API 5B: Thread Gauging, Standoff and Gauge Control

API 5B is the API specification for threading, gauging and thread inspection of casing, tubing and line pipe threads. It defines the thread form and its dimensions, the gauges used to measure a cut thread, and the practice for using them. Where API 5CT says what the pipe must be, API 5B says whether the…

API 5B is the API specification for threading, gauging and thread inspection of casing, tubing and line pipe threads. It defines the thread form and its dimensions, the gauges used to measure a cut thread, and the practice for using them. Where API 5CT says what the pipe must be, API 5B says whether the thread on the end of it is acceptable.

At a glance

Question	Short answer
What does it govern?	Thread form and dimensions, gauge requirements, and gauging practice
Which threads?	Round thread, buttress, extreme-line and line pipe threads
What does it not govern?	The pipe itself. Grade, wall and testing sit in API 5CT for casing and tubing, and in API 5L for line pipe
The measurement	Standoff — where the gauge sits relative to a reference on the thread
Gauge control	A traceable hierarchy from working gauges up to reference masters
Who meets it	Threading shops, repair facilities and anyone accepting re-cut threads

The four thread elements it puts tolerances on

A thread is a set of dimensions, and each can drift while the others stay correct.

Element	What it is	What goes wrong if it drifts
Thread form	The profile shape and flank angles	Flanks do not bear evenly; contact concentrates where it was not intended
Lead	Axial distance advanced per thread	Cumulative error along the engagement; the joint fits at one end and not the other
Taper	Rate at which the thread diameter changes along its length	Interference is wrong; the connection makes up too shallow or too deep
Thread height	Depth of the profile	Reduced engagement, or crest-to-root interference

The specification puts numbers and tolerances on all four, then defines the gauges that confirm the result on the shop floor. Standoff is the quick reading that tells you whether the thread as a whole came out large or small. It does not replace the element checks — API 5B still calls for lead, taper and thread height to be measured with their own gauges, because standoff cannot see them individually.

Standoff: the number the gauge actually gives you

The gauge is made up hand-tight on the connection — a defined condition, not simply turned until it will not move — and what is recorded is where it stopped relative to a defined reference on the thread. That distance is the standoff.

Both ends of a connection are gauged, and the reading runs in opposite directions. A ring gauge goes onto the pin; a plug gauge goes into the box. On an external thread checked with a ring gauge, cut the thread slightly large and the ring stops earlier; cut it slightly small and it runs on further. On a coupling the sign reverses: a box cut large has more material removed, so the plug runs further in. Which way the number moves depends on which member you are gauging, and that is a routine source of misread standoff.

Because it collapses several elements into one number, a thread can be within standoff tolerance and still carry a defect the standoff does not separate. Standoff is a necessary check, not a complete one.

The reference the standoff is measured from also differs by thread type, which is why gauging round thread and gauging buttress are not the same procedure with a different tool — see why buttress and round threads are gauged from different references.

The gauge hierarchy, and why it exists

A gauge is a measuring instrument, so the obvious question is what measures the gauge. API practice answers it with a chain.

Level	Role	Where a shop meets it
Reference master gauges	The top of the chain, held centrally under API control	Never directly. It is why the chain has an end
Master gauges	Certified against the reference level; used to certify working gauges	Held by gauge suppliers and larger facilities
Working gauges	The plug and ring gauges used on the shop floor every day	In your hands, wearing slightly with every use

The chain matters because a working gauge is hardened steel that gets threaded onto abrasive, doped, sometimes damaged connections many times a day. It wears, and a worn gauge accepts threads it should reject.

• The recertification interval comes from the gauge supplier’s certificate and your own quality manual. Keep the certificate date and the next-due date visible at the gauge cabinet, not filed in an office.
• A dropped gauge is out of service until re-certified, whatever it looks like.
• Store gauges clean, dry and lightly oiled in their cases. Rust on a gauge thread transfers to every connection it touches.
• Log which gauge accepted which batch. When a customer rejects a joint, the first question is which gauge passed it.

Where API 5B and API 5CT meet

The two specifications are routinely confused because they arrive together on the same certificate.

Question	API 5CT	API 5B
Governs	The pipe: grade, chemistry, dimensions, testing, marking	The thread: form, dimensions, gauges, inspection
Typical failure it catches	Wrong grade, wall out of tolerance, missing NDE	Thread cut out of tolerance, or gauged with a worn gauge
Who is responsible	The mill	Whoever cut the thread — often a third-party shop
What a buyer checks	Mill certificate against the order	Gauge certification and the threading record

Responsibility for the thread usually sits with a different company from the one that made the pipe. Pipe frequently arrives as 5CT product and is threaded, or re-threaded, somewhere else entirely, so the grades, connections and PSL levels on the mill certificate tell you nothing about the quality of a thread cut later by someone else.

