I'm installing a GlowShift water temp gauge in my 1978 Oldsmobile Delta 88, and I have the sensor, wiring, and related tools now. I'm installing the sensor into one of the unused port plugs on the radiator. The larger plug here is where I'm installing the aftermarket sensor (leaving the original one on the block alone):

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Question is, the sensor is obviously much smaller than the port plug size. What adapter will I need? The thread size on the sensor is 1/8"-27 NPT, and I have no idea what it is on the radiator (no info on that in the service manual as the radiator is clearly not original). There is a large selection of thread adapters and I don't know which one will fit.

 

That's a pretty lousy picture. Where are you going to install the temp sensor? Is the circle at the top a flange molded into the plastic/aluminum of the radiator/shroud? Is the circle at the bottom some sort of plug?

I suggest you remove the plug from the radiator and take it to the hardware store to determine the thread size. FWIW, I know the transmission cooler lines are inverted flare fittings.

 

The top plug is where I'm intending to install the sensor. I've removed it once before, nothing is attached to it nor does it seem to have any purpose. The bottom plug is a much smaller one that I was curious about, but I don't think I'm going to mess with it.

I'm going to have a drain a bit of coolant to access the top plug. Do I risk letting air into the cooling system from this? I don't have to drain much, just wondering if I'll have to bleed the cooling system again. The car is in my driveway so the radiator is the highest point in the system.

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Took this months ago, back when the water pump was leaking (hence the low coolant level) and yes the radiator is icky and will be replaced very soon. But in the meantime, I want to install the temp gauge. The plug in question can be seen on the left, inside of there ( the round stub-looking thing).

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Based on the pics it seems the top plug just has normal pipe threads on it. Take out the plug after draining some coolant and take it to the hardware store to buy an adapter. Bring along your sensor so you can be sure everything fits.

When you drain a bit of coolant from the radiator you shouldn't have to worry about belching the coolant system as long as you don't run the car with low coolant.

 

If the rest of the inside of your radiator looks like your picture, the temp sensor is the least of your worries.

 

That plug is not NPT it's straight thread. I don't think that's a good spot for a temp sensor but if you're dead set on that location I would ground down that tab to a flat surface and then drill and tap for an 1/8 NPT.
That spot is not continually covered in fluid and may give erroneous readings. What is wrong with putting in in the block or the intake manifold? Those fittings would be NPT and getting a bushing that is in those sizes with an 1/8 inch NPT hole in them would be an easy task.

 

Ooops, my bad.

If the top port isn't submerged in coolant all the time perhaps you should use the bottom port, or one on the engine/intake?

As Eric mentioned too, that is a pretty nasty radiator. The one in my 71 98 is old and grimey and nowhere near that dirty.

 

An easy way to tell if things are straight cut threads or NPT is to see how it sits in the hole. If they sock down flush to the surface with no gap it's a straight cut thread. Anything that sits proud of the surface would be NPT. Of course this assumes some gorilla hasn't socked it down to within an inch of it's life. In those cases the hole usually cracks. This can be avoided (somewhat) by using a softer plug than the parent material the hole is cut in, IE brass plug in cast iron.

 

I hadn't considered that the port in question may not be continuously submerged in coolant, so a different location will be pursued instead (especially given that I'm going to replace the radiator anyway). I'm going to T off the original temp sensor in the block. I want to keep the stock sensor but also add the aftermarket one.

Basically I want to see how hot the engine is currently getting with that old nasty radiator still installed, and then after the new one is installed.

 

You can't T off the original location because the sensors have to be submerged in the coolant flow to get an accurate reading.

 

Don't know about your year, but intake may have additional port. Putting it in the radiator is not a good idea anyway, You want to know the temp of coolant in the block.

 

Can always pull out a block plug by the engine mount and go in there. Always submerged and it's 1/4 inch NPT.

 

Any recommendations on location then? Also shouldn't the sensor be installed before the thermostat or it might give erroneous readings?

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Try one of these, pick one that fits your hose.

 

One good thing about the Chevy engines is the plug in each head, one is for gauge, and other for TCS.

Hey, that gives me an idea...does a 78 have the TCS system he can disable and remove the sensor for it and use its hole, or did they go on to bigger and better by then?