Thanks Ross - that is as I thought. In contacting both Wolf's and Roy Martin, I am advised that they can renew the gauge and sender to factory spec, including a new 5/8" follower nut, so I am okay. Thanks for the offer on the one you have; I'd have gladly bought it from you had I struck out with the repair options or decided to do it myself. For the reasonable fee that these guys charge, it doesn't seem prudent to mess with it. I will do the retap on the hole and hopefully get a nice set of threads out of the deal. I spent the day finalizing the engine detachment and am pretty much ready to lift it out. After it is 'on the hard', so to speak, I will tackle the drilling and tapping. Thanks again for weighing in on this... Appreciate ALL the responses.

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.

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)

Stainless steel T-bolts would be a good option.

https://www.amazon.ca/Pro-Tie-33735-T-Bolt-Stainless/dp/B00D4MKYRK?th=1&psc=1&source=googleshopping&locale=en-CA&tag=googcana-20&ref=pd_sl_175k72kdnt_e

Pat, re the serrations on the hose-bib end of the pipe, I agree with the gasket shellac approach and with the low pressure, it will give you an adequate seal. One thing to note: most folks install 2 clamps on the hose and so often, you see them line up the gear housings. While this looks nice and neat, it is not the best method to ensure a good seal. Any gear clamp or bolt type clamp, when tightened on a hose, imposes a 'teardrop' force on the hose, with the tip of the teardrop being the the tightening point. Having them lined up creates a leak-path, while opposing the clamps to have one gear housing or tightening bolt on either side of the hose will ensure a better seal. Just the musings of an old hose guy, but trust me, it works!!

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)

Roy Martin - got him. Will call first thing on Monday. Thanks David. If I strike out there, Howard, would you be able to suggest an aftermarket equivalent that would be a candidate for the 'ice/salt bath'and sleeve solder??? Something that would go in the hole and seal in the same fashion with the hollow follower nut? I did the Ebay query and could not come up with anything that even resembled the pics you sent me of the original Packard unit. Thanks a lot for your help. Chris

Thanks Howard, that is an interesting article. After reading it through and checking all the pics, I think I could do the sleeve-solder bit, but not so sure about filling the sensor bulb with ether and immersing it in ice water and SALT, what's with the salt????
If anybody has a sensor bulb in their 'for sale' pile, I'd sure be interested. Preferably with the retaining nut, but if not, we are dealing with standard fine thread and I think it would be fairly straight forward to shorten a 5/8"-fine bolt and drill a hole through it to make one.
I will continue to pursue Wolf's and will also scour the regular sources for a complete gauge with capillary tube and sensor already attached, although I think the odds of that are pretty long.

Steve, That is probably good advice. My 91 year old Dad said the same thing!! I guess I'm just stubborn and hate to give up. I went out and bought the 5/8-18 tap and a 37/64" bit and have the hole drilled nice and evenly to a half inch. Because it is so far back in the head, it is difficult to get at it squarely with the drill and tap, so I was going to leave it until I had the engine out and drill and tap the hole then. In drilling it out as much as I have, I've drilled into the sending unit and would really like to get the threads properly cleaned up and then make sure I get any parts and pieces of what's left of the sender bulb out of the head.

I've emailed Wolf's and not gotten a reply. Anybody have any suggestions on sources for the sender unit and/or the gauge line repair???

Pat, I just re-cored my rad and when I purchased new upper and lower hoses, I was able to get the exact size and bend radius but the new hoses did not have the helical spring coils in them and, as is suggested here, they are required. I ended up taking the old ones out of the used hoses, and spent quite some time cleaning off all the scale and mineral deposits on them. I found the coils were a really tight fit in the new hoses, so I placed them in really hot water and got them nice and supple, then lubed them thoroughly with dish soap and tied a couple of cords, diametrically opposed on the end of the coil, and had someone hold the hose and I pulled the springs squarely through until they left just enough of a cuff to go over the rad and water pump barbs and they gave the hoses excellent rigidity. The springs want to elongate of course, so you have to kind of stuff the trailing end in as you go, with a stick of something smooth that won't damage the tube of your new molded bends, If your old hoses don't have the springs, I think you can buy new ones at Napa or some place like. A thorough rinse afterward so your rad isn't blowing bubbles and you're ready for antifreeze. This may or may not apply in your case, and it is a bit of a bother to do, but thought I'd share my little exercise anyways, just in case it could be of some use! LOL