API Thread Specification: Types, Dimensions and Chart

API thread specifications define the types, dimensions, and requirements of threaded connections used for oil and gas casing and tubing. Common API thread types include API Round Thread and Buttress Thread. This guide explains API thread types, dimensions, thread charts, and common fittings.

1. API Thread Types

1.1 API Round Thread

There are short round thread casing and the long round thread casing points. There are some advantages about round thread casing as follows:

Improve the abrasion caused by resistance when the thread tightening
Tighten the screw, the tooth tip provides a reasonable space for foreign particles and dirt
The circular arc tooth is not sensitive to local scratch or dent damage

Round thread due to its easy processing, good sealing performance and low price, is widely used in casing connection. In addition, it’s simple in site maintenance and use. In the test of different specifications of the round thread casing, use appropriate specifications of the thread gauge to test, when necessary carries on corresponding processing to the data.

1.2 Buttress Thread

In oil field tubing, buttress thread is a pipe thread form designed to provide a tight hydraulic seal. The thread form is similar to that of Acme thread the force is transmitted almost parallel to the axis and thread is about the same strength as standard v threads.

The advantages of buttress thread:

1) Buttress thread can withstand large tensile or compressive load, especially having sufficient tensile strength.

2) The design of the thread parameters are optimized to ensure that at least there is a tight fit around the thread, there is no gap, so that the sealing capacity of the terminal is greatly improved.

1.3 Premium Thread

Structure: Improved thread that is not in accordance with API standards (such as Hydrill, Tenaris, etc.), often with metal sealing surface, no thread grease required.
Application: Ultra-high pressure, high corrosion or gas sealing requirements (such as shale gas wells, deep-sea pipelines).

2. API Thread Specification Parameters

API thread specification parameters define the mechanical and dimensional requirements that keep casing and tubing connections interchangeable and reliable in the field. These parameters generally fall into three categories:

2.1 Size Range

API threaded connections cover a wide range of pipe sizes, from around 1.050 inches for oil pipe up to 20 inches for large-diameter casing. The applicable API specification defines the thread form and dimensions for each size.

2.2 Tolerance System

Each API thread specification sets the allowable deviation in thread diameter, pitch, and tooth angle. These tolerances are what allow connections manufactured by different suppliers to be interchangeable and to make up reliably in the field, without relying on match-fitting individual joints.

2.3 Sealing Design

Sealing performance depends on the thread type. Round threads typically rely on thread grease (dope) to fill the clearance between mating threads and prevent leakage. Buttress and other trapezoidal-profile threads achieve an initial mechanical seal through closer tooth-to-tooth contact, which is then reinforced with thread compound to complete the seal.

3. API Thread Chart and Dimensions

API Thread Type	Thread Profile	Typical Application
API Round Thread	Round profile	Casing
API Buttress Thread	Buttress profile	Casing and tubing
Premium Connection	Enhanced sealing design	Demanding well conditions

API thread dimensions vary by pipe size, thread type, and applicable API specification. Always refer to the relevant API standard for exact dimensions and tolerances.

4. API Thread Fittings and Applications

API threaded connections are matched to the pipe's service conditions. Round thread fittings are commonly used where good sealing performance and easy field maintenance are priorities, while buttress thread fittings are selected for casing and tubing strings that need to withstand higher tensile or compressive loads. Premium connections are reserved for wells with ultra-high pressure, corrosive fluids, or strict gas-tight sealing requirements. Some casing and tubing applications also use internal flush thread connections, which maintain a smooth internal bore for improved flow characteristics.

For related connection types used in drilling operations, see our guide to drill pipe connection types.

5. FAQs

What is the full form of API thread?

API stands for American Petroleum Institute. An API thread is a threaded connection manufactured to API specifications, used to join casing, tubing, and other pipe in oil and gas well construction.

What are the main types of API threads?

The main types are the API Round Thread and the API Buttress Thread. Round thread is widely used for casing connections due to its good sealing performance and ease of maintenance, while buttress thread is used where higher tensile and compressive strength is required.

What is the difference between API Round Thread and Buttress Thread?

Round thread has a rounded profile and is easier to process and maintain in the field. Buttress thread has a flatter, near-axial load-bearing profile that provides greater tensile and compressive strength, making it better suited to higher-load casing and tubing connections.