The Old Tradesman

The tips library

Sweat it right

Solder doesn't get pushed into a copper joint. It gets pulled in, and every tip on this page is about giving it a clean, tight, hot place to go.

Tip 1

Ream to the full bore, then take the burr off the outside

The trick

Every cut end gets reamed back to the full inside diameter of the tube, and the burr on the outside comes off too so the tube slides into the cup.

Why. That little curl the cutter leaves inside makes turbulence and faster flow right at the joint. The Copper Development Association says that is how erosion-corrosion gets started.

The tell. Run a fingertip around the inside of the cut. A ridge you can feel, or a bright ring of curl, means it isn't reamed. On soft tube, an end gone oval means you leaned on the reamer too hard.

A half-round or round file, a pocket knife or a deburring tool all do it. On soft tube, go easy. The handbook warns that too much pressure deforms the end, and a sizing tool (a plug and a collar) brings it back.

When to stop. Soft tube pushed out of round goes back round with a sizing tool, or gets cut back to round copper. Stub-out too short to cut back? Extending it is a plumber's job.

Sources

Tip 2

Clean a hair past the cup depth, then keep your hands off it

The trick

Sand the tube end for a distance slightly more than the depth of the fitting cup. Clean the cup with abrasive cloth, a pad or a fitting brush that fits it. Then no bare hands and no oily gloves on either one.

Why. Solder only wets bright copper. The handbook says skin oil, lubricating oil and grease impair the soldering, and dirt left behind can interfere with the capillary draw and weaken the joint.

The tell. Put it together and look at the cup edge. A band of bright copper should show just past it. Dull copper right at the lip says you cleaned short.

When to stop. Don't sand until it slops. Take off too much metal and the handbook says the loose fit makes a poor joint. A fitting that rattles gets a new fitting or a fresh end, not extra solder.

Sources

Tip 3

Mind the gap: four to six thousandths

The number

Capillary action pulls solder best when the gap between tube and fitting is 0.004 to 0.006 inch. That's a snug slip fit, not a sloppy one.

Joint gap, cut awayA copper tube inside a fitting cup, cut away. The gap between them, 0.004 to 0.006 inch, fills with solder. A snug gap fills all the way; a loose gap leaves voids.TUBE IN THE CUP, CUT AWAYTUBEwater sideFitting cupGap: 0.004 to 0.006 inSolder fills itSnug: pulledin all the wayLoose: voidsand leaksGap drawn far bigger than life.

Why. Solder doesn't get pushed in, it gets pulled in, and a narrow, even gap does the pulling. Open it up and the pull falls off. The handbook says too loose a fit causes trouble, worst on larger fittings.

The tell. Dry fit it before you light anything. It should slide on snug with no rattle. A fitting that wobbles on the tube is telling you the joint will come out starved.

Support the tube and fitting square while you heat so the gap stays even all the way around. A joint hanging crooked is tight on one side and wide open on the other, and the open side is where it leaks.

When to stop. Over-sanded tube or a cup that rattles: cut a fresh end or get a new fitting. Feeding extra solder into a loose joint doesn't fill it.

Sources

Tip 4

Thin coat of flux, both parts, with a brush

The number

Brush a thin, even coat on the tube end and inside the cup as soon as they're cleaned. Never with your fingers. The handbook calls for a flux that meets ASTM B813.

Why. Flux takes off the last trace of oxide and keeps clean copper from oxidizing again while it heats. What's left over keeps working, and the handbook says in extreme cases flux residue can eat clear through the tube or fitting wall.

The tell. Green crust or pitting around an old joint, or flux running out of a fresh one in streaks. Somebody loaded it on.

For a sense of scale, the handbook figures flux at about 2 ounces per pound of solder. A thin coat really is thin.

When to stop. Fluxed but not soldered by quitting time? The copper folks' Soldering School says a prepared joint gets finished the same day, so don't leave it overnight.

Sources

Tip 5

Start the flame on the tube, then sweep into the cup

The number

Flame on the tube first, square to it. Then move onto the cup and sweep back and forth between cup and tube over a distance about equal to the cup depth. Never point the flame into the mouth of the cup.

Heat order for a sweat jointStep 1: flame on the tube, square to it. Step 2: move to the cup and sweep between cup and tube over about one cup depth. Never point the flame into the mouth of the cup; it burns the flux.HEAT ORDER, SIDE VIEWcup depth1Flame on the tubefirst, square to it2Then onto the cup. Sweepcup to tube, about onecup depth back and forthNever into thecup mouth: itburns the flux

Why. The tube carries the first heat into the cup and spreads it evenly. A flame aimed into the cup face burns the flux, and burnt flux won't let the solder in.

The tell. Black flux at the lip and solder that balls up instead of disappearing. On a level joint, a dry spot at the top means somebody heated the top. Heat rises: work the bottom and leave the top alone.

