I would like to install another temp gauge in addition to the stock temp gauge. There is one small (1/8pt) at the thermostat housing for the stock sensor. I have no other holes in this area for an additional sensor. I do have what looks like a 3/8pt plug in the passenger side of the block about 2/3 of the way back. It's is the area of the oil sending unit for more clearification.
Is this port in the coolant area and can I install my additional senor here?

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

I believe you are referring to the coolant cleanout plug installed on the passenger side of the block towards the rear. That does go into the water jacket, but I don't know how accurate of a temp sensing ocation it owuld be. You would first have to make sure that the sensor was not covered by sludge.

That would be the drain plug, being at the bottom of the block close to where the cooler coolant enters, youd have unstable temp readings. A gauge there would tell little of the actual temperature and would always read lower than the original. As long as your slant is not computer controlled ( computer relies on temperature reading to determine mixture and spark advance), then invest in a good quality mechanical or electrical replacement in place of the original.

I believe Lou said he used that port for a temp gauge but because its only 1/2 way through the coolant flow path, its roughly 20 deg cooler then the one by the thermostat.

Easiest way is to drill and tap the thermostat housing for whatever sensor you want. You won't see the engine warming up on your gauge necessarily but once the thermostat opens then it will reveal engine temp. Thats what I did for my gauge because I'm using the stock location for the sender going to efi computer.

Other options - I've seen people poke a hole in the upper radiator hose and put a sensor there - I've also seen fittings that have two male hose barbs and a 3/8" npt fitting at the top that you tee into the radiator hose. Check summit for these, not sure if they are available in proper diameter for our radiator hoses. Theres also a decent amount of metal meat in that area of the head, you maybe able to drill and tap a hole for a sensor on the face of the head towards the radiator, but make sure it doesn't get in way of the belt.

Why have more than one? All this drilling and tapping and cutting hoses, geez, asking for trouble or what. The next question will be " I installed two gauges now BOTH say my engine's OVERHEATING! I wonder why"

The in dash gauge is sticking sometimes. It's sensor is 1/8 npt. I have yet to see an aftermarket mechanical temp sensor that will fit in this hole. Fixing the in dash sensor would require removing it and sending it out for repair or getting another one like it, both of which would cost more time and money than I am ready to spend on this little problem. Repairing the gauge myself I could probally do with a little alteration, but getting it out and putting it back and to have it still not work would be more than I could take. Let alone their not really accurate, even when they don't stick. A mechanical gauge seems to me a good inexpensive accurate way to monitor temp. At this time.
Any other coolant ports on this engine that would give a close to accurate temp or at least a stabil temp.

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

My 85' 225 has a smaller bolt just to the right of the temp sensor, directly over the thermostat housing (I be darned if Ill ever see it come off though) probably for venting the system when being refilled I dont know if it exists on your older model or not

there is a thermal vacuum switch on my car right above the thermostat housing that we bypassed and are going to use for a temp guage

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1975 Plymouth Duster,225/6 2bbl. and duels, 904, 7 1/4
1986 Dodge Charger Shelby, 2.2 Turbo I, A525

Autometer and Dakota Digital both make temp gauges that come with 1/8" npt senders. These are electrical senders, but for $40 (autometer analog readout) you really can't complain about price.... The dakota digital ones are closer to $100 if you want a fancy digital readout but you have to place them right or else glare will kill visibility.

This is one of the many autometer styles....
http://store.summitracing.com/partdetai ... toview=sku