Compression valves vs. sweating copper for a beginner faucet swap?

I went thru this exact nightmare last summer with a 1970s Kohler setup. Those rusted mounting nuts are a beast—I ended up using an oscillating tool with a metal-cutting blade to carefully slice through the nut beacuse my basin wrench kept slipping. If your cabinet is alredy showing signs of water damage, I’d suggest you bite the bullet and learn to sweat the lines. Compression fittings are okay, but I’ve seen them weep over time if the copper isn't perfectly round or if there is a tiny scratch in the metal. For the dishwasher line, just make sure you leave at least three inches of clean pipe above the tee. I use a simple MAPP gas torch and some Oatey No. 95 flux. Once you get the hang of it, a soldered joint is basically permanent. If you’re nervous about the heat near the wood, get a flame protector cloth to hang behind the pipe. It gives way more peace of mind than a mechanical connection that might start a slow drip when you aren't looking.

I honestly think telling a beginner to sweat pipes inside a cramped, water-damaged cabinet is asking for a house fire. I’ve seen way too many scorched floorboards from someone trying to get a torch angle right behind a deep sink bowl. If your cabinet base is already rough, the last thing you want is an open flame near that old, dry wood. Instead of sweating or struggling with basic compression rings, grab a couple of SharkBite push-to-connect shut-off valves. People talk trash about them, but I’ve had them on my main manifold for six years without a single drip. For that dishwasher branch, they make a 1/2-inch by 3/8-inch dual-outlet stop valve that lets you run the faucet and the dishwasher lines independently. You just cut the pipe clean with a simple Ridgid tubing cutter, deburr the edge, and push it on. It saves you from the frustration of a leaky solder joint that you can't even see properly. Just make sure you mark the depth on the copper with a Sharpie before you push, so you know it's seated all the way. It’s way more reliable for a DIYer than trying to learn a specialized skill while lying on your back in a dark cupboard.

Forget the torch and the cheap compression bits. If you want a seal that’s actualy permanent without burning your house down, just pick up some SharkBite Max Push-to-Connect fittings. Since you’re worried about the dishwasher branch, they make a specific 1/2 inch x 1/2 inch x 3/8 inch compression tee that just slips right onto your cut copper. I’ve used them in dozens of cramped cabinets and they don't leak if you deburr the pipe properly. It’s way more reliable than trying to sweat a joint in a dark corner when you're a novice.

Oh wow, I'm just getting started with DIY plumbing and this thread is a lifesaver! I'm leaning towards the soldering idea becasue it sounds so permanent, but the fire risk mentioned defintely makes me nervous. Since you guys are talking about kitchen sinks, does anyone know if these same rules apply to outdoor spigots? I noticed my hose bib is leaking like crazy and I'm wondering if I can use the same flux and solder for stuff that stays outside in the freezing cold!

Spot on about the soldering being the way to go for a permanent fix, expecially with that old moisture damage. One thing people usally forget when they're learning to sweat lines in a tight spot like that is to pull the cartridges out of the new faucet before you start heating anything up. If you leave them in, the heat travels up the copper and can melt the plastic or rubber seals inside the brand-new fixture before you even turn the water on. Also, keep a soaking wet rag handy to wrap around the base of the pipe to act as a heat sink.

I had this same mess with the rusted nuts and found that a Dremel with a reinforced cutoff wheel saved my sanity. It gets into those tight spots behind the sink better than a full-sized oscillating tool. Once I got the old faucet out, I skipped the torch and the SharkBites and used high-quality brass compression valves. The trick that worked for me was using a tiny bit of pipe dope on the sleeve before tightening. It’s been three years and not a single weep. Just make sure you use two wrenches so you don't twist that old copper when you're cranking them down.