Coolant temp sensor in the intake right by the thermostat?

[runmikeyrun]

Hi guys. I know I’ve had a lot of questions lately. Just trying to make sure I don’t screw anything up.

Our fire department’s 1942 Ford fire engine had the flathead replaced with a Y block somewhere along the way. Since I still have the stock ‘42 gauge cluster I need the flathead temp sender to read properly.

I received the flathead sender today. However, it is physically too large and long to thread into the block where the sender normally sits. I could piece together a series of adaptors to make it fit, but it will be sticking 5” up from the head and I feel that will not read the temperature accurately.

Up front near the thermostat is a plug that I believe is the same size/thread pitch for my sender (the Allen key plug in the pic) I also think the area is deep enough for my sender to fit. Since this location has coolant that is exiting the engine I feel it’s an appropriate place to read the temperature.

Any reason why I can’t put my sensor there? I’m not in a position to drill/tap/etc the head where the sensor usually goes.

Thanks!





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[dmsfrr]

The allen keyed plug would normally be one of the fittings for the heater hoses, the other is on top of the passenger side of the water pump. Obviously the heater isn't being used.

'57+ heads use a smaller temp sensor as you found out. (photo)
IF the Flathead temp sensor is the same size as y-block thru '56 it's 3/8 *pipe thread*, and is the same size as the allen plugged hole behind the thermostat. The longer larger flathead temp sensor should(?) clear the back end of an oem style replacement thermostat. It's known to be a problem with heater hose fittings with a long tube on the bottom when using high-flow after-market thermostats.

Pretty sure the water temp readings there will be lower than in the head but once you know where the needle 'normally' rides in the gauge you should be ok.

FYI, I've had the drill and re-thread the temp bushing done in the head (but with the intake manif not on the engine) and the actual cutting was easier than I expected.

It looks like the engine had top-end oiling problems and had a copper tubing oil kit put on it.

[runmikeyrun]

Thanks for the super fast response. I gave it a shot, it does fit. Drilling and tapping etc is out of the question in this case, so like you said I’ll see where it normally rides on the gauge and go with it. I won’t get a chance to start/Run it tonight but we’ll see how it does.

Thanks again.


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[FRANK PKNY]

I THINK YOUR SOLUTION WOULD BE FINE. As Runmikeyrun says just find your norm and go from there.

[paul2748]

Be careful about putting a thermostat at that location - make sure it will not interfere with the thermostat operation. There have been some problems with this

[runmikeyrun]

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Originally Posted by paul2748

Be careful about putting a thermostat at that location - make sure it will not interfere with the thermostat operation. There have been some problems with this

Which direction is the thermostat mounted? I can see that possibly being an issue. I wonder how big the thermostat is inside of there?


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[dmsfrr]

Since the spring loaded thermostat valve moves into the intake manifold as it heats and opens it would need about 1/2 inch (?) cold clearance to not hit the temp sender, which would keep it from opening fully.
With some luck and a good flashlight you might be able to guesstimate the existing clearance close enough.

[runmikeyrun]

Originally Posted by dmsfrr

Since the spring loaded thermostat valve moves into the intake manifold as it heats and opens it would need about 1/2 inch (?) clearance to not hit the temp sender, which would keep it from opening fully.

Thanks for that visual. I’m not exactly sure on my clearance, or the way you worded that lol. Are you saying there will not be enough room for the thermostat to open?


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[dmsfrr]

If I had a loose intake manifold I'd measure the actual inside distance for you, but that depends on how far your thermostat fits into the manifold, if it's different than this one.
The depth of the rear side of this (closed) thermostat is 3/4 inch.
The copper colored core inside the spring (photo) moves toward the screwed-in water fitting as the thermostat opens.
But I don't know how far the center core moves when fully heated.

I held this one against the intake manifold on my 292 and eyeballed the space between the back side of the thermostat and heater hose hole at 1/2 inch. It looks to be just-enough to fit. Anything you can do to double check that with the parts you have would of course be a very good idea.

