Help identifying a plumbing thread

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Long story, but I seem to have volunteered myself to repair the firebox in an old woodstove. It's a steel fabrication with a water jacket around it. The water jacket is leaking.... somewhere...

No chance of buying a new one. The model was obsolete 20 years ago!

First job, is to cap off the ends of the pipe connections so I can get some pressure into the water jacket and try to find the leak. There are three connectors that I need to blank off. All are parallel male threads. Two are relatively easily identifiable as 1" BSP. However, the third one has me stumped!

The outside diameter of the thread is 34.88mm The pitch is "about" 1.8mm. A 1.8mm thread gauge is a better fit than a 1.75mm thread gauge and a much better fit than a 2mm thread gauge.

So, 1.814mm would be 14 threads per inch. I wouldn't mind betting that's the pitch, but the outside diameter of the thread has me stumped! 1" would be 33.25mm - which is too small. Also, the pitch of a 1" BSP is 11 threads per inch and this is definitely finer than that. 1 1/8" BSP has an OD of 37.9mm (which is too big and also the wrong pitch).

The only other piece of information I can offer is that the bore of this connector is designed to accept a pipe with an outside diameter of 28.37mm (or just slightly less in fact - say 28mm).

All thoughts gratefully received!
 
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What came off it? Not a threaded union by any chance? I've never found any information on the thread of the joined parts of a threaded union. Obviously it doesn't usually matter as you have both halves, but the thread could be whatever the manufacturer chooses (obviously the female threaded ends are BSP). The 28mm bore would be fairly close to the OD of a 3/4" steel pipe, so possibly a 3/4" threaded union.
The part is likely screwed into the boiler. If you removed it you could at least take it to the shop. Might come out OK with a bit of heat, but you don't want to risk making things worse.
 
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Do you know the make of stove,a photo of boiler and stove front if possible would be good . Ive repaired / rebuilt quite a few boilers over the years and you need very little pressure to find the leak. a wine making bung cut to block pipes if you want to get on with it, one with a hole to attach a a bike pump or similar
 
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Maybe it's like a compression fitting ? 28 pipe goes in and a nut+ olive on the thread.
Looks like you could be right
 
Thanks all!

Nige, you were spot-on! I ordered a 28mm compression fitting and the ring on it is a perfect fit, thanks!
 
The text on the nut gives it away!! BS864-2 was the (obsolete) British Standard for compression fittings. 28MM is self-explanatory.

Yes but this thread is a bit confusing because the OP (@Avocet) is not who posted the photos (@lostinthelight)... I think... So @Nige F got it before those images were uploaded... I was confused... @lostinthelight did you step in with those images?
 
Yes but this thread is a bit confusing because the OP (@Avocet) is not who posted the photos (@lostinthelight)... I think... So @Nige F got it before those images were uploaded... I was confused... @lostinthelight did you step in with those images?
I went into the workshop as I knew I had loads of old fittings kicking around specifically to find something 34.88 diameter which I did...then found a nut to fit.
When I re read the thread and saw Nige F ‘s post I thought Id better post the pics to confirm his suggestion
hope that makes things clearer?
 

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