Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I think this type of solder to threaded adapter may be what you’re referring to (in model engineering size), but without a picture that’s only a guess.

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A fitting on one of my engines.
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You may find the type fitting(s) you are thinking of in the resources listed below. There are many other online resources where you will find helpful information. Once you have identified the fitting(s),you might want to post pictures here.

https://www.ajreeves.com/

https://www.munotmetalloys.com/blog/everything-you-need-to-know-about-copper-pipe-fittings/

https://pdf.directindustry.com/pdf/nibco/copper-fitting-catalog/12637-123275.html

https://maccmodels.co.uk/

https://www.ekpsupplies.com/steam-fittings-and-parts.html



As others have said, PM Research is a very good resource.

https://pmmodelengines.com/pipe-fittings/