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Allis Chalmers Discussion Forum

WD water temp gauge thread size?

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01-26-2015 12:54:34

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Has anyone had to drill out and re-tap the threads on the water temp gauge/upper rad hose fitting on a WD? If so, what size tap is the right one to use?

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04-02-2015 12:43:40

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 12:54:34  
The fix to this issue was actually pretty simple, in case someone else runs into this problem in the future. A clue came to me by accident when I was searching for more info on thread sealing techniques, especially 5/8-18 threads. A hit came up on Google that read something like "5/8-18 fittings use straight threads not designed for sealing. These fittings seal using o-rings". Well, there is an o-ring that comes on this gauge, but it is located above a lip on the gauge stem that the 5/8-18 flare nut tightens down against, and it's a very thin o-ring. It would never seal against the outside of the hole that the gauge fits into, but it will seal the nut to the stem of the temp gauge. What I did was install a second o-ring below the lip, so it seals between the lip on the gauge stem and the step it sits on in the rad inlet. After carefully tightening the nut just enough to compress the o-ring a bit, voila! no leaks. Case closed.

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03-26-2015 12:00:45

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 12:54:34  
So after messing with the new thermostat to get it to fit the housing, and getting the housing to seal (different post), the next issue is that the gauge leaks at the threads, probably because I had to drill out the old one and re-tap the 5/8-18 threads. As I was drilling the final hole for the tap, the old threads, which had stuck fast through all the drilling up to then, came loose. It was only a very small bit of thread, but of course the new hole wasn't perfectly centred in the old one. I went ahead and cut the threads, but it seemed a little too easy to do.

When I installed the gauge I knew right away that the threads were too loose, so I put some thread sealer on there and hoped for the best...but it leaks when you put the rad cap on and pressure builds up. It's just a bit too much of a leak to just put in a can of stop leak and wait for it to close up.

There are a couple of things I can still do, and when I get it right I'll post the fix here, but in the meantime I'm open to suggestions and recommendations!

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01-28-2015 06:31:05

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 12:54:34  
Thank you Bob, much appreciated!

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Bob Bancroft

01-26-2015 14:05:31

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 12:54:34  
If I recall correctly, there is a tapered seat down in there, so you wouldn't be able to run a tap very far in without ruining the seat. Maybe a bottom tap would be somewhat useable.

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01-26-2015 20:20:38

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 Re: WD water temp gauge thread size? in reply to Bob Bancroft, 01-26-2015 14:05:31  
Thanks Bob - There is a step about 3/16" below the threads. The temp gauge has a lip that seats against that step, and the nut tightens down onto both, kind of like a flare nut on a brake line (it actually is a flare nut on the temp gauge). Either a plug tap or a bottoming tap seem to work fine because of the 3/16 of space between the end of the threads and the step. What I can't tell for sure is the thread size. I've already threaded it 9/16-18 and that is way too tight, the nut won't even engage the threads. 19/32-18 is a real oddball size and seems like it would be too tight as well. I'm leaning towards 5/8-18 because it's a common size and a 5/8-18 nut I have threads onto the gauge nut with no sloppiness, although the hole on that nut looks huge, compared to the nut on the temp gauge. I just wanted to check here to see if anyone's had recent experience with it and knew offhand what the thread size would be.

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Bob Bancroft

01-28-2015 06:16:52

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 20:20:38  
Yes- 5/8-18

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Bob Bancroft

01-27-2015 16:47:23

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 Re: WD water temp gauge thread size? in reply to icb, 01-26-2015 20:20:38  
If I can remember tomorrow when I'm in the shop, maybe I can figure it out.

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