How to Use a Boring-Welding Machine for Excavator Arm Repair?

Last week, a Cat 320 rolled into my shop that had been idle for 10 days. Another shop "fixed" his boom pin hole for $800, and it blew out on day three. That lazy repair cost him $12,000 in lost work. I’ve been turning wrenches for 14 years, and 7 out of 10 boring-welding jobs fail because guys skip the basics. This is exactly what I do in my shop to make repairs last 4+ years, not 4 months.

Pre-Work That Stops Comebacks Cold

90% of bad jobs die before you plug in the welder. Worn holes are warning signs, not just wear and tear.

Tear the whole arm off the machine. Don’t try to do this on the excavator—you’ll never get proper alignment. Strip every pin, bushing, and line. Lay it on a level steel table; shim it if you have to. A wobbly table means a wobbly bore.

Scrub everything until the rag comes out white. Grease under welds causes porosity that will fail in weeks. Then check for cracks. I use a magnetic particle tester, but WD-40 and baby powder works too—cracks will show up as dark lines. Grind out any crack completely before welding; welding over it just traps stress.

Measure the hole at 12, 3, 6, and 9 o’clock. More than 0.5mm out of round? Bore it. Most importantly, measure the distance between all pin hole centers. Off by more than 0.3mm? You’ll have to correct alignment when boring.

Boring & Welding: The Secret to Zero Warping

Warping is your worst enemy. Heat one side too much, and your perfect bore will turn oval as it cools.

Mount your boring machine tight with four bolts, torqued evenly. Check runout with a dial indicator—no more than 0.02mm. A wobbly bar makes tapered holes that eat bushings.

Bore out 2-3mm past worn metal until you hit bright solid steel. Leave exactly 1.5mm welding allowance—more causes too much heat, less leaves no room for error.

Now use the intermittent opposite quadrant method I learned from a 70s-era mechanic. Divide the hole into four parts. Weld 1 inch at the top, then immediately 1 inch at the bottom. Wait 2 minutes. Weld left, then right. Wait 2 minutes. Repeat layer by layer until full. This distributes heat evenly and eliminates warping.

Use E7018 low-hydrogen rod at 120-140 amps. And the hardest rule: let it cool completely before boring. At least 4 hours, preferably overnight. Boring warm steel guarantees shrinkage and a bad hole. I’ve made this mistake twice—never again.

Finishing & Checks That Guarantee Your Work

Once stone cold, bore to exact OEM specs at 0.1mm per revolution. Fast feeds leave rough surfaces that destroy bushings. Hone for 30 seconds to get a Ra 2.0 finish—smooth enough to seat, rough enough to hold grease.

Press in bushings with a hydraulic press. Never hammer them. Chill bushings in dry ice for 10 minutes first—they’ll slide in effortlessly with no binding.

Do three non-negotiable checks:

Stretch piano wire between end holes—must pass dead center through every hole

Pin should slide in like butter through soft peanut butter

Reassemble, run full range of motion 20 times, then check for uneven wear

Final Thoughts

This isn’t rocket science, but it’s unforgiving. One shortcut and you’ll be redoing the job for free. Slow down, do it right the first time. In this business, a reputation for repairs that last is worth more than any quick buck.

Got a horror story about a botched boring job? Drop it in the comments. I check every day.