1968 300, any way to have engine temp lights AND a gauge?

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
Does anyone know the NPT of that "extra hole/second hole" that we're talking about...the one circled in the pic above? I'm wanting to install a temp gauge sensor there...but it looks quite small.

[EDIT: It's 1/8th NPT, same as the oil sender fitting, which makes it useless for all the typical gage sensor bulbs that need to screw into a much larger hole.]

So I'm going look into making a tee that goes in the larger hole on top and can accept the OEM sender on one end and a gage sensor bulb on the other, but I doubt there's enough real estate in that tight, busy area.
 
Last edited:

Larry Jett

New Member
Joined
Jul 22, 2016
Messages
11
Reaction score
10
Location
Newark CA
Does anyone know the NPT of that "extra hole/second hole" that we're talking about...the one circled in the pic above? I'm wanting to install a temp gauge sensor there...but it looks quite small.

[EDIT: It's 1/8th NPT, same as the oil sender fitting, which makes it useless for all the typical gage sensor bulbs that need to screw into a much larger hole.]

So I'm going look into making a tee that goes in the larger hole on top and can accept the OEM sender on one end and a gage sensor bulb on the other, but I doubt there's enough real estate in that tight, busy area.
The DakotaDigital hidden guage in the ash tray has a sending unit for the 1/8 inch factory plug as well as the 3/8 inch hole.
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
So does the AutoMeter gauge I used.

Can you provide some details, model numbers, etc for this AutoMeter, please?

After getting lots of parts to make a 1/2 inch Iron Pipe Thread tee into the water pump housing to hold both the OEM Cold/Hot sender for the dash idiot lights, I decided that wasn't a proper solution because the sensors/senders wouldn't really be in the hot coolant stream. I estimate that they would probably read 15-20 degrees low at best, and be in a permanent air pocket, too. Probably never get accurate readings, so why do it

So I'm going for the Cold/Hot idiot light sender in the big, 1/2 inch pipe thread hole, and looking for something to go in the 1/8th hole in the front of the housing.
 

Big_John

Illegitimi non carborundum
FCBO Gold Member
Joined
May 21, 2013
Messages
15,727
Reaction score
21,082
Location
Marcellus, NY
Does anyone know the NPT of that "extra hole/second hole" that we're talking about...the one circled in the pic above? I'm wanting to install a temp gauge sensor there...but it looks quite small.

[EDIT: It's 1/8th NPT, same as the oil sender fitting, which makes it useless for all the typical gage sensor bulbs that need to screw into a much larger hole.]

So I'm going look into making a tee that goes in the larger hole on top and can accept the OEM sender on one end and a gage sensor bulb on the other, but I doubt there's enough real estate in that tight, busy area.
If you use an electrical gauge, rather than a mechanical gauge, the size of the sender should be 1/8 NPT. When you use a mechanical gauge, that bulb on the end of the Bourdon tube is going to be larger and need a 3/8 or 1/2 NPT location. I have an electrical temperature gauge in my 300 along with the stock cold/hot lights. Light sender is in stock location and the gauge sender is in the "extra" NPT hole.
 

Ripinator

Old Man with a Hat
Joined
Jul 31, 2013
Messages
5,506
Reaction score
6,156
Location
Germantown, MD
Can you provide some details, model numbers, etc for this AutoMeter, please?

After getting lots of parts to make a 1/2 inch Iron Pipe Thread tee into the water pump housing to hold both the OEM Cold/Hot sender for the dash idiot lights, I decided that wasn't a proper solution because the sensors/senders wouldn't really be in the hot coolant stream. I estimate that they would probably read 15-20 degrees low at best, and be in a permanent air pocket, too. Probably never get accurate readings, so why do it

So I'm going for the Cold/Hot idiot light sender in the big, 1/2 inch pipe thread hole, and looking for something to go in the 1/8th hole in the front of the housing.

This photo illustrates how the small electrical temp sender is installed:

img_20180908_110844385-jpg.jpg


Do you see the green wire with the little red crimp terminal? Also, please note that the gauge has another wire that is connected to a power source that is present when the ignition key is on the 'ON' position. I wired mine to the 12-volt side of the ballast resistor. I'll try and look up the purchase info for my setup and post it here soon.
 

Ripinator

Old Man with a Hat
Joined
Jul 31, 2013
Messages
5,506
Reaction score
6,156
Location
Germantown, MD
This photo illustrates how the small electrical temp sender is installed:

View attachment 230951

Do you see the green wire with the little red crimp terminal? Also, please note that the gauge has another wire that is connected to a power source that is present when the ignition key is on the 'ON' position. I wired mine to the 12-volt side of the ballast resistor. I'll try and look up the purchase info for my setup and post it here soon.

Here is a link for the sender I purchased from Summit:

AutoMeter Traditional Chrome Analog Gauges 2532
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
Did you even go to the link I provided?

Of course I did. That fitting is at Depot, along with many others. The problem is the size of the bulb with my existing gage that was plugged into the 1/2 boss. It's 1 1/4 long, and has to be in it's own adapter (captivated on the thermo-couple wire) so it'll seal to whatever is coming out of the engine. So....that would put the whole thing poking out near the belts, etc. And it wouldn't really be in the water flow.

You should see the pile of Depot brass I bought yesterday trying to make something I liked. But a tee makes for a big, ugly "christmas tree" at that 1/2" boss on the housing, AND there's no water flow going past either the two-prong idiot light sender or the gage sender. I got it plumbed up...and then abandoned the idea. That's why I want to move to a gage with a 1/8 sender.
 
