Jim, I think your in better shape then you think.

Your sending unit is the same as mine and your starting with a smaller size top fitting.

Figure out your adapter needed as they are made just for this.

https://www.ebay.com/sch/i.html?_fr...w=water+temp+gauge+adapter&_osacat=0

Something like this...

So you are thinking I start with an 1/8" NPT fitting out of the original sender hole,then another adaptor to take it up the smaller of the 2 fittings I have now and do away with the 1/2" NPT adaptor that came with the gauge correct? Let me know what you find for fittings and size.Thanks !!

Looks like we crossed post.Thanks for the link and I'll give it a try.Thanks again !!!

I will find them before the days out.

some of mine

it's been a while since i installed a mechanical temperature sensor.
with that said, the end bulb fitting threads into an adapter that sandwiches the bulb between the fittings, thus sealing the connection. here's where i think it gets a kink.
if i remember right, the fitting on the bulb end is STRAIGHT pipe threads, and so are the female threads in the adapter. this allows for the correct tightening of the bulb to the adapter fitting.
trying to rig up 73 adapter fittings to get to 1/8npt is going to look pretty hoaky at best, and won't be very accurate.
i would look at the water pump housing or intake manifold to see if there is a place that could be safely tapped for the gauge's fitting.
a guy could possibly fabricate a fitting from hex or square brass or aluminum stock, that could T into the inlet heater hose. it wouldn't be that hard to do, plus it would look better than a hardware store plumbing piece.

Good points, mainly on the ugly.

All my fittings came from summit in one form or another. I used two, and the first has the proper taper for the sending unit seal ring, accuracy and sealing was/has never been a problem. The strictest tech tracks never said fix it.

Im sure he can do it with one.

How do I know how accurate the manual unit is installed this way?

Look at this pick, thats the t stat for my 2nd (backup) ele. fan and is said to be a 180 degree unit, the few times ive got that warm, manual unit said right at 178/180. Real world testing, not "I think" or "It might".

You cheat (LOL) and used the heater return opening which I believe is 3/8 NPT. Looks nice though

Only other option without stacking fitting would be to drill and tap to a larger NPT if there is a enough material around the opening to do so.
Welding a bung on may be a possibility or t/sat spacer block with NPT openings. Depends on how clean appearance wise you want it.