MIG welding is the fastest way to fabricate or repair an exhaust system. Most aftermarket and repair exhaust work uses mild steel pipe in 16-18 gauge (0.048-0.063"), which MIG handles with 0.030" wire, 75/25 gas, and settings around 17-19 volts. Set up the pipe, tack it in four spots, and weld downhill around the circumference. A complete exhaust joint takes 5-10 minutes with MIG versus 20-30 minutes with TIG.
For stainless steel exhaust systems, switch to ER308L wire with 98/2 argon/CO2 or tri-mix gas. The settings and technique are similar, but you need to manage heat more carefully because stainless retains heat longer and warps more easily than mild steel.
Exhaust Pipe Materials
Mild Steel (Most Common)
The majority of stock and aftermarket exhaust systems are mild steel, sometimes with an aluminized coating for corrosion resistance. Mild steel exhaust pipe is cheap, easy to weld, and available everywhere. Standard wall thickness runs 16 gauge (0.063") for mufflers and pipes, with 18 gauge (0.048") on some lighter-duty sections.
Weld mild steel exhaust with ER70S-6 wire and 75/25 argon/CO2 gas. Standard MIG setup, nothing special.
The downside of mild steel is corrosion. Exhaust systems run through heating and cooling cycles with condensation forming inside the pipes. This moisture, combined with combustion byproducts (sulfuric acid, hydrochloric acid, water vapor), eats mild steel from the inside out. A mild steel exhaust on a daily driver lasts 3-7 years depending on climate and driving conditions.
Aluminized Steel
Aluminized steel has a thin aluminum-silicon coating that resists corrosion better than bare mild steel. Many OEM exhaust systems use aluminized pipe. You can MIG weld aluminized pipe the same as mild steel, but the aluminum coating burns off in the weld zone, which eliminates the corrosion protection at the joint. Weld-through primer or high-temp paint after welding helps, but the weld area will still corrode first.
The aluminum coating produces fume when welded. Keep ventilation adequate.
409 Stainless Steel
409 is a ferritic stainless used in many OEM and aftermarket exhaust systems. It costs more than mild steel but resists exhaust corrosion much longer (8-15+ years). Weld 409 with ER309L wire and 98/2 argon/CO2 gas. The dissimilar filler (309L on 409) provides a ductile, corrosion-resistant weld.
409 stainless is magnetic (unlike 304/316), so a magnet test alone won’t distinguish it from mild steel. Check the pipe markings or consult the manufacturer.
304 Stainless Steel
304 stainless is used on performance exhaust systems, headers, and show-quality builds. It’s the most corrosion-resistant option but also the most expensive and hardest to weld without discoloration. Weld with ER308L wire and 98/2 or tri-mix gas. Keep heat input low to minimize the blue and purple heat tint.
For visible 304 stainless exhaust, TIG welding produces far better-looking results than MIG. The stacked-dime TIG bead on a stainless header is part of the aesthetic. MIG is functional but won’t win any beauty contests on exposed stainless.
321 Stainless Steel
321 stainless adds titanium for stabilization against carbide precipitation at high temperatures. It’s used in turbo manifolds, downpipes, and header collectors where temperatures regularly exceed 1,000 F (538 C). Weld with ER347 wire. This is specialty fabrication territory.
Wire and Gas Selection
| Exhaust Material | MIG Wire | Shielding Gas | Polarity |
|---|---|---|---|
| Mild steel / aluminized | ER70S-6 (0.030" or 0.035") | 75/25 Ar/CO2 | DCEP |
| 409 stainless | ER309L (0.030" or 0.035") | 98/2 Ar/CO2 or tri-mix | DCEP |
| 304 stainless | ER308L (0.030" or 0.035") | 98/2 Ar/CO2 or tri-mix | DCEP |
| Mild steel to stainless | ER309L | 98/2 Ar/CO2 | DCEP |
Use 0.030" wire for 18-gauge pipe and thinner. Use 0.035" for 16-gauge pipe and heavier sections like flanges and collectors.
