What Is a Drill String Drift? And Why You Shouldn't Ignore It

I learned what a drift was the expensive way.

My first year handling drill string orders at our mining supply operation, I saw the spec sheet for a new DTH hammer: 4-inch. I ordered 4-inch rods, 4-inch bits, everything looked fine on paper. The first job site call came at 4:30 PM on a Friday.

"The rods won't fit the hammer."

Turned out the hammer had a 4-inch outer diameter, but the pin connection was 3.5-inch API Reg with a specific drift diameter I'd never checked. The rods I ordered had a box end with a smaller ID than the hammer's pin required. That mismatch cost $2,800 in return shipping, restocking fees, and a 3-day rental of a different rig. The project manager wasn't happy. I wasn't happy.

So let's talk about what drift actually means in the context of Atlas Copco tooling and why it matters more than you probably think.

No Single Answer: Your Scenario Determines the Priority

There isn't one "correct" drift specification that fits every operation. It depends on:

• Your hammer type and connection standard
• How deep you're drilling
• Whether you're using water, foam, or air for cuttings removal
• What brand of rods and bits you're pairing with Atlas Copco hammers

I'm going to break this down into three common scenarios. Find the one that matches your situation.

Scenario A: You're Mixing Brands (Most Common, Most Risky)

This is where I see the most mistakes—because no one likes to admit they're mixing Atlas Copco hammers with cheaper Chinese or refurbished rods to save money.

Here's the thing: Atlas Copco hammers (like the COP series, or the Secoroc DTH hammers) are designed to specific API standards. But the drift diameter—the smallest internal diameter through the tool joint that the cuttings must pass through—can vary by as little as 1/16 of an inch between brands. And that 1/16 inch difference will cause a blockage if you're drilling through sticky formations or using foam.

What I do now: Before any mixed-brand order, I take a physical drift mandrel (basically a rod with a known diameter) and check the box end of every rod against the pin end of the hammer. It takes 30 seconds per joint and has saved us from at least 5 mismatches in the past 3 years.

If you don't have a drift mandrel: Use a caliper. Measure the ID of the rod box end and the OD of the hammer pin at the narrowest point (usually at the shoulder). The ID of the box must be at least 0.010 inches larger than the OD of the pin for a safe fit.

Scenario B: You're Perfectionist About Cuttings Removal

Most buyers focus on tensile strength and thread type. They completely miss the fact that drift diameter determines how fast you can move cuttings out of the hole.

We had a job in Q3 2023 where we were drilling 150-foot holes in decomposed granite. The air flow was adequate on paper—800 CFM at 350 psi. But we kept getting stuck at about 90 feet. Pulling the string, checking everything, putting it back down, getting stuck again.

Turns out the drift on our rods was 0.125 inches narrower than the hammer's internal bore. That tiny restriction was creating a bottleneck at every tool joint. The air speed through the drift was too high, causing turbulence that actually trapped the cuttings instead of blowing them out.

Fix: We switched to rods with a drift diameter that matched or exceeded the hammer's internal bore. Problem solved, drilling speed increased by about 15%. On a $3,200 dayrate rig, that's meaningful.

Per Atlas Copco's own literature—and I've confirmed this with their tech support—drift diameter should be matched to the hammer's internal bore, not just the connection size. The tool joint is the narrowest point in the entire string. If you choke it, you choke your production.

Scenario C: You're Only Using Atlas Copco Tooling (The "Safe" Route)

If you're running a full Atlas Copco string—hammer, rods, bits, all from the same OEM—drift is usually not something you have to stress about. Atlas Copco designs their components to match each other.

But—and this is where I made my second mistake—even within OEM-matched strings, drift changes when you add a crossover sub or a shock sub. I once ordered a used Atlas Copco hammer that came with a crossover sub that reduced the drift by nearly 0.2 inches. That sub was meant for a different hammer model. The result: same problem I described in Scenario B, but with a string where everything looked compatible.

So even if you're staying all Atlas Copco, check the drift at every connection point in the string. Don't assume that because the threads mate, the drift is adequate.

How to Figure Out Your Scenario

If you're setting up a string right now and wondering what to check:

1. Get the manufacturer's spec sheet for your hammer. Look for "drift diameter" or "minimum bore" at the pin connection.
2. Get the spec sheet for your rods. Look for "box bore" or "pin bore"—whichever is smaller is your drift.
3. If the rod drift is smaller than the hammer drift, you have a restriction at every tool joint.
4. If you're adding any adapters or crossover subs, they're often the worst offenders. Measure those separately.
5. If you're in a hurry and can't get spec sheets, ask your distributor for drift numbers. If they can't provide them, that's a red flag.

My rule of thumb: I aim for a drift that is at least as large as the hammer's indicated bore. If I have to go smaller, I'm okay with a reduction of up to 1/8 inch for shallow holes (under 50 feet), but for deeper drilling? I want it equal or larger.

I cannot stress this enough: drift is one of those specs that looks minor on a PDF but causes a major headache at the drill site. And it's not just about compatibility—it's about efficiency. If your air flow is fine but your penetration is slow, check your drift before you spend money on a bigger compressor or a new hammer.

I've been doing this for about six years now—or rather, five and a half—and I still find myself checking these numbers more carefully than I used to. Because one $2,800 lesson was enough for me.