Question about silver brazing

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tornitore45
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Question about silver brazing

Post by tornitore45 »

I build my engine's valves in two pieces. The valve head is threaded on the stem and silver brazed.
The tread is loose and I can file a flat on the thread to avoid the spiraling down of the solder.

My question is: Should the flux go inside the thread or just out on the head?
I wonder if the brazing alloy can displace the flux inside the thread
Mauro Gaetano
in Austin TX
Richard_W
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Re: Question about silver brazing

Post by Richard_W »

The flux should go on where ever you want the silver solder to stick. The silver solder will wick up into assembled parts provide you have put the flux where you want the solder to go. I have made custom hydraulic fittings with a loose fit and had the silver solder wick all the way that you could see it inside the fittings at the seam. Yes the silver solder will follow the thread if you have put flux on the threads. I have never tried silver soldering on a blind hole where the air and flux couldn't be displaced. One would have to make a test part and then cut it in half to see how far the silver solder traveled up the threads.
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Harold_V
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Re: Question about silver brazing

Post by Harold_V »

I agree with Richard. In my experiences, silver solder will flow anywhere there's flux added, and how it spreads is heat dependent. When heating by torch, when you're near the temperature at which solder flows, the solder will follow the torch.

H
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10KPete
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Re: Question about silver brazing

Post by 10KPete »

I agree with Harold and Richard and will add that given clean parts, only a light smear of flux is required. On blind holes that are more than one diameter deep I will drill a small hole at the blind end to let the solder 'out'. A hole .03" diameter is enough...

Pete
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Harold_V
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Re: Question about silver brazing

Post by Harold_V »

Yep, I've often done the weep hole as well, which eliminates any possibility of the heated flux (which contains water) from expanding, separating the pieces to be soldered.

H
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tornitore45
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Re: Question about silver brazing

Post by tornitore45 »

Thank you all. In this case the blow hole is not feasible, but the depth of the "nut" is just one diameter. I have a loose enough fit.
In the past, I did not thread the parts but did turn down the stem partially to the proper clearance leaving a thin portion full size to register the head. The register did not let me see if the solder wicked all the way.
With the new approach I was not sure if the solder would follow the much longer thread spiral all the way to the other side.
This time I wanted do do it right and ask someone who knows.
Mauro Gaetano
in Austin TX
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Harold_V
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Re: Question about silver brazing

Post by Harold_V »

When in doubt, start at the top, then move past the depth that requires solder. Once it's fluid on top, it will follow the torch.
The bleed hole isn't generally a requirement for threads, just for parts that are a slip fit. There's ample clearance for solder to flow with threads, as the major and minor diameters don't touch one another.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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