[runmikeyrun]

Originally Posted by dmsfrr

If I had a loose intake manifold I'd measure the actual distance for you, but that depends on how far your thermostat fits into the manifold, and if it's different than this one.
The depth of the rear side of this (closed) thermostat is 3/4 inch.
The copper colored core inside the spring (photo above) moves into the intake manifold space / toward the screwed-in water fitting, as it opens.

I held this one against the intake manifold on my 292 and eyeballed the space between the back of the thermostat and the heater hose hole at 1/2 inch. It looks to be just-enough to fit. Anything you can do to double check that with the parts you have would of course be a very good idea.

I REALLY appreciate you doing that, thank you. I think I will pull it apart to verify next time I have a chance.

Thanks again.


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[dmsfrr]

Just to have something for you to compare to, and why I think 1/2 inch clearance between the thermostat & temp sensor will be enough.

I heated this one in boiling water and the center piece sticks out 3/8 inch when hot. It has no brand markings I could read and looks pretty generic. It does have 'USA' on it.

[runmikeyrun]

Originally Posted by dmsfrr

Just to have something for you to compare to, and why I think 1/2 inch clearance between the thermostat & temp sensor will be enough.

I heated this one in boiling water and the center piece sticks out 3/8 inch when hot. It has no brand markings I could read and looks pretty generic.

Oh wow ok! I’m going to give it a go then. I’m leery of pulling the thermostat, I’d hate for a bolt to break off, but that is the only way to assure it’s actually going to clear. Maybe I will start the truck and warm it up some, then when the intake is hot I’ll just crack the bolts loose. Once the engine cools back down I’ll pull the housing.


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[dmsfrr]

I'd be more inclined to just drain the radiator down a bit and put the temp sensor in the currently plugged hole on top then refill and go with it. With any luck you'll be able to see into the hole when the plug is out, to double check the available space.
Removing the thermostat (an admittedly small can-of-worms?) could be saved for later, just in case.

edit -
Measure this distance on your manifold (photo) I got 1 & 1/8 on mine. That 'should' give you a fraction over 1/8 clearance from the heated thermostat to the temp sensor, since the sensor shaft has a slightly smaller diameter than it's threads.

[runmikeyrun]

That sounds like a good idea. The coolant is kinda rusty so it probably wouldn’t hurt to pull the bottom hose and let some out.

Thanks again for all your help. I don’t mind screwing up my own stuff but this is the department’s vehicle.


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[dmsfrr]

Originally Posted by runmikeyrun

... The coolant is kinda rusty so it probably wouldn’t hurt to pull the bottom hose and let some out. ...

Rust tends to build up in the block around the rear cylinders, clog things up and the engine runs hot.
A good flush out wouldn't hurt.
(except for getting crusted in-place radiator hoses to re-seal and not drip)

[runmikeyrun]

Originally Posted by dmsfrr

Rust tends to build up in the block around the cylinders, clog things up and make the engine hot.
A good flush out wouldn't hurt.
(except for getting crusted in-place radiator hoses to re-seal and not drip)

The last part is what I was concerned about, but I’d rather replace a radiator hose than extract bolts from the intake manifold. Also, if the hoses are bad enough to not re-seal then they should probably be replaced lol. They’re probably pretty ancient


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[paul2748]

I meant the sender unit - make sure it does not interfere with the thermostat

[runmikeyrun]

Originally Posted by paul2748

I meant the sender unit - make sure it does not interfere with the thermostat

I gotcha lol


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[runmikeyrun]

Well, I pulled the sensor out and stuck my pinkie finger in there, being careful not to get it stuck! I could not feel the thermostat. I also put a screwdriver down the hole and pushed it so it swung forward. Still did not hit the thermostat. Maybe it doesn’t have one installed? [emoji23] it’s always possible.

I drove it to a local cruise-in last night and ran fine with no issues. Like previously mentioned the gauge read a little cooler, between C and the middle line once fully warmed up, but expected since it is the coolant temp and not the head temp.

Thanks guys for all your help.


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[dmsfrr]

Nice looking Fire Engine. Thumbs up!