MIG Settings for Exhaust Pipe
| Pipe Gauge | Wire | Voltage | Wire Speed (IPM) | Gas Flow (CFH) |
|---|---|---|---|---|
| 18 ga (0.048") mild steel | 0.030" | 17-18 | 230-280 | 25-30 |
| 16 ga (0.063") mild steel | 0.030" | 17-19 | 260-310 | 25-30 |
| 16 ga (0.063") mild steel | 0.035" | 18-20 | 250-300 | 25-30 |
| 14 ga (0.075") collector/flange | 0.035" | 19-21 | 280-340 | 25-30 |
| 18 ga 304 stainless | 0.030" ER308L | 16-18 | 210-260 | 25-30 |
| 16 ga 304 stainless | 0.030" ER308L | 17-19 | 240-290 | 25-30 |
These settings produce a short circuit transfer arc that works well on thin-wall tubing. If you’re getting burn-through, reduce wire speed by 20-30 IPM. If the wire is stubbing and not melting, increase voltage by 1 volt.
Joint Types for Exhaust Work
Slip Joint (Most Common)
One pipe slides inside another with 1-1/2" to 2" of overlap. The outer pipe is either expanded slightly or a larger-diameter section is used as a sleeve. This is the easiest joint for exhaust work because it requires minimal fitting and provides a lap joint that’s easy to weld.
Weld around the outside edge of the overlap. You can run a continuous bead around the circumference, or tack in four spots and stitch between them if heat management is a concern. The overlap provides built-in backing for the weld.
Butt Joint
Two pipes meet end-to-end with no overlap. This requires precise cutting and fitting because any gap on thin-wall tubing leads to burn-through. Butt joints are used when you need a smooth interior (for exhaust flow) or when joining pipes of the same diameter without a sleeve.
Tack in four spots (12, 3, 6, and 9 o’clock positions). Then weld between tacks, alternating sides to distribute heat evenly. On thin tubing, use stitch welds with cooling time between passes.
V-Band Clamp Joint
Two flanges welded to the pipe ends, joined by a V-band clamp. This creates a removable connection that’s popular on performance exhaust, turbo downpipes, and anywhere you need to disassemble the exhaust. Weld the flanges to the pipe, then the clamp holds the system together without welding the joint.
Flange Joint
A flat or formed flange welded to the pipe end, bolted to a mating flange. Common at the header-to-pipe connection and at the catalytic converter. Flanges are thicker than the pipe (1/8" to 3/16" typical), so bump your settings up slightly for the flange-to-pipe weld. Concentrate more heat on the flange side to avoid burning through the thinner pipe.
Welding Technique for Round Pipe
Welding around a pipe is different from welding a flat joint because the position changes continuously as you travel around the circumference. At 12 o’clock you’re welding flat. At 3 and 9 o’clock you’re welding horizontal. At 6 o’clock you’re overhead.
Downhill Method (Preferred for Thin Pipe)
Start at 12 o’clock (top dead center) and weld downhill to 6 o’clock (bottom) on one side. Return to 12 o’clock and weld downhill on the other side. The two beads overlap at the top and bottom.
Downhill is the preferred method for thin-wall exhaust tubing because:
- Fast travel speed reduces heat input
- Gravity helps the puddle flow ahead of the arc
- The bead stays thin and flat, matching the thin-wall pipe
Maintain a 10-15 degree drag angle (gun pointing slightly downhill ahead of the puddle). Adjust your body position as you move around the pipe to keep a comfortable arm angle. This usually means repositioning yourself at least once per side.
Tack and Stitch Method
For pipes that are difficult to access or where you can’t rotate the pipe:
- Tack at 12, 3, 6, and 9 o’clock.
- Weld 1" stitches between tacks, skipping around the pipe.
- Fill in the remaining gaps after the stitches cool.
This method takes longer but gives you the most control over heat input and allows you to work in tight spaces where a continuous bead isn’t practical.
Rotating the Pipe
If the pipe is off the vehicle and you can mount it in a vise or fixture, rotate the pipe to keep the weld at the top (flat position) at all times. This is the easiest way to weld exhaust pipe. Roll the pipe as you weld so you’re always depositing metal in the flat position. The bead quality is better, and burn-through is less likely because gravity works in your favor.
Dealing with Rusty Exhaust Pipe
Exhaust pipes rust from the inside out. By the time the outside shows rust, the inside may be significantly thinner than the original gauge. Welding on rusted exhaust pipe is frustrating because:
- The true wall thickness is unknown. A pipe that started as 16 gauge may be 22 gauge or thinner where it’s rusted.
- Rust contamination causes porosity. Even if you grind the surface, embedded oxides and moisture in the pitted metal outgas during welding.
- Heat burns through rust-thinned areas instantly.
