What is the typical clearance between the outer diameter and inner diameter of a hollow-stem auger?

• A. 1-2 inches
• B. 6-7 inches
• C. 4-5 inches
• D. 3-4 inches

Answer: (C)

The key idea here is the wall thickness of a hollow-stem auger—the difference between the outside diameter and the inside bore. This clearance must be enough to give the stem structural strength while leaving a usable inner passage for the sampling apparatus and any fluids or cuttings to move through.

In typical hollow-stem augers used for drilling and sampling, that wall thickness is about four to five inches. That size provides a sturdy, rigid tube that resists bending and wear, yet keeps a reasonably sized inner bore for the equipment and operation. A thinner gap, like one to two inches, would make the wall too fragile and the inner passage too small; a gap as large as six to seven inches would make the wall unnecessarily thick and heavy and would reduce the inner bore more than needed. The four to five inch range hits the practical balance, which is why it’s the best choice.