The sender identified as "1964-up" in my previous post was original equipment in '74 and '78; the first year for any different temp sender was '82, when a unit with the same 1/8"-27 NPT thread but 100Ω at 220°F was substituted.

The later '70s heads have several holes at the top-front leading into the coolant passage. There was one for the temp gauge sender, and one or two for coolant temp sensors (or switches) used as part of the emission control systems of the day. The sensors have a 3/8"-18 dryseal thread. Do you see where this is going yet? I think you're trying to fit the (smaller) temp gauge sender into the (larger) emission control coolant temp sensor/switch hole. This in itself wouldn't necessarily explain your low reading, but as Rock pointed out, the sender can only measure the temp of fluid that's actually touching it, so if the adaptor moves the sender out of active coolant flow, and as Dakight alludes, that Teflon stuff may be enough to increase resistance and cause a low reading.

I think the correct '64-'81 temp sender from a reputable supplier, installed in the correct hole, will get your temp gauge working properly...as long as the gauge itself and the instrument cluter voltage limiter are in good shape, and the thermostat's not opening too soon!

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Guys,

Slant Sx Dan runs wrong so rarely I don't recall the last time...I act in reliance on his stuff! But...

The resistance at 220 degrees is not the one I consider the most important and I wonder why the have always used it. I think the resistance from 150 to 180 degrees is more important than the one at 220 degrees because I want my thermostat opening when it should and I want to know when it is open. When you start up each time, don't you feel good every time you see your temp go up and up, pause for moment, then stabilize at whatever operating temp you picked? Once it has gone over my 180 or 200, I already know something is possibly wrong, I don't need to see 220.

Dan is correct as usual in pointing out the posted values at 220 and I concede it could be a personal philosophical point that I prefer to know what is going on as the temp increases, rather than know it is higher than I want at the end. So I was examining curves in the 150 to 212 range (being limited to a glycol/water combination drained from my radiator and heated in a container open to atmosphere at sea level (during a week at the beach) . I believe it may have been Dart270 who installed temp sensors in other parts of his block and I would llike to do that too. I think it was DD who put the one in the thermostat housing and it works GREAT for me!.

Last, Dan makes a point I forgot about...I don't use stock gages. I use what I consider to be a much more reliable gage from an industrial boiler adn also run a new Autometer gage. So the resistance effects on stock gages were something I never wondered about and I am glad to have that pearl in my knowledge stash (Thanks Dan)

rock
'64d100

I never trust any gauge until I have had experience with it. The main thing is, where does the gauge run when things are right? When your engine boils over, was the gauge in the red yet, or sitting down lower? Where is the gauge reading when you finally get heat out of your heater? That's why I love my old cars. It would be a shame to get rid of them just as I got to know them like a brother. Each sender is possibly a little different, and each gauge seems to be for sure, as well as the voltage regulator in the dash. Don;t worry about it. Just get it to work, and learn it's ways.

Sam

_________________

Yesterday afternoon.
I think you're probably right, but the figures at 220°F are how the senders are classified. Over a decade ago I fiddled around with an ohmmeter and a thermometer and a handful of senders, but I didn't keep what I scribbled down
Well, you can achieve that by putting the sender in the thermostat housing/water outlet (the DD method -- I agree, works great.)
I'm sure your boiler gauge is very much more precise and accurate than anything the factory installed, especially given that the factory gauges aren't calibrated ("C" and "H" don't really count as calibrations!). I like the factory appearance of stock gauges, so I make so with them as best I can.
Glad to help out. Wanna know something funny? There's a 1960 TSB for the Dart & Plymouth. Seems there were complaints of overheating. The cars were running at the right temperature, but the temp gauges were indicating too high. The factory advised dealers to install a resistor of specified value in the temp sender wire and wrap it in electrical tape!

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.