Re-cut threads, and what limits them

Re-threading is normal and legitimate. A damaged connection is machined off and a new thread cut, and the joint goes back to work. What changes is where responsibility for that thread now sits, and how much material is left to work with.

Factor	Effect to consider	What the team should use for a decision
Upset remaining	On upset ends this governs, not joint length. Machine past the upset run-out and no compliant thread can be cut	How much upset is left to cut into, measured before the job is accepted
Remaining length, plain end	Each re-cut shortens the joint	The measured length against the minimum in the inspection standard named in the customer’s order
Gauge certification at the cutting shop	A thread is only as good as the gauge that accepted it	The certificate date and traceability chain, requested with the quote
Thread type actually cut	A re-cut may not reproduce the original connection	The threading record naming the thread type, not the tally’s original description
Post-thread handling	A perfect thread damaged in handling is a rejected thread	Whether protectors go on before the joint leaves the machine

Handling the joint after threading is part of the threading job. A thread that passes gauging and is then dropped, dragged or stored uncovered arrives at the rig as a reject, which is why how OCTG is handled and stored belongs in the same procedure rather than a different department’s.

The boundary of what gauging can certify

A gauged thread is a thread confirmed to be the right shape. That is a statement about geometry at rest, before the connection has ever been assembled.

Everything that decides whether that geometry survives assembly sits outside API 5B. Surface finish, the compound applied to the thread, cleanliness, alignment at stab and make-up speed are governed by the make-up procedure, not by anything a gauge certificate covers. Two connections can gauge identically and behave completely differently on the bench.

So a threading shop that holds only gauge certificates holds half an evidence file. The gauge certifies the thread before it is made up; a recorded torque-turn curve certifies what that thread actually did under load, per joint, against the values the connection is specified to. When a customer rejects a joint, the shop holding both can answer the rejection and the shop holding one cannot. That second record is what a controlled make-up bench produces.

Before you specify equipment for this work

A threading or inspection facility is quoted from a different set of inputs than a pipe order. These are the ones that change the answer:

• Thread types cut or handled — round, buttress, extreme-line, line pipe, or a licensed connection named by its licensor
• Which gauge sets the shop holds, and the certification interval they run on
• Whether re-cutting is done in house, and on upset or plain-end product
• Largest connection handled, and the torque it has to reach
• Grades passing through, including any chrome or corrosion-resistant material
• Joints per shift at peak, not annual throughput
• The acceptance document the customer expects at handover

Those seven answers are enough for our engineering team to tell you what the work needs, and whether your existing bench already covers it.

Frequently asked questions about API 5B

What is API 5B?

The API specification covering threading, gauging and thread inspection for casing, tubing and line pipe threads. It sets the thread form and dimensions, specifies the gauges, and defines the practice for using them so a thread cut in one shop is acceptable in another.

What is the difference between API 5B and API 5CT?

5CT governs the pipe — grade, chemistry, dimensions, testing and marking. 5B governs the thread on the end of it. A joint can be fully compliant 5CT product carrying a thread that fails 5B, and the two are often the responsibility of different companies.

What is thread standoff?

The distance between where a gauge, made up hand-tight, stops on a connection and a defined reference on that thread. Because the thread is tapered, that single linear reading reports whether the thread was cut large or small at the gauge plane.

Does standoff move the same way on a pin and a box?

No, and this is a common misread. A ring gauge on a pin cut large stops earlier. A plug gauge in a box cut large runs further in, because more material has been removed. The direction depends on which member is being gauged.

Which threads does API 5B cover?

Round thread, buttress, extreme-line and line pipe threads. Licensed premium connections fall outside it entirely — those are proprietary designs gauged to the licensor’s own procedure rather than to API practice.

How often should thread gauges be certified?

On the interval stated by the gauge supplier’s certificate and your quality manual, and immediately after any drop or suspected damage regardless of date. A worn gauge accepts threads that should be rejected, so an out-of-date certificate quietly widens your tolerance.

Does a gauged thread guarantee a good make-up?

No. Gauging certifies geometry before assembly. The variables that decide what happens during assembly are governed by the make-up procedure and the record it produces, which is a different document from the gauge certificate and has to be kept separately.

How many times can a connection be re-cut?

There is no count. On upset ends the limit is how much upset remains to cut into; machine past the run-out and no compliant thread can be cut. On plain-end product the limit is remaining length against the inspection standard named in the order.

Closing checklist

• Read standoff hand-tight — a strong wrist and a weak one will not report the same thread
• Know which member you are gauging before reading the number; the sign reverses between pin and box
• Treat standoff as a screening reading, and keep the lead, taper and thread-height checks as well
• Ask the threading shop for gauge certification dates before the work, not after a rejection
• Put a dropped gauge out of service until re-certified, whatever it looks like
• Check remaining upset, not just joint length, before authorising another re-cut
• Confirm the threading record names the thread type actually cut
• Fit protectors before the joint leaves the threading machine
• Keep the gauge certificate and the make-up record together — they certify different halves of the same joint