul 15, 2017 9:39 am

I was recently dry fitting an in-head thermostat to a rebuilt head. My notes from years ago say that a gasket goes on both sides of the thermostat flange. When I did the dry fit, a single gasket took up the available between the coolant outlet casting and the head. I'm thinking a single gasket between the outlet casting and the thermostat flange probably does the total job. I'm also thinking this is a job for RTV. Any thoughts?
Richard
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Re: In-head thermostat gasket

Post by jseabolt » Sat Jul 15, 2017 10:40 am

I would think there should be a gasket on both sides.

I've seen some in-head thermostats use a gasket that "wraps around" the lip of the thermostat like this one.

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Re: In-head thermostat gasket

Post by friedman » Sat Jul 15, 2017 1:12 pm

The gaskets tend to be thick rubber rings and using two won't fit as you noticed. I believe I only use one and can't for the life of me remember if I put it on the top or bottom of the thermostat. I'd keep away from any sealant material that comes out of a tube.

Richard, if you drop by and buy me lunch I'll pull the gooseneck off my spider and see what I'm running.
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Re: In-head thermostat gasket

Post by fp55scca » Mon Jul 17, 2017 11:35 am

rridge wrote:
Sat Jul 15, 2017 9:39 am
I was recently dry fitting an in-head thermostat to a rebuilt head. My notes from years ago say that a gasket goes on both sides of the thermostat flange. I'm also thinking this is a job for RTV. Any thoughts?
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--I believe the correct gasket/configuration for the in-head thermostat is dependent on which T-stat housing is being fitted. There is the single outlet (1438, 1592, and the '74 1756), then there were several outlets that were designed in the T-configuration for the external thermostat. In all cases, if installing an in-head thermostat, I place the thermostat in the head first, then insert the appropriate gasket, then fit the T-stat housing. Here's what works for me:
--Early, single outlet thermostat housing: These housings have a very shallow lip on the base where it meets the head, so a thicker gasket works best for me, rather than trying to fit two gaskets. I use an O-ring style here: Auto Zone part number 25598; or search for an O-ring for a 1970-1992 Audi 4 cylinder.
--The T-configuration outlets, designed with an external thermostat originally, have a deeper lip on the base of the housing, so a single flat style rubber gasket works here. Both the thermostats themselves, and the flat rubber gasket are the same as a small block Chevy (350) thermostat and gasket. Our vendors also sell a version of the flat gasket.
Jim Scurria
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1972 Fiat 124 Spider
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1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: In-head thermostat gasket

Post by rridge » Mon Jul 17, 2017 12:23 pm

Thanks Jim. I like the O-ring suggestion. The attached photo shows another comparison of the two coolant outlet casting styles. The FI casting is original to my car. You can see the corrosion that developed on it due to a leaking gasket. The lower single outlet casting is the one I've run for some years.

I'm still puzzled by the note to myself about using two gaskets, one above and one below the thermostat flange. Many years ago I had a Mazda GLC that developed a thermostat housing leak while still under warranty. The explanation was that there had been a reaction between the metal of the thermostat flange and the aluminum head material. The factory coolant had insufficient corrosion inhibitors to stop it. That made me appreciative of modern stainless steel thermostats.
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Comparison of Coolant Outlets Castings (2).jpg
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Richard
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Re: In-head thermostat gasket

Post by fp55scca » Mon Jul 17, 2017 5:27 pm

--Richard, there was a time when I used two thermostat gaskets, as well as the wrap-around one that James posted, even with the old style single-outlet. Been using the SS "Super-Stant" thermostats, which tend to help with corrosion. The large O-ring has worked particularly well for me with the early style single outlet.

--Another problem I've encountered with both styles is corrosion on the 4 hold-down bolts. I now grease the 4 bolts thoroughly before inserting them into the housing.

--In other instances, I've found replacement bolts that were just slightly longer than originals, causing the bolts to bottom out before the required hold-down pressure was achieved. Sometimes difficult to notice, because it feels like the requisite torque is being achieved, but usually results in a leak. In the past, I have been guilty of using a small amount of RTV, until I discovered the hold=down bolt discrepancy. A few small washers under the heads of the longer bolts usually does the trick.
Jim Scurria
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Re: In-head thermostat gasket

Post by rridge » Sun Jul 23, 2017 1:01 pm

Jim,

Just an update. I discovered that the O-ring you use, Auto Zone part number 25598, is actually a Fel-Pro part number and applications continued through 2010 on the VW Golf, so fresh stock is available.

I ordered the Felpro piece and did a quick test fit with a Stant Superstat, the o-ring and a single outlet casting. With the thermostat in the head, the clearance between the top of the head and the top of the thermostat flange on my head was 6mm. My coolant outlet has a 3mm deep lip. The O-ring was 3.5mm. So those two add to 6.5mm in a 6mm hole and gives positive contact on both sides of the o-ring.