Last edited:

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
If you use an electrical gauge, rather than a mechanical gauge, the size of the sender should be 1/8 NPT. When you use a mechanical gauge, that bulb on the end of the Bourdon tube is going to be larger and need a 3/8 or 1/2 NPT location. I have an electrical temperature gauge in my 300 along with the stock cold/hot lights. Light sender is in stock location and the gauge sender is in the "extra" NPT hole.

That's what I'm planing to do, now that I've given up on the Big Bulb and its necessary adapter (so it seals the bulb, available as /12 or 3/8 NPT).

Is your electrical temp gage digital or "normal". I'd like to have it in my 3-gage panel under the dash, with oil and volts and don't want digital if it's avoidable.
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
This photo illustrates how the small electrical temp sender is installed:

View attachment 230951

Do you see the green wire with the little red crimp terminal? Also, please note that the gauge has another wire that is connected to a power source that is present when the ignition key is on the 'ON' position. I wired mine to the 12-volt side of the ballast resistor. I'll try and look up the purchase info for my setup and post it here soon.

That's what I'm looking for, but with A/C I think I'll prolly have to go to the horizontal boss on the other side. Yes?

Thanks for any info.
 

Big_John

Illegitimi non carborundum
FCBO Gold Member
Joined
May 21, 2013
Messages
15,727
Reaction score
21,082
Location
Marcellus, NY
That's what I'm planing to do, now that I've given up on the Big Bulb and its necessary adapter (so it seals the bulb, available as /12 or 3/8 NPT).

Is your electrical temp gage digital or "normal". I'd like to have it in my 3-gage panel under the dash, with oil and volts and don't want digital if it's avoidable.
"Normal" or what people refer to as "analog" readout. Excuse the crappy pic... These are the small Auto Meter gauges. I used them because they were small, electric and most important, I had them on the shelf.

fa85MEh.jpg
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
Thanks John. The AutoMeter website is a mess for finding anything...they want to sell $2200 dash kits for restomod Camaros....

Now I kinda know what to look for.

Oh, and I used "normal" on purpose, as a nod to the period of our old cars....when things were normal. :D
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
Here's my solution for a proper appearance for the sensor. I took a three-prong headlight connector from Autozone and made it a two pronger, and a dash of black Fusion spray. I don't like modern crimp connectors in their tutti-fruitti colors where they can be seen, same goes for zip-ties.

IMG_0577.JPG
IMG_0578.JPG
 

Big_John

Illegitimi non carborundum
FCBO Gold Member
Joined
May 21, 2013
Messages
15,727
Reaction score
21,082
Location
Marcellus, NY

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
What temperatures do the idiot lights actually engage at? I’ve always assumed if it got to the point where the hot light came on - you’re screwed anyway.

Here's an idea . . .
Get a "heat gun" and use it to check the temperature at the thermostat housing as the "COLD" light goes out. Probably in the neighborhood of 175 degrees F or so? The "HOT" light will probably trigger at about 240 degrees F, which is a bit under the boing point of a 50/50 coolant mixture with a 15lb radiator cap (which I believe is 260 degrees F or so?).

It's in the FSM on what temperatures turns off the COLD light.
I can't recall if the FSM states specifically when the HOT light comes on.

I just did a stovetop test of my new Cold/Hot sender. The Cold switch opened at a surprisingly low 110F, which turns the Cold light OFF.

I cranked the heat to a rolling boil of 228F (and I'm 9 feet above sea level) and never got the Hot switch to close, which would turn the Hot light ON. This makes sense, as the boiling point of pure water with a 16 psi cap would be around 250F. Antifreeze raises the boiling point, of course, but I think we can assume 240-250 is the setpoint for the Hot light to come on, unless someone knows different facts.

So, we can conclude that when your red light comes on, it's time to cook some crabs and shrimp ....

IMG_3360.JPG
 

73 T&C

Senior Member
FCBO Gold Member
Joined
Mar 19, 2013
Messages
2,048
Reaction score
3,128
Location
Coral Gables, Florida
Trace 300 Hurst....
Love the test.... you are a man deserving of admiration in my book! Well done!....

now we know!

Javier
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
I doubt I deserve the Nobel Prize for my thermometer work, but it's good to know the **COLD** number. It seems low at 110, but seeing as our giant lumps of cast iron (and water) take a long time to warm up, that temperature number kinda makes sense. At that water temp, the combustion chambers are already pretty warm, and the heat riser and choke systems are mostly satisfied. It's FAR from sophisticated, but at least the owner--48+ years ago--figured he could get underway with some amount of throttle response.

Not much, but some. :(
 

Trace 300 Hurst

Professional Tinkerer
FCBO Gold Member
Joined
May 27, 2018
Messages
1,910
Reaction score
2,290
Location
Ponte Vedra Beach, FL
Also--I'll be using the OEM Cold/Hot sender screwed into the 1/2 NPT bung, but I'll also have the Autometer "AutoGage" 3-instrument unit (PN 2391, $72 at Sumitt) ) that Big John mentioned above. It has Oil Press (mechanical), Volts, and Water Temp (electrical). The water temp fitting is 1/8 NPT, allowing it to be plugged into either of the two small bungs discussed above, while you can retain your big Cold/Hot sender.

Cake. Eat it. Too.
 
Top