Last week we looked at core lifters — the small part that determines whether a well-selected bit and barrel actually deliver recovered core. This week we step back to a decision that gets made before any of that: which drill rod series to run in the first place. The BQ, NQ, HQ, and PQ designations show up on every quote and every spec sheet, but the reasoning behind choosing one over another is worth spelling out clearly.

Wireline drill rod sizes follow the DCDMA (Diamond Core Drill Manufacturers Association) system, where each size designation corresponds to a fixed set of outer diameter, inner diameter, and coupling dimensions. The letter prefix — B, N, H, P — indicates the general size class, moving from smaller to larger: B is the smallest of the four common series, N and H are the mid-range workhorses, and P is the largest standard size in general exploration use. The "Q" suffix specifically denotes the wireline series, as opposed to conventional (non-wireline) rod of the same size class.

Because rod, core barrel, bit, casing, and core lifter all need to be size-matched within the same series — a point we've returned to throughout this whole series of articles — the rod size chosen at the start of a project effectively locks in the size of every other component that follows it down the hole.

BQ is the smallest standard wireline series and produces the smallest core diameter of the four. It's typically chosen when hole size needs to be minimized — for tight access sites, for very deep holes where reducing rod weight and torque requirements matters, or for programs where a smaller core diameter is acceptable for the assay or geotechnical purpose at hand. The tradeoff is a smaller core sample and, in some ground conditions, less stability than a larger rod provides.

NQ is the size most exploration programs default to, and for good reason: it strikes a practical balance between core diameter, rod strength, hole size, and cost. It produces a core diameter large enough for most assay and logging purposes while keeping rod weight and drilling costs more manageable than the larger series. If a project brief doesn't specify a particular constraint pushing toward BQ or HQ, NQ is very often the right starting assumption.

HQ steps up in both rod and core diameter from NQ. The larger diameter improves rod stability in deviated or difficult ground, and the larger core diameter is often preferred for geotechnical work or situations where a larger sample improves the reliability of structural logging. The tradeoff is heavier rod, higher torque and pump capacity requirements, and generally higher cost per meter than NQ.

PQ is the largest of the four common series and is typically specified when the project genuinely needs the largest practical core diameter — often for detailed geotechnical investigation, or where sample volume itself is the limiting factor for downstream testing. PQ rod is correspondingly heavy and demands a rig with sufficient hoisting and torque capacity to run it efficiently, which is worth confirming before committing to the series on a smaller or lighter rig.

Each rod size has a matched casing size that telescopes correctly through it — running HQ rod inside NQ casing, for example, simply doesn't work, and mismatches here are one of the more disruptive planning errors on a multi-hole program where casing needs to be pre-ordered ahead of the drilling schedule. When a hole needs to be cased and then continued at a smaller diameter below the casing shoe, this telescoping relationship is what determines how far the size can be stepped down while still fitting the originally planned rod and bit through the casing string.

In practice, the choice usually comes down to three questions: what core diameter does the assay or geotechnical program actually require, what depth and ground conditions is the hole expected to encounter, and what rig capacity is available on site. A program chasing maximum depth with manageable rig requirements tends toward BQ or NQ; a program prioritizing sample size or stability in difficult ground tends toward HQ or PQ. Getting this decision right at the planning stage avoids the far more expensive problem of discovering partway through a program that the rod size doesn't suit the ground conditions or sampling requirements.

As with every component we've covered in this series, rod size isn't a decision made in isolation — it sets the size class for the bit, barrel, reaming shell, core lifter, and casing that follow it, which is exactly why it's worth getting right before the first rod goes down the hole.