However, the weight of the outlet casting is almost enough to compress the O-ring before even torquing the joint. Also I know that nitrile rubber shrinks with heat and age. So I'm back looking at two flat gaskets 2mm (1/8") each that can be compressed from 4mm to 3mm when the casting is torqued down to the head or a thicker O-ring. What is the flat gasket you show in the picture with the O-ring?
Richard
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Re: In-head thermostat gasket

Post by fp55scca » Sun Jul 23, 2017 8:38 pm

rridge wrote:
Sun Jul 23, 2017 1:01 pm
++With the thermostat in the head, the clearance between the top of the head and the top of the thermostat flange on my head was 6mm. My coolant outlet has a 3mm deep lip. The O-ring was 3.5mm. So those two add to 6.5mm in a 6mm hole and gives positive contact on both sides of the o-ring.
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--Richard, the last question first: the flat gasket is a Fel Pro 35094. The card says "Fel Pro" but that could still be an Auto Zone number. In any case, you can always just ask for the rubber gaskets for a GMC 350.

--A couple of the numbers don't align with my measurements, but first is the depth of the thermostat opening in the head. Mine is about 6mm with nothing in the opening. But the lip on my single thermostat housing outlet is only 1.5mm. I measure a Stant thermostat at roughly 1.25mm, and the O-ring I'm using is 4mm. So, in my case, it's 4mm + 1.25mm + 1.5mm = 6.75mm. So in my case that's 6.75mm in a 6mm hole, and because the O-ring is round, and on top of the thermostat, the lip on the housing spreads the o-ring as well to seal it.

--In your case, with a 4mm O-ring, a 1.5mm Stant stat, and a lip at 3mm, that should be 8.5mm in a 6.5mm hole? What am I missing here?
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: In-head thermostat gasket

Post by rridge » Mon Jul 24, 2017 8:46 pm

Alright Jim, you've made me get out my high end Harbor Freight digital caliper and re-measure all dimensions.
--Richard, the last question first: the flat gasket is a Fel Pro 35094. The card says "Fel Pro" but that could still be an Auto Zone number. In any case, you can always just ask for the rubber gaskets for a GMC 350.

The two gaskets I'm using are both FelPro. The O-ring which measures 3.9mm is p/n25598. The flat gasket is 2.9mm and p/n35094.
Both look like the one's you are holding in your shot.
--A couple of the numbers don't align with my measurements, but first is the depth of the thermostat opening in the head. Mine is about 6mm with nothing in the opening. But the lip on my single thermostat housing outlet is only 1.5mm. I measure a Stant thermostat at roughly 1.25mm, and the O-ring I'm using is 4mm. So, in my case, it's 4mm + 1.25mm + 1.5mm = 6.75mm. So in my case that's 6.75mm in a 6mm hole, and because the O-ring is round, and on top of the thermostat, the lip on the housing spreads the o-ring as well to seal it.
My coolant outlet casting has a 1.8mm deep lip. That measured with it sitting on a glass plate and using a feeler gauge stacked to the point that there is a slight drag on withdrawal. I'll bet you have the same. However, my chamber hole is 7mm empty and about 6mm with the stat. The stat lip varies in thickness due to the ridges. Where the O-ring sits on the outer lip, the thickness is only 0.8mm. The flat gasket sits on the ridge which measures 1.1mm. So the big difference between us is the chamber depth. The head I'm working with is European, a late 132 head. My original head was destroyed by corrosion in the chamber area which is why I'm OCD on the sealing.
--In your case, with a 4mm O-ring, a 1.5mm Stant stat, and a lip at 3mm, that should be 8.5mm in a 6.5mm hole? What am I missing here?
So, with a bit more precision, my buildup is a 7mm hole depth filled by a 1mm stat lip, a 3.9mm O-ring and 1.8mm casting lip for a total of 6.7 mm. Not going to work. On the other hand a 2.9mm flat gasket on either side of the thermostat lip builds to 8.7mm and that's not going to work either. So I back where I started looking for gaskets. Much better to find this out now with a dry build. Appreciate your inputs and cross checks.
Richard
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Re: In-head thermostat gasket

Post by fp55scca » Tue Jul 25, 2017 4:16 pm

--Richard, I see your issue. Also, the fact that we're both using a trusty Harbor Freight digital caliper helps to explain some of the variance in measurements!? These things are just about useless, except for ballpark figures! :lol:

--One possible solution for your setup with the single outlet would be to have a machinist take about 1mm+ off of the housing to render a deeper lip. Maybe a lip of 2.8 - 3.0mm like the T-configured housing. This would permit the use of the o-ring, which I've found to be a far more reliable seal for our Fiat housing. Just a thought.

--When you look at most in-head thermostat housings for automobiles, we see they use both a thermostat gasket, or o-ring, and a thermostat housing gasket. We're trying to solve the whole sealing process with a single gasket. I think this is where the RTV came into use with the single outlet.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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