Drill Pipe Thread Types & Tool Joint Connections: NC, IF, FH, REG Selection Guide

A drill string can be thousands of feet of strong steel and still fail at a single joint. Every connection carries torque, tension, and pump pressure at once, so the string is only as strong as its weakest one. Drill pipe thread types stand between a working string and a fishing job.

In the oil and gas industry, that choice decides how much torque reaches the drill bit, how much pressure you lose downhole, and how long the pipe survives repeated drilling operations.

At Imex Canada, connection selection is one of the first questions when a drilling program specifies pipe, and getting it wrong is expensive to fix once the string is in the hole. This guide covers REG, IF, FH, NC, and premium threads, plus the criteria that decide which connection threads belong in your string.

Key Takeaways

• Every connection carries torque, tension, and pressure at once, so the string fails at its weakest joint.
• Bigger bore means better hydraulics but less steel and lower torque capacity.
• REG fits older strings, IF favors flow, FH balances bore and strength, and NC dominates modern pipe.
• Legacy XH, SH, and WO sizes can look interchangeable with NC but must be checked pair by pair.
• Match connection ratings to pipe body capacity, and go premium for laterals, HPHT, or deepwater.
• Imex Canada supplies drill pipe meeting or exceeding API Spec 5DP from a large Edmonton inventory.

What Drill Pipe Threads Do in the Drill String

Every connection has three jobs at once. It transmits torsional loads from the surface to the drill bit, carries the axial loads of the string below it, and seals the fluid path so mud stays inside the pipe at high pressures.

Threads are the part of the string the drilling industry watches most closely. They see cyclic loading as the string rotates and bends, made up and broken out hundreds of times, all in heat and corrosive fluid. Thread damage and fatigue show up here first.

The connection lives on the tool joint, a heavier forged section welded to each end of the pipe body. Connection choice belongs with pipe size, weight, grade, and length, not as an afterthought once the pipe is bought.

Anatomy of a Rotary-Shouldered Connection

Modern drill pipe uses rotary-shouldered connections: a tapered thread that stops against a shoulder rather than tightening on the threads alone. The parts:

• Pin: the male threaded end.
• Box: the female threaded end that receives it.
• Torque shoulder: the flat face where the pin and box seat are against each other.
• Tool-joint bore: the inner diameter that drilling fluid flows through.
• Thread roots and crests: the engaging profile, where root radius controls how stress concentrates.

The torque shoulder deserves more attention than most crews give it. Torque is carried and sealed there, not solely in the threads, so a galled face leaks or fails before the thread profile wears out. Too little make-up torque and the shoulder never seats; too much and you yield the pin.

Two dimensions tie the connection to everything else: the inner diameter sets pressure loss, and therefore drilling efficiency, and the outer diameter of the tool joint must clear the hole. Tool joint dimensions sit in API Spec 7-1, and the pipe itself in API Spec 5DP.

Drill Pipe Thread Types Compared

The American Petroleum Institute (API) standardized most of these families in API Spec 7-2, which governs threading and gauging of rotary shouldered connections.

Thread type	Typical characteristics	Common use
API Regular (REG)	Small tapered connection; ID reduced at the joint	Older, smaller strings
API Internal Flush (IF)	Larger bore than REG for better hydraulics	Common conventional drilling
API Full-Hole (FH)	Large bore and robust connection	Heavy-duty drilling
NC (Numbered Connection)	API rotary-shouldered family: NC38, NC40, NC46, NC50	Most modern drill pipe
FH / XH / SH	Older API designations	Legacy and inherited strings
Premium / proprietary	Maker-specific geometry for torque, fatigue life, or hydraulics	HPHT, directional, deepwater

API Regular (REG) — The Bore Restriction Trade-Off

The defining trade-off is inner diameter. The connection narrows the flow path at every joint, and each restriction costs pump pressure at the bit. REG persists because it is compact, strong for its size, and already cut on much existing equipment, including many bit connections and drill collars.

API Internal Flush (IF) — Bore for Hydraulics

If connections open, that flow path goes up. A bore closer to the pipe body reduces pressure loss, enhancing drilling efficiency by putting usable pressure at the bit. The trade-off is wall thickness: less steel means lower torsional capacity than a heavier connection of the same size.

API Full-Hole (FH) — Large Bore With Strength

Full-hole keeps a large bore while using enough tool joint material to stay robust, suiting programs that need volume through the string.

The geometry differs from NC: the 2-7/8, 3-1/2, and 4-1/2 FH connections use the V-040 thread form at 5 threads per inch on a 3-inch per foot taper. Only 5-1/2 FH and 6-5/8 FH remain preferred sizes.

NC (Numbered Connection) — The Modern Standard

NC dominates new pipe. An API-numbered connection uses one thread form across the range, the V-038R, and the number refers to size, not to a maker.

The R denotes a radiused root, and the number is that radius: 0.038 inch. Spreading load around a curve rather than into a corner gives the NC series its fatigue advantage over the flat-rooted forms it replaced.

Legacy Designations (XH, SH, and Older FH) — The Interchangeability Risk

Extra Hole, Slim Hole, Wide Open, and older Full-Hole designations turn up in inherited strings and rental fleets. Many map onto an NC size exactly, as the NC size table shows. Others do not, and API Spec 7-2 warns that some pairs mate without making a connection of adequate strength:

• NC38 and 3-1/2 WO: different pin lengths.
• NC44 and 4 OH (Open Hole): different taper.
• NC26 and 2-3/8 WO, NC31 and 2-7/8 XH, and NC35 and 4 SH: pitch diameters within 0.06 inch of each other.

