What Is the Theory of Drift? A Used Terex Equipment Buying Lesson

I’m not a heavy equipment mechanic. I’m the office administrator who handles purchasing for a 70-person site construction company—roughly $450,000 a year in equipment and parts orders across nine vendors. I’ve been in this role since 2020, and I still ask our lead operator before every major decision. But after a near-miss with a used Terex excavator in 2024, I had to learn enough about hydraulic drift to ask the right questions.

The Problem I Thought I Was Solving

Our project manager asked me to find a used compact excavator. The lead operator wanted a 2016 Terex R070T—a 7-ton tracked machine with around 2,900 hours—from a dealer two states away. It was priced reasonably, and the photos looked clean. I also found an older Terex dozer 82-30 from a local seller. It was cheap, and part of our crew thought it could be useful for rough grading. On paper, the decision was about model year, hours, and price.

It wasn’t. It was about drift.

So, What Is the Theory of Drift?

Search “drift” and you’ll get clips of a 2024 Bentley GT sliding sideways at a track event. That looks cool, but it’s not what I mean. Search “what is the theory of drift” and you’ll get geology articles about continental plates. The principle is actually similar: slow movement under pressure, until something has to give.

In hydraulic equipment, drift is the slow movement of a cylinder when you didn’t ask it to move. When you raise a boom or a blade and neutralize the controls, internal seals are supposed to hold the oil in place. If the seals are worn—or the oil is dirty, or the cylinder is scored—pressure leaks past the seal. The attachment settles. It might be an inch in five minutes, or five inches. That’s drift.

You could hold a second congress on fluid power to argue about all the formulas behind it. For a buyer, the exact math matters less than knowing the machine has internal wear that a clean paint job won’t show.

Why Drift Changes Everything on a Used Terex

On a Terex dozer 82-30, blade drift can ruin fine grading. The operator spends more time correcting the blade than moving dirt. On the 2016 Terex R070T, boom drift is even more uncomfortable to watch, because a bucket or attachment can move closer to a worker or a trench wall without warning.

Now, I’m not a hydraulic engineer, so I can’t tell you exactly which seal or spool is to blame from a phone photo. What I can tell you from a purchasing perspective is that drift is a signal. It tells you the machine has internal leakage, and internal leakage usually means a repair bill is coming.

Part of me wanted to ignore it. Another part remembered a $4,800 cylinder rebuild on a loader I bought in 2023—not a Terex, but the lesson applies everywhere. The machine looked great at auction. It drifted badly by the time it reached our yard. That cost us a week of rental income and a late schedule.

The Real Cost of Buying the Wrong Machine

Let’s put numbers on it. A hydraulic cylinder rebuild on a compact excavator like the R070T can run anywhere from $2,500 to $7,000, depending on the cylinder, labor, and whether the pump is compromised. For an old Terex dozer 82-30, parts availability is the bigger risk. Terex still supports legacy models through its parts network, but sourcing parts for a machine of that age isn’t as fast as it used to be. Our dealer in Ontario told me to budget for longer lead times if we bought the 82-30.

Ignore drift and you’re not just paying for repairs. You’re paying for downtime. You’re paying for the operator who has to fight the controls. You’re paying for the safety issue when a loaded bucket settles toward someone’s boots. In my experience, those costs add up faster than the difference between a good price and a suspiciously good price.

The Drift Test I Now Ask For

Here’s the thing: drift isn’t a hidden flaw. It’s measurable. You just have to ask to see it before handing over money.

1. Warm the machine to operating temperature. Cold seals can hide internal wear.
2. Raise the boom or blade a few feet off the ground.
3. Hold the position, then shut off the engine and neutralize the controls.
4. Mark a reference point on the cylinder rod or the attachment, and time it.
5. Watch for movement over five to ten minutes. Terex service manuals for the R070T have a maximum drift spec for this check, and a dealer can look it up for you.

If the seller won’t run the test, that’s a red flag. I’d walk. If they do run it, and there’s a little drift, that’s not automatically a deal-breaker. It depends on the price and what’s causing it. But now you can negotiate from a position of information instead of hope.

Our dealer also recommended a hydraulic oil sample before finalizing any used equipment purchase. That’s one of those checks I used to skip because it felt like extra work. It’s not. (Should mention: our 2024 vendor consolidation project taught me to verify the basics before trusting a shiny invoice.)

What I’d Do Differently

If I could go back, I wouldn’t have spent so much time comparing spec sheets and photos. I’d have asked one question first: “Does it drift?” The 2016 Terex R070T we looked at? It drifted 3 inches in five minutes on a warm machine. The dealer was honest about it, adjusted the price, and the repairs cost less because we knew going in. The old Terex dozer 82-30 is still at that local seller’s lot, as far as I know. I never called back. Maybe that’s fine—somebody else will buy it and learn the same lesson I did.

Knowing the theory of drift doesn’t make me a mechanic. It makes me a better buyer. That’s good enough for me.