Standard-size rotary drill bits end in a threaded pin — a male API Regular connection at the top of the bit. The bit sub or drill collar above it carries the matching box (female) thread. The bit diameter decides which pin the bit is built with: an 8-1/2 inch bit carries a 4-1/2 REG pin, a 12-1/4 inch bit carries a 6-5/8 REG pin. These are API rotary shouldered connections, standardized in API Spec 7-1 (drill stem elements), and manufacturers build to the same pairings. The chart below is the full mapping.
Read the chart by bit diameter. The connection size (for example 4-1/2 REG) is the nominal size of the thread, not of the bit — a 4-1/2 REG pin sits on bits from about 7-5/8 to 9 inches.
| Bit diameter | Standard pin | Common bit sizes in this range |
|---|---|---|
| 3-5/8″ to 4-1/2″ (92.1 to 114.3 mm) | 2-3/8 REG | 3-3/4″, 3-7/8″ |
| 4-5/8″ to 5-5/8″ (117.5 to 142.9 mm) | 2-7/8 REG | 4-3/4″, 5″, 5-1/2″ |
| 5-7/8″ to 7-1/2″ (149.2 to 190.5 mm) | 3-1/2 REG | 6″, 6-1/8″, 6-1/4″, 6-1/2″, 6-3/4″ |
| 7-5/8″ to 9″ (193.7 to 228.6 mm) | 4-1/2 REG | 7-7/8″, 8-1/2″, 8-3/4″, 9″ |
| 9-1/2″ to 14-1/2″ (241.3 to 368.3 mm) | 6-5/8 REG | 9-7/8″, 10-5/8″, 12-1/4″, 13-3/4″ |
| 14-3/4″ to 26″ (374.7 to 660.4 mm) | 7-5/8 REG | 16″, 17-1/2″, 22″, 26″ |
Bits smaller than about 3-5/8 inch generally sit below the API Regular range and use thread systems specific to their drilling method; confirm those against the tooling they run on.
Most of the chart is uniform across manufacturers. At three points the industry genuinely splits, and a bit at these sizes can arrive with either connection:
If your job uses one of these sizes, do not assume. Confirm the pin on the bit's specification sheet and the box on your bit sub, or state both when ordering and let the supplier match them.
State three things: the bit size, the connection size, and the word pin— for example, "8-1/2 inch bit, 4-1/2 REG pin." The bit always carries the pin; the bit sub above it carries the box, so there is no need to specify pin or box for the bit itself, but stating it removes doubt.
One confusion is worth clearing. API threads come in families: Regular (REG), Internal Flush (IF), and Full Hole (FH). Bits use Regular almost universally. IF and FH — and the numbered NC connections such as NC38 or NC50 — are drill-pipe tool-joint connections, used higher in the drill string. If a form asks for a bit with a "4-1/2 IF" connection, that request almost always describes the drill pipe, not the bit; the bit itself will still be a REG pin, and the bit sub bridges the two.
Use a crossover sub. A crossover sub is a short threaded joint with a box that matches the bit pin on one end and a connection that matches your drill string on the other end. They are standard stock items, made for exactly this situation — a new bit size entering a string built for another connection. When ordering one, state both connections in full, including pin or box for each end — for example, "box 4-1/2 REG down, box NC50 up."
Yes. The connection follows the bit diameter, not the cutting structure. An 8-1/2 inch PDC bit and an 8-1/2 inch tricone bit both carry a 4-1/2 REG pin, and the same holds across the chart — with one caution: the 9-1/2″ to 9-7/8″ boundary zone above is where PDC and tricone practice can differ. If you have the bit program but not the connection details, send the bit sizes on WhatsApp and we confirm the pairing with the quote.