Interchangeability runs pair by pair, not family by family.

Premium and Proprietary Connections — Beyond API Margin

Where API geometry runs out of margin, various manufacturers offer premium connections. A non-API connection may add a secondary shoulder or an extra torque shoulder, modify the thread profile, or rework stress relief to raise torque capacity and fatigue life.

They deliver superior performance at a higher cost and are not interchangeable between different manufacturers.

API Thread Designation Is Not the Same as Connection Type

An API thread designation identifies a standardized size and thread geometry. A connection type describes the whole engineered joint: shoulder arrangement, stress relief features, and rated performance.

Take NC50. That designation tells you the size and thread form of an API rotary-shouldered connection. A maker can also offer a proprietary connection at a similar size with different API-declared torque ratings and fatigue behavior.

The consequence shows up on the tally sheet. Two lines reading the same nominal size may not mate, and if they do, they may not carry the same load. 

Matching numbers is not matching performance. Confirm the designation and the connection type against the exact specifications before you buy or make up.

Common NC Connection Sizes

API Spec 7-2 treats NC23 through NC70 as preferred connections. The table covers the sizes you meet most often and the designations each replaces.

NC size	Interchangeable older designation	Thread form	Threads per inch	Taper (in/ft)
NC26	2-3/8 IF, 2-7/8 SH	V-038R	4	2
NC31	2-7/8 IF, 3-1/2 SH	V-038R	4	2
NC38	3-1/2 IF, 4-1/2 SH	V-038R	4	2
NC40	4 FH, 4-1/2 DSL (Double Streamline)	V-038R	4	2
NC46	4 IF, 4-1/2 XH, 4 WO	V-038R	4	2
NC50	4-1/2 IF, 5 XH, 5-1/2 DSL, 4-1/2 WO	V-038R	4	2

The series also includes intermediate sizes such as NC44 and continues through NC56, NC61, and NC70 as pipe size and load capacity rise.

Mid-range sizes carry most programs: NC50 pairs with 5-inch drill pipe, usually on a 6-5/8 by 2-3/4 inch tool joint. Stepping up increases bore, torque capacity, and tool joint OD together, and that last one is the constraint people forget.

How to Select the Right Drill Pipe Connection

Work these criteria together, because they push against each other.

• Pipe size: the connection must match tool joint and pipe body geometry, which sets the shortlist.
• Pipe weight and grade: the chemical composition and heat treatment behind a pipe’s mechanical properties set its tensile strength and yield strength, which the connection should not undercut.
• Required torque: directional and extended-reach work demands high torque, where standard connections run out of room first.
• Inner diameter: your hydraulics constraint. Total pressure loss across every joint against pump pressure and flow rate at depth.
• Drilling depth: deeper wells mean more cumulative axial loads and more fatigue cycles on every connection.
• Operating conditions: high pressures, corrosive fluid, and dogleg severity all drive increased wear.

Matching Connection Capacity to Pipe Body Capacity

The goal is balanced design: a connection weaker than the pipe body builds a failure point into every joint. Aim for ratings at or just above pipe body capacity in torsion and tension, the same principle used when sizing drill collars and bit connections.

When Standard API Connections Aren’t Enough

Three signals push a program toward premium threads: torque demand above the make-up torque API declared for the connection, common on long laterals; fatigue in high-dogleg wells, where bending concentrates at the connection; and deepwater or HPHT work, where one failure costs more than upgrading the whole string.

Mixed-String and Crossover Considerations

Many working strings carry more than one connection type. Where two meet, you need a crossover sub, and every crossover is another connection to inspect.

Record what sits where, mark the pipe, and inspect transitions often. Mismatched make-up torque is a common source of thread damage.

Quick Reference: Choosing by Application

• Conventional vertical drilling: pipe size and bore decide it, and a mid-range NC connection covers most programs.
• Heavy-duty programs: prioritize robustness and torque capacity, checking outer diameter against hole clearance.
• Legacy string maintenance: identify the existing designation, then check the equivalence table before ordering.
• Directional and extended reach: torque and fatigue cycles lead, where premium geometry earns its cost and protects operational efficiency.
• Deepwater and HPHT: operating conditions dominate, so follow the load and pressure case, not the price sheet.

Sourcing Drill Pipe With the Right Connection

Choosing the connection is one job; finding the pipe that comes with it is another. Imex Canada supplies drill pipe that meets or exceeds API Spec 5DP, sourced from global mills and held in a large inventory in Edmonton, Alberta.

Every piece goes through quality control before it ships: mill test reports covering chemical composition and mechanical properties, dimensional verification against API 5DP, and connection inspection verifying thread integrity and gauge requirements.

Third-party inspection is available on request, with full documentation and traceability from manufacturing through delivery.

Getting Drill Pipe Connection Selection Right

Connection selection is a systems decision. Geometry, load, torque, and hydraulics all pull on the same choice, and optimizing one alone is how strings end up with a weak link. Drill pipe sits inside the oil country tubular goods supply chain, where API standards and industry standards give a common language but not a guarantee.

Verify the designation, the connection type, and the make-up torque against the exact specifications before the joint goes in the hole. Treat gauge checks as the step for ensuring compatibility rather than assuming it.

If you have questions about drill pipe connections, just contact us; we are happy to help.