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http://www.partdeal.com/isspro-nut-adapter-for-mechanical-temp-gauge-1-2in-14m-r7850.html

Link is not coming through, try partdeal.com. ISSPRO nut adaptor for mechanical temp. gauge.
Part No.: R7850
See if that works.

Quote:

Chris, I don't know if I can help with the info on the exact unit I bought because it has been 5 or more years and ebay doesn't seem to keep purchase history more than 3 years. I bought a triple unit similar to this one but was a different brand and in all black. One of my gauges says Actron but not sure that was the brand for the entire set.https://www.ebay.com/itm/2-52mm-Chrome-Panel-Oil-Pressure-Water-Temperature-Amp-Triple-Gauge-3-in-1-Kit/372334961942?epid=17016542816&hash=item56b0e4a516:g:RJkAAOSwxada-o6e

The nut on the temp sensor shown in the 4th and 7th photos without any adapters threaded right into the head but threads were slightly short so it needed an O ring behind the sensor for a tight seal between the head seat and sensor flange before the nut would tighten down to stop a leak. I can't say for sure that holds true for other gauge sets sold today or if they all use copper tubing that could solder to the old gauge line.

There are other single gauges that look to have similar adapter sets.https://www.ebay.com/itm/Sun-East-SE-AG201-Mechanical-Water-Temp-Temperature-Gauge-2-Diam-130-280-F-NOS/183270836953?hash=item2aabcb0ad9:g:MFUAAOSwaZFa~yEW

If the temperature guy or Wolf can fix yours that would be the best suggestion by far. I notice that some of the gauge sets say they accommodate 1/2 NPT threads. With the 5/8-18 thread BDeB mentioned, that would work out to about the 5/8 diameter but not sure how NPT works out on the 18 fine thread part. I don't have a spare head to experiment with. The item tolandis mentions might fit but can't say for sure.

Thanks Howard and Todd. I can only say I am forever grateful and awed by the help available on this site. I just hope to someday be knowledgeable enough to be 'you guys' and help out those coming up in the hobby, like me.

I hope I am not off track on the thread size with the 5/8"-18 UNF thread. When I look closely at the head, I can see the outline of the old nut and, having drilled it out to 1/2", there is still a good-size 'ring' of material left. The correct drill bit for a 5/8-18 tap is supposed to be 37/64" which is 5/64's bigger than a 1/2, so I am thinking that is the correct size. The tap will then 'bite' to 40/64 (5/8") and should give me pretty good thread 'meat'. Open to any correction or suggestion here as I am certainly no machinist! Todd, I did look up the R7850 and it is 1/2", but there is a R7853 that says 5/8" and although the picture doesn't show it very well, it looks like it would be the correct one.
I am taking your advice Howard and will check out Wolf's and Roy Martin, (the Flackmaster) to see if I can get somebody to do this for me. I will post my findings and progress. Any comments to my approach are certainly welcome. (and thanks Todd, for the offer to make one on your lathe; most appreciated)

I checked and I do not have a nut in my piles. Try not to damage the threads in the head.

1/2 NPT is way too big. The outside diameter of 1/2" pipe is 0.840 inches.
3/8 NPT is close with pipe outside diameter of 0.675 inches but still larger than 5/8 " (0.625 inches)
I tried a 3/8 NPT pipe nipple in the head and it was too large to go in although the thread count is also 18 per inch.
Drilling the nut out will also be tricky as there is an internal shoulder at the bottom of the threads that provides a seat for sealing the flange on the sensing bulb.
Proceed with caution.

Hello again Packard newbie, neither of my Packard's have the type of situation you are facing with your temp ga., but after all I have read in this post, and I'm certainly not qualified to do any of the work you are contemplating or an expert, but from a personal perspective I would ask myself if I mess-up the drilling, easy out and removal of the part(s), and if there is damage done to the head, is it an easy fix or does it go downhill from there?

Just took one of those nuts out of a Stude Champion. PM me with address if you want it.

Well, now I'm thoroughly confused!!!! Brian, you said the thread size was 5/8-18 fine. I went and bought a tap that size and it is, obviously, 5/8"OD. I have the hole drilled out to 1/2" and I can see the collar in behind the hole. If I put a thin, flat screwdriver in at an angle, it catches on the collar. The sender unit for these gauges look like they are about 1/2" in diameter and a 5/8" follower nut would sound about right. Why does this suddenly sound 'way too big'??? 5/8" is .625, nothing to do with pipe thread, which is huge, compared to. I did get with Wolf's and they do the repair for $165 which is okay and they replace the entire capillary tube from the back of the gauge to a new sender bulb. When I mentioned 5/8-18 for the follower nut, they knew right away what I was talking about - it seemed commonplace to them.
Can somebody 'set me straight' here before I ruin the head???
What am I missing???
Also, Ross - can you measure the OD of the threads on the nut you have so we can see if I have already drilled it too big??
Keep in mind Brian, when I say I have the hole drilled to 1/2", I am not saying 1/2" NPT, just a 1/2" bit (.50)

My comment on way too big was in response to the 1/2 NPT (1/2" Pipe Thread) connection mentioned by HH56 in post #21 for some aftermarket replacement gauges. A gauge with that size connection at the sensing bulb would be too big and the wrong thread for the opening in the Packard head.

Okay Brian, I get it. One thing in my favour is that I did talk with Roy Martin in South Burlington today and he says some people DO inadvertently drill out the retaining collar and if that happens, he has a sender bulb with an adapter that will fit that size hole with just female thread. (seals on the threads, I presume) and so, while I will try and get this thing back to 'as it was at the beginning' WITH the collar, all is not lost if I don't. Either way, I will be tapping it to 5/8-18 UNF thread and will be able to salvage my dash gauge. That would have really bummed me out to lose the original temp. register.