Folks say heat the fitting, not the pipe. That's half right. The handbook starts on the tube, then moves to the cup. On a level joint it says don't preheat the top directly, because that's where the flux burns first. How long you preheat depends on the size of the joint.

When to stop. Joint in a wall or ceiling next to wood, insulation or wiring, and you can't get a heat shield behind it? That's a plumber's joint, or a flameless electric soldering tool.

Sources

Tip 6

Let the copper melt the solder, and build it from the bottom

The trick

Touch the wire to the joint, never the flame. No melt? Pull it away and keep heating. Once it melts, start a little off center at the bottom, torch at the base of the cup just ahead of the wire, up one side to the top, back to the start with a little overlap, then up the other side.

Why. Solder runs toward heat. If the copper melts it, the joint is hot enough to draw it in. The bottom sets first and acts as a dam, so the sides and top fill instead of running out.

The tell. Solder that beads up and drips off is sitting on a cold joint. When the joint is full, small drops show up behind where you're feeding.

When to stop. Copper that won't melt the solder no matter how long you heat it usually has water in the line. If the shutoff won't stop the trickle, put the torch down and shut the water at the main.

Sources

Tip 7

Feed about a diameter of wire, and stop when it drips

The number

The copper handbook's table, in 1/8 inch wire: a 1/2 inch joint holds about 1/3 to 1/2 inch, a 3/4 inch joint about 2/3 to 1 inch, a 1 inch joint about 1 to 1-1/2 inches. Call it a length of wire about the size of the pipe.

How much solder a joint takesLengths of 1/8 inch wire solder drawn to one scale: about 1/3 to 1/2 inch for a 1/2 inch joint, 2/3 to 1 inch for 3/4 inch, 1 to 1-1/2 inches for 1 inch. Fill only, no waste. Stop feeding when drops show all the way around.WIRE PER JOINT, ONE SCALE1/2 in pipeabout 1/3 to 1/2 in3/4 in pipeabout 2/3 to 1 in1 in pipeabout 1 to 1-1/2 in01/2 in1 in1-1/2 in1/8 in wire, 0.004 to 0.006 in gap.Fill only, no waste. Drops all theway around: stop feeding.

Why. Those numbers are what fills a 0.004 to 0.006 inch gap, and that's all the joint can hold. Wire fed past that has nowhere to go but off the fitting or inside the pipe.

The tell. The handbook says small drops show up behind where you're feeding once the joint is full to that point. Drops all the way around, quit feeding.

The table figures 1/8 inch (No. 9) wire solder and the ideal gap, with no allowance for waste. The handbook's own weight column adds 100 percent for waste and loss, and it says actual use depends on workmanship. So the inches are the fill, not what leaves the spool.

When to stop. Joint keeps swallowing wire well past the table with no drops? The gap is too wide. Stop feeding, cut it out and remake it with a fitting that fits.

Sources

Tip 8

Soldering a valve: crack it open and heat the tube

The number

Valve partly open before the flame goes on, and the heat goes mainly into the tube, not the valve body. The valve maker's sheet comes first, and heat-sink gel helps.

Sweating a valve inA valve between two copper tubes with its handle partly open. The torch flame is on the tube, not the valve body, where the seats and seals are. Heat-sink gel helps. Follow the valve maker's sheet first.SWEATING A VALVE INHandlepartly openHeat goesinto the tubeBody: keep the flame off.Seats and seals live here.Heat-sink gel helps.Valve maker's sheet first.No-lead brass: preheat the tube more,until the flux starts to bubble.

Why. The seats and seals sit right next to the joint. Open, the cavity isn't sealed up while it heats, and heat fed through the tube gets into the cup without cooking the body.

The tell. A brand new valve that drips at the stem or won't shut all the way got cooked going in.

Newer no-lead brass takes heat differently. The Construction Specifier says to preheat the tube more than you used to, until the flux at the face of the joint starts to bubble and clean, and to preheat the fitting from the back of the cup toward its face.

When to stop. Gas valves are not on this page. Gas piping is licensed work with a pressure test.

Sources

Tip 9

Let it cool on its own, then wipe the flux off

The trick

No wet rag on a hot joint, no water, no bumping it. Let it cool by itself. Once it's cool, wipe the leftover flux off the outside with a wet rag.

Why. The copper folks' Soldering School says shock cooling with water may stress the joint. Flux left on the outside keeps working as a corrosive.

The tell. Green stain or pitting around an old joint is flux nobody wiped.

When to stop. Leaks when the water comes back on? Don't heat it with water in the line and pile on solder. Drain it, take it apart, clean it and make it again.

Sources

The Old Tradesman is a cartoon character. Every tip on this page stands on the sources listed under it. He is the voice. They are the authority. The drawings are diagrams, not photographs.

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