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SAE J2123: A Practical Guide to the Numbering System for Ground Thread Taps
The SAE J2123 Recommended Practice provides a standardized method for identifying ground thread taps used in inch and metric sizes. Developed to streamline cataloging and supply, this numbering system ensures consistent tool descriptions across the industry. This article explains the key elements of J2123, including the two numbering formats, position-by-position coding, and important design considerations.
Overview and Format Selection
J2123 covers ground thread taps for inch sizes up to 9.999 in and metric sizes up to 99.9 mm, with a maximum pitch of 99.9 TPI (inch) or 9.99 mm (metric). It excludes ACME, Buttress, and similar thread forms. The standard provides two distinct formats based on the tap’s overall length conformance to referenced dimensional standards (ASME B94.9, ISO 529, etc.).
⚠️ As noted in the standard, caution must be taken when assigning codes to prevent specifying taps that cannot be physically or economically manufactured. The code number must always reflect the actual tool construction.
The 21-position format is used for taps with overall lengths that comply with the referenced standards. The 26-position format is reserved for extension taps, pulley taps, and other taps whose overall lengths do not meet those referenced standards. The table below summarizes the differences.
| Feature | 21-Position Format | 26-Position Format |
|---|---|---|
| Application | Taps conforming to standard overall lengths | Taps with non-standard overall lengths (extension, pulley) |
| Number of Characters | 21 | 26 |
| Additional Information | Standard dimensions | Extra characters for extended length and other variables |
Detailed Position Encoding Guide
The first two positions are always TT (Tool, Tap). Position 3 indicates the tap type, with separate codes for taps without and with through oil holes. The table below lists the assigned codes.
| Code (w/o oil hole) | Tap Type | Code (w/ oil hole) | Tap Type |
|---|---|---|---|
| 1 | Standard Straight Thread | A | Standard Straight Thread with oil hole |
| 2 | Taper Pipe | B | Taper Pipe with oil hole |
| 3 | Straight Pipe | C | Straight Pipe with oil hole |
| 4 | Pulley | D | Pulley with oil hole |
| 5 | Nut | E | Nut with oil hole |
| 6 | Thread Forming | F | Thread Forming with oil hole |
| 7 | STI (Screw Thread Insert) | G | STI with oil hole |
| 8 | Extension | H | Extension with oil hole |
| 9 | Short Thread Length | I | Short Thread Length with oil hole |
| W | Taper Pipe Small Shank | Y | Taper Pipe Small Shank with oil hole |
| X | Straight Pipe Small Shank | Z | Straight Pipe Small Shank with oil hole |
Position 4 encodes the measurement system, hand of thread, and the applicable industry standard for general dimensions.
| Code | Description |
|---|---|
| C | Right Hand, Inch, ASME/ANSI |
| L | Left Hand, Inch, ASME/ANSI |
| M | Right Hand, Metric, ASME/ANSI |
| N | Left Hand, Metric, ASME/ANSI |
| U | Right Hand, Metric, DIN |
| V | Left Hand, Metric, DIN |
| W | Right Hand, Metric, ISO |
| Z | Left Hand, Metric, ISO |
| J | Right Hand, Metric, JIS |
| S | Left Hand, Metric, JIS |
Positions 5 through 12 define the nominal size (diameter) and pitch. The format depends on whether the tap uses metric or inch threads. For metric taps, position 5 is an ‘M’, followed by a three-digit diameter (to one decimal), an ‘X’, and a three-digit pitch (to two decimals). For inch taps, the diameter is given as a four-place decimal and the threads per inch to one decimal. Zeros fill open positions.
🛠️ Always use zeros for all open positions in the size-pitch fields. For example, a metric tap with M8 x 1.25 becomes M080X125.
Engineering Design Insight: When assigning codes, always verify that the combination of type, size, pitch, and dimensional standards is physically achievable. For example, specifying a very fine pitch on a large diameter may not be standard or cost-effective. Refer to the latest revisions of ASME, ISO, JIS, and other referenced standards to ensure compatibility. The code must reflect the actual tool construction — including oil hole presence — to avoid mismatched specifications.
Frequently Asked Questions
1. When should I use the 26-position format instead of the 21-position?
The 26-position format is required when the tap’s overall length does not comply with the standard lengths referenced in J2123 (such as ASME B94.9 or ISO 529). This includes extension taps and pulley taps. If the tap meets standard overall dimensions, use the 21-position format.
2. How are metric nominal size and pitch encoded in positions 5–12?
Position 5 is an ‘M’. Positions 6–8 represent the diameter in millimeters to one decimal place. Position 9 is an ‘X’. Positions 10–12 represent the pitch in millimeters to two decimal places. All positions are filled with leading zeros as needed. Example: M1.6 x 0.35 is encoded as M016X035.
3. What is the significance of Position 4?
Position 4 serves triple duty: it indicates the measurement system (inch or metric), the hand of thread (right or left), and the applicable industry dimensional standard (ASME, DIN, ISO, or JIS). Choosing the correct code ensures the tap will be manufactured to the correct standard and thread orientation.
4. Can I use J2123 to number ACME thread taps?
No. J2123 is explicitly intended for ground thread taps and specifically excludes ACME, Buttress, and similar thread forms. Use dedicated standards for those thread types.
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