Farmall M Thermostat

farmall pete · Feb 22, 2021

Photos of a replacement thermostat on an M (actually an MD, but same configuration). The tractor did not have a thermostat in it so I bought this new one. It doesn't look like it will function correctly when installed, as the neck of the thermostat seats into the outlet for the bypass (see 3rd photo). So when the thermostat is closed, the bypass will not function correctly. (The bypass is integral to the thermostat housing, in the long narrow slot alongside the main round coolant passage)

My understanding is the original thermostats had holes in the flange around the spring to allow some coolant by when the thermostat is closed.

Also, the "button," which normally faces the hot side of the thermostat appears to be on the wrong side, facing the radiator.

Anyone have experience with these replacement thermostats? Thanks!

rustred · Feb 22, 2021

first of all, your second picture is the top of the thermostat. i am not quite following all these pictures you did not show which way you
think it goes. that long section is the bottom and goes down. for some reason i thought there was another rubber piece that the stat fits
in. that slot is the bypass. no holes were in the stat as far as i know.

farmall pete · Feb 22, 2021

Hi Rustred, that's the problem. The thermostat will only go in one way, with the long section facing up (toward the radiator) to mate with the round receiver in the top of the housing. The original thermostats installed this way too, but I think the button was oriented opposite of the new one.

pete 23 · Feb 22, 2021

That is the correct thermostat for that tractor. I can't tell from here but
it looks like it is long enough to go up into that machined hole in the upper
housing which it must to work. I never saw one too short but you never know
now days.

As far as the bypass, coolant goes right through the center of that
thermostat and then into that passage along side you described. Therefore the
sensing button will always be exposed to the coolant no problem.

The thing is with those rusty old housings is that coolant can go right on
past the outer side of the tube of thermostat so they don't open the
thermostat for coolant to flow to radiator.

They finally got smart later in life on the diesels and changed the housing
some and added a seal that the tube had to enter . I have had a housing
machined and a seal installed on one I worked on but not all housings will
accept a seal.

It worked out good on that one. My own gas tractors I could not do it so I
did some jury rigging to block that flow.

Realize, that tube has to move when thermostat opens so tube has to be free
to move up into housing, just that if it is too loose coolant goes past the
outside.

farmall pete · Feb 22, 2021

Hi Pete, thanks so much! I did not understand that the tube moves upward as the thermostat opens. But I tested it in hot water and indeed that is the case.

If I understand correctly, the tube is not seated into the bypass hole when the engine is cold, so coolant can flow through the bypass. When the engine warms up, the thermostat opens and the tube raises into the bypass hole and seals the bypass, so coolant then runs through the thermostat and into the main coolant passage. It may not seal completely because everything is corroded, but that is the general idea.

farmall pete · Feb 22, 2021

On second thought, I guess I don't understand it correctly. I took a measurement, and the thermostat tube is 1/16" into the circular bypass hole in the casting when the engine is cold. I realize it will leak around the edges because the hole is corroded. If I put my mouth over the tube end of the thermostat and blow, there is no way for air to get out. So how can the bypass work when the thermostat is closed?

pete 23 · Feb 22, 2021

OH boy. I'm looking at your pictures and I figured those hole went right on
th rough. Been many years since I had one in my hand. Don't really know what
to tell you when you don't have an old one to go by.

Of course, the original style was made different but I have put in several
new ones over the years.

IH had a mixup back in the 80's where they were sending out regular
automotive style thermostats but were quick to have a recall on those as they
blocked the bypass completely. They warmed up fast but with no bypass
circulating a lot of hot spots can develop before circulation took place.

DanMD · Feb 23, 2021

(quoted from post at 23:16:45 02/22/21) OH boy. I'm looking at your pictures and I figured those hole went right on
th rough. Been many years since I had one in my hand. Don't really know what
to tell you when you don't have an old one to go by.

Of course, the original style was made different but I have put in several
new ones over the years.

IH had a mixup back in the 80's where they were sending out regular
automotive style thermostats but were quick to have a recall on those as they
blocked the bypass completely. They warmed up fast but with no bypass
circulating a lot of hot spots can develop before circulation took place.

I installed the thermostat pictured in this thread when I got my SMD. My tractor will warm up at an idle in 10 minutes to operating temperature and stays there.

Is this some kind of indication the thermostat is blocking the bypass? If so, is it harming anything?

farmall pete · Feb 23, 2021

Well, this is a puzzle. I'm going to pull an old thermostat out of a parts tractor and compare the two. Hopefully that will get to the bottom of this, and I'll follow up with some photos on here.

J.ReinkeFarms · Feb 24, 2021

(quoted from post at 18:16:45 02/22/21) OH boy. I'm looking at your pictures and I figured those hole went right on
th rough. Been many years since I had one in my hand. Don't really know what
to tell you when you don't have an old one to go by.

