Drill String Thread Identification

What the dimensions mean

An API rotary shouldered connection carries its load on the flat shoulder, not on the threads — the threads only pull the shoulders together. That is why the shoulder face, not the thread, is the part worth protecting.

Telling the three profiles apart

Where two connections share a diameter, the shape of the thread itself is the tiebreaker — and you can read it by eye and by fingertip.

Five steps, two tools

1. Work out pin or box

   The pin is the male thread on the outside of the tube end. The box is the female recess. The tables quote pin dimensions, so if you only have a box, use the counterbore column instead.

2. Count the threads per inch

   Rule flat along the crests, parallel to the axis, and count the crests in a full inch. For a straight rock drill thread, measure crest to crest in millimetres instead — that is a pitch, not a TPI.

   Needs: steel rule or tape · thread pitch gauge if you have one

3. Decide tapered or straight

   Caliper the pin OD at three points along the threads. Shrinking steadily toward the tip means a tapered API connection. Constant means a straight rope or T thread — do not try to read a taper off it.

4. Measure the large diameter and the thread length

   For a taper, measure the OD at the large end of the pin nearest the shoulder, and measure the threaded length separately. Sit the caliper jaws on sound crests a few threads back from the tip; the last threads are usually worn or galled and read undersize.

   Needs: vernier or digital caliper

5. Match the combination, not one number

   Take TPI plus taper plus diameter into the tables together, or into the lookup at the top of this page. Any one of the three on its own leaves you with several candidates.

What catches people out

• “2-3/8” is not a thread. One supplier’s thread protector chart lists API IF 2-3/8, API Reg 2-3/8, Mayhew Regular 2-3/8 and Mayhew Jr 2-3/8 as four different connections under the same nominal callout. Never trust the sticker or the tally book — measure it.
• The whole NC series shares one TPI and one taper. NC26 through NC50 are all 4 TPI, 2 in/ft. Diameter is the only thing that separates them.
• API Reg 2-3/8 to 4-1/2 also share one TPI and taper (5 TPI, 3 in/ft), and 6-5/8 Reg shares its 4 TPI and 2 in/ft with the entire NC series.
• Full Hole sits almost on top of NC on diameter. 3-1/2 FH measures 3.884 in at the pin large diameter and NC38 measures 3.891 in — seven thousandths apart, well inside field measurement error. What separates them is the thread count: FH is 5 TPI and 3 in/ft, NC38 is 4 TPI and 2 in/ft. The same trap sits at 4-1/2 FH against NC46. Skip the thread count here and you will get it wrong.
• Worn threads lie. Galled or belled tips read undersize. Measure back from the tip and take the best of three readings.
• Pipe threads are not drill string threads. NPT and BSP on swivels, hoses and casing accessories seal fluid; they carry no drilling load.

Published dimensions

Every row carries a confidence mark. Figures marked verified were found in two or more independent published sources that agree. Where no source published a figure the cell says so — nothing on this page is estimated to fill a gap.

API Regular (Reg)

The workhorse of water well work — drill pipe, subs, and the backhead of almost every DTH hammer. Pin large diameter and counterbore agree to three decimals across an API-attributed table and an API gauge maker.

API IF / NC series

NC is the modern name for the older IF (Internal Flush) connections — the same thread, renamed. All five sizes share 4 TPI and a 2 in/ft taper, so inside this family diameter is the only thing that tells them apart.

API Full Hole (FH)

A longer, deeper pin than Reg or NC at a similar diameter, for more thread and shoulder contact. Watch the diameter collision with NC38 and NC46 — it is the trap described above.

Mayhew

Not an API thread, and still in service on smaller water well rigs. We could not source a single published dimension for it. The rows are here so the gap is visible rather than quietly left out.

Rope (R) and T threads

Top hammer and percussive extension rods, common across southern Africa. These are cylindrical — no taper, no make-up shoulder — and are specified by nominal diameter and pitch in millimetres.

Which thread comes on which hammer

Down-the-hole hammer backheads are not a separate thread family — they are almost always plain API Regular, picked by hammer diameter. This is the pairing the major hammer makers publish.

The bit end of a hammer is a different matter: bits fit a proprietary splined shank (DHD, QL, SD, Mission and so on), not a thread. Shanks are matched by hammer model rather than measured — take the model number off the hammer body.

Threads from next door

Wireline and coring rod threads — the AW, BW, NW and HW families, and the matching AQ, BQ, NQ and HQ wireline sizes — belong to exploration and geotechnical rigs rather than production water well work. You will meet them on a hired-in or converted exploration rig, on core barrels and the casing used to seal off unstable overburden, or in a second-hand rod string that started life on a mineral job. They are straight, coupling-jointed rods sized by outer tube diameter, so none of the pin diameter logic above applies to them. Casing and tubing threads — API 8-round, buttress and the rest — are a separate subject again.

What this guide does not know

This reference was compiled from 33 published sources — API gauge makers, hammer and rod manufacturers, and drill pipe spec sheets — and cross-checked figure by figure. We have not bought a copy of API Spec 7-2 to check against the standard itself. Where sources disagreed or went silent we left the gap visible rather than filling it with a plausible number, because a wrong thread dimension is worse than no dimension. These are the known holes:

Use this to identify, not to guarantee a fit. Measurements here are an identification aid. Before you make up a string, confirm the mating thread physically — with gauges if the joint matters — and never force a doubtful connection.

Where the numbers came from

1. Mills Machine — Tool Joint Thread Chart
2. API Reg Thread Forms (attributed to API Spec 7, Table 25, 40th ed. 2001)
3. Gauge Training — NC 26
4. Gauge Training — NC 31
5. Gauge Training — NC 38
6. Gauge Training — NC 46
7. Gauge Training — NC 50
8. Gauge Training — 2 3/8 in Reg
9. Gauge Training — 2 7/8 in Reg
10. Gauge Training — 3 1/2 in Reg
11. Gauge Training — 4 1/2 in Reg
12. Gauge Training — 6 5/8 in Reg
13. Gauge Training — 3 1/2 in FH
14. Gauge Training — 4 1/2 in FH
15. Leanoms Drill — Drill Pipe technical sheet
16. Patterson Services — NC26 drill pipe data sheet
17. Quail Tools — NC31 spec sheet
18. Quail Tools — NC38 spec sheet
19. Quail Tools — NC46 spec sheet
20. Quail Tools — NC50 spec sheet
21. M-P.com — Drill Pipe Thread Protectors size chart
22. Hole Products — DTH Hammers and Bits catalogue
23. Mincon — 4 in DTH hammers
24. Mincon — 5 in DTH hammers
25. Mincon — 6–7 in DTH hammers
26. Mincon — 8–9 in DTH hammers
27. Rockmore International — Standard Thread Connections
28. Boart Longyear — Top Hammer Drilling Tools catalogue, 2015
29. Rock-Drillbits — Drill Rod Sizes
30. RockHound — T38 vs T45 vs T51 drill rod
31. Hole Products — 2-3/8 in Mayhew Jr rotary drill rod
32. WIHO Industrial — Types of drill pipe connection threads
33. Litian — T38 vs T45 vs T51