Making It Work
Grind the weld area. Remove all surface rust, scale, and coating from the joint area. Get to bright metal for at least 1" on each side of the weld. A flap disc or wire wheel works. If you can see through the metal after grinding, you’ve found a spot that’s too thin to weld. Cut it out and add new pipe.
Test the wall thickness. Tap the pipe with a screwdriver. Solid pipe rings. Rust-weakened pipe thuds. If you can poke through with a screwdriver, that section needs replacement, not repair.
Lower your settings. If the pipe is thinner than expected from rust, your normal settings blow right through. Start at the lowest settings your machine offers and work up.
Use a sleeve. Rather than trying to butt-weld rusty pipe, slip a new section over (or inside) the damaged area and weld to sound metal on both sides. This bypasses the rusty zone entirely and is the most reliable repair method.
Back Purging Stainless Exhaust
When MIG welding stainless exhaust pipe, the inside of the pipe oxidizes at welding temperatures. This shows up as a dark, rough scale on the inner surface (sugaring). On a performance exhaust, this rough interior surface disrupts gas flow and can break off and enter the engine or turbo.
Back purging fills the inside of the pipe with argon to prevent oxidation. For exhaust work:
- Plug the pipe ends near the joint with aluminum foil, tape, or purpose-built purge plugs.
- Insert an argon supply line through a small gap and flow argon at 5-10 CFH.
- Purge for 3-5 minutes before welding (or until an oxygen analyzer reads below 1%).
- Maintain the purge throughout welding.
For non-critical daily-driver stainless exhaust, many fabricators skip the purge. The inner oxidation doesn’t affect structural integrity and won’t be visible. For turbo applications and show-quality builds, purge every joint.
Exhaust Manifold and Header Welding
Cast iron exhaust manifolds are covered in the MIG welding cast iron article. Short version: it’s doable with nickel wire and preheat but not a job for beginners.
Tubular steel headers are welded from thin-wall tubing (16-18 gauge) and flanges (1/8" to 3/8" thick). The challenge is the dissimilar thickness at the tube-to-flange joint. Aim your arc toward the flange and let heat transfer to the thinner tube. If you point the arc at the tube, you’ll burn through while the flange stays cold.
Stainless headers (304 or 321) require careful heat management. The tight bends and clustered tubes create heat buildup zones where multiple tubes are close together. Alternate between tubes, welding one joint and letting it cool while you work on another.
Common Exhaust Welding Problems
Burn-through. The most common problem on thin-wall exhaust. Reduce settings, increase travel speed, use downhill technique. On slip joints, the overlap provides some protection. On butt joints, you’re exposed.
Porosity. Contaminated base metal (rust, oil, exhaust deposits) or inadequate gas coverage. Grind all surfaces clean. Check gas flow and nozzle.
Cracking at the weld joint. Exhaust systems experience thermal cycling (heat up when driving, cool down when parked). This cyclic stress can crack welds, especially at stress risers like undercut or incomplete fusion. Use smooth, well-fused beads with no sharp transitions at the toes.
Leaks. Pinholes in the weld, incomplete fusion, or missed spots. Test the exhaust with the engine running and feel for leaks. A smoke test (block the tailpipe and use a smoke machine) reveals tiny leaks that you can’t feel by hand.
Distortion. Thin pipes warp from welding heat, causing alignment problems. Use tack-and-stitch technique, alternate sides, and let each section cool before moving to the next.
Safety for Exhaust Welding
Work on exhaust systems outdoors or in a well-ventilated shop. Exhaust pipes contain residual carbon monoxide, unburned hydrocarbons, and combustion byproducts. Heating these residues releases toxic fumes. If you’re welding under a vehicle, use a fume extractor or fan to move air across the work area.
Galvanized and aluminized coatings produce toxic fumes (zinc oxide, aluminum oxide) when welded. Grind off coatings where possible and use respiratory protection.
Exhaust components on a vehicle are near fuel lines, brake lines, electrical wiring, and the gas tank. Verify the routing of all nearby components before striking an arc. A stray spark or conducted heat can melt a brake line or ignite fuel vapor. Keep a fire extinguisher nearby and an escape plan in mind.
If the vehicle is on jack stands, verify stability before getting underneath to weld. A vehicle falling off stands while you’re under it with a welding gun in your hand is a worst-case scenario. Use quality stands rated for the vehicle’s weight and place them on solid, level ground.