If you're building a custom suspension or a heavy-duty steering setup, you're probably going to spend some time welding heim joints into your link tubes. It's one of those tasks that seems straightforward until you realize how easy it is to warp the threads or ruin the integrity of the rod end. Whether you're working on a rock crawler, a race car, or even a high-end lawn tractor, getting these joints secured properly is the difference between a reliable rig and a mechanical failure at the worst possible moment.
Technically, you aren't usually welding the heim joint itself. You're welding a threaded insert, often called a "bung," into a piece of tubing. The heim joint then threads into that bung. However, the heat from that weld can wreak havoc on the fitment if you aren't careful. Let's walk through how to handle this without making a mess of your project.
Start with the Right Materials
Before you even pull the trigger on your welder, you've got to make sure your parts actually like each other. Most heim joints and bungs are made from either mild steel or 4130 chromoly. If you're building something for high-stress off-road use, chromoly is usually the go-to choice because of its strength-to-weight ratio.
You need to make sure the outer diameter of your bung matches the inner diameter of your tubing. A sloppy fit is a recipe for a weak weld. Ideally, the bung should slide in with just a tiny bit of resistance. If it's rattling around in there, you might need to find a different tube size or have someone turn down a custom insert on a lathe.
Also, check your threads. Make sure you know if you're working with right-hand or left-hand threads. It sounds like a "no-brainer," but I've seen plenty of guys weld two right-hand bungs onto a single link, only to realize later they can't adjust the length of the bar without taking the whole thing apart.
Cleaning and Prep Work
If you want a weld that actually penetrates, you can't be lazy about prep. Most tubing comes with a layer of mill scale—that dark, flaky coating—and bungs might have some machining oil on them.
Grab a flap disc on an angle grinder and take the tube end down to shiny metal. You should also put a slight bevel on the end of the tube. This creates a little "V" groove where the tube meets the shoulder of the bung, allowing the weld puddle to sit deeper into the joint. It's the best way to ensure you aren't just laying a bead on top of the metal.
Once you've ground everything down, wipe the area with some acetone or brake cleaner (the non-chlorinated stuff, please). Any grease or residue left behind will contaminate the weld, leading to porosity—which is just a fancy way of saying your weld will look like Swiss cheese and have about the same amount of strength.
The Heat Management Problem
The biggest mistake people make when welding heim joints (or rather, their bungs) is overdoing the heat. When you get that bung red-hot, the metal expands and can slightly distort. Once it cools back down, you might find that your heim joint won't thread in anymore because the internal threads have shrunk or gone out of round.
A common trick is to thread a "dummy" bolt or an old, sacrificial heim joint into the bung while you weld. This acts as a heat sink and helps the bung hold its shape. However, you have to be careful here. If you use a real heim joint with a Teflon or nylon liner, the heat will melt that liner instantly, and you'll have just ruined an expensive part. If you do use a bolt, don't leave it in there too long, or you might accidentally weld the bolt to the bung if the penetration is deep enough.
Personally, I prefer to weld the bung "empty" but do it in stages. I'll throw four solid tacks around the circumference to keep it straight, then weld it in quarters. Weld one-fourth of the way around, move to the opposite side, let it cool for a minute, and repeat. It takes longer, but it saves you from having to chase the threads with a tap later.
TIG vs. MIG for Bungs
You can use either a MIG or TIG welder for this, depending on what you have in the shop and what the application is.
TIG welding is the gold standard here. It gives you way more control over the heat and results in a much cleaner, stronger joint. If you're working with chromoly, TIG is almost a requirement so you can manage the heat-affected zone and ensure the metal doesn't become brittle. Plus, a nice stack of TIG "dimes" just looks better on a custom build.
MIG welding is perfectly fine for mild steel setups, like a basic 4-link on a trail rig. It's faster and easier for most hobbyists. Just make sure your settings are dialed in. You want to see that puddle "wetting out" into both the tube and the bung. If the bead looks like a cold rope sitting on top of the metal, you need to turn up the heat or slow down your travel speed.
Cooling Down Naturally
Once you've finished the weld, put the torch down and walk away. Don't be tempted to dunk the hot part in a bucket of water or blast it with compressed air to cool it down faster.
Quickly quenching the metal can make the weld brittle and prone to cracking under stress. This is especially true for chromoly or high-carbon steels. Just let it air cool on the bench. It gives the molecular structure of the steel time to settle down properly. Once it's cool enough to touch with your bare hand, that's when you can grab your heim joints and start the final assembly.
Final Fitment and Anti-Seize
After everything is cooled off, it's time to see if the heim joints actually fit. If you did your job right, they should thread in by hand. If they're a little tight, you might need to run a thread chaser or a tap through the bung to clean out any minor distortions or bits of spatter that found their way inside.
Before you thread them in for good, always use anti-seize. Steel-on-steel threads have a nasty habit of "galling" or seizing up over time, especially if they're exposed to salt, mud, or moisture. A little bit of silver or copper anti-seize on the threads will save you a massive headache a year from now when you need to adjust your alignment.
Safety and Testing
I know it sounds like a lecture, but remember that these joints are usually holding your vehicle's axles or steering in place. If a weld fails while you're doing 60 mph on a washboard road, things are going to get ugly fast.
Always inspect your beads for any signs of cracking or undercut. If you aren't confident in your weld, grind it out and try again, or take it to a professional. There's no shame in having a pro welder handle the critical suspension components if you're still learning the ropes.
Welding heim joints correctly really comes down to patience and prep. If you take the time to clean the metal, manage your heat, and let the parts cool naturally, you'll end up with a setup that can handle whatever abuse you throw at it. Now, get out to the garage and start building!