Of course, the original style was made different but I have put in several
new ones over the years.

IH had a mixup back in the 80's where they were sending out regular
automotive style thermostats but were quick to have a recall on those as they
blocked the bypass completely. They warmed up fast but with no bypass
circulating a lot of hot spots can develop before circulation took place.

Ok, now I'm confused. Based on your description, when the thermostat heats up the upper part moves up and blocks the bypass. If you used an automotive style thermostat wouldn't the bypass be open all the time since there would be nothing to move up to stop it?

DanMD · Feb 25, 2021

DanMD · Feb 25, 2021

DanMD · Feb 25, 2021

Reading through this, I'm guessing the thermostat is made incorrectly with no holes allowing coolant to flow past the thermostat coil, thus blocking the bypass completely. This jives with what Pete said and makes sense with the manual. I believe the top of the tube should be inside the machined casting at all times per the manual.

Can you drill some holes in it without breaking it? I guess I better open mine up and look at it as well.

DanMD · Feb 25, 2021

Looks like Stant makes a version of this thermostat. Maybe there is a through flow design for a bypass system.

https://www.rockauto.com/en/parts/stant,13436,,0

farmall pete · Feb 28, 2021

This diagram is really clear on how the thermostat works. It shows coolant flow for the thermostat both in the open and closed position. The closed position in shown in the upper left, and the open position in the main diagram.

When the thermostat is closed, there needs to be holes in the thermostat to still allow some coolant to flow through it and into the bypass circuit.

When the thermostat is open, the tube on the end of it lengthens to close off the hole in side of the casting, thus shutting off the bypass circuit.

So my thermostat (and lots of other replacement ones out there) does not function correctly, because there are no holes to allow coolant through when the thermostat is closed. I'm going to go hunting for a good used thermostat. I'm surprised they goofed up the replacement ones this badly.

P.S: Putting in a regular automotive thermostat, with no neck on it, wouldn't work either. The coolant won't circulate properly and the bypass circuit wouldn't circulate any coolant through the engine when the the thermostat is closed. Instead, coolant would just be circulated through the water pump and radiator.

DanMD · Feb 28, 2021

So is it possible to modify the thermostat in your picture to let it work correctly? I ordered the Stant I linked above. I hope it is correct or if not, at least modifiable.

farmall pete · Feb 28, 2021

I'm not sure it could be modified because I think it's more complicated than drilling holes in the plate; there are multiple layers in there. When you get your Stant it would be great if you could post some pictures. The only picture I've been able to find of them is the side view.

J.ReinkeFarms · Feb 28, 2021

So looking at your diagrams, from what I gather - and could still be way off, is that an automotive thermostat does not "completely block" the bypass as I have read, it leaves it open all the time because there is nothing to block off that smaller tube in the upper water pipe. This then leads to a less efficient cooling system and if you work the tractor hard it could have overheating issues.

farmall pete · Feb 28, 2021

I think more importantly, an automotive thermostat would not allow the bypass to work at all when the thermostat is closed. You would just have coolant circulating through the water pump and radiator in that case, and no circulation through the engine.

J.ReinkeFarms · Feb 28, 2021

Thank you for being patient with me. NOW it is making sense, just took a while to get through my hard head. So the automotive thermostat doesn't really block the bypass itself, it blocks the coolant from getting to the bypass when it is closed. Coolant has to go through the bypass portion fo the thermostat, which is what get the coolant in contact with the spring to make it all work.

So that aftermarket one you have would be like an automotive one, blocking all flow when closed but does not have any coolant in contact with the spring to make it open. The aftermarket one, when open - if it does open, will block the bypass return passage where the automotive one has nothing to block the bypass to keep it from recycling coolant.

I'm not sure I'm explaining it correctly but it is finally making sense. Now when I go back and read some of your previous messages they make more sense. Thank you.

DanMD · Mar 5, 2021

(quoted from post at 17:46:54 02/28/21) I'm not sure it could be modified because I think it's more complicated than drilling holes in the plate; there are multiple layers in there. When you get your Stant it would be great if you could post some pictures. The only picture I've been able to find of them is the side view.

Great news, I laid my hands on a Steiner IHS913 tonight that is a Gates thermostat and you can blow through it. I also received my Stant 13436 today and it looks like the Steiner/Gates unit and you can blow through it as well. BTW, the Stant is marked as made in France.

farmall pete · Mar 6, 2021

That's great Dan! Could you post a picture looking in the pointed end of the thermostat? I'm curious what the hole configuration is.

DanMD · Mar 6, 2021

(quoted from post at 14:20:00 03/06/21) That's great Dan! Could you post a picture looking in the pointed end of the thermostat? I'm curious what the hole configuration is.

farmall pete · Mar 6, 2021

Thanks Dan. Looks darn close to the one I have with the exception of the holes. Good to know there are still sources.

Farmall M Thermostat

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