This reference shows representative preferred diameter and threads-per-inch (TPI) combinations for American 7°/45° buttress threads. It includes eight nominal diameters from 0.5 to 24 inches, along with each size’s preferred pitches and the first-preference pitch converted to inches and millimetres. It is a partial lookup: use it to orient a selection, then verify the full thread designation and dimensions against the drawing and current ASME B1.9 before manufacturing or inspection.

Buttress Thread Diameter and Pitch Combinations

Parameters and definitions

Nominal major diameter is the named outside size in inches. TPI is the number of thread pitches per inch. Pitch is the axial distance between adjacent thread forms, so pitch in inches per thread equals 1 divided by TPI. The millimetre conversion uses 25.4 millimetres per inch.

The first-preference pitch is one preferred choice for that diameter; the other pitches in the row are also preferred combinations. “Preferred” does not mean a thread automatically fits a mating part. Diameter and pitch describe only part of a complete thread specification.

How to use the chart

Start with the nominal diameter required by the drawing. Compare the listed TPI values for that diameter and use the conversion columns only to interpret pitch spacing. If a required diameter is not shown, consult the complete standard rather than interpolating from a nearby row. The eight entries are representative examples, not the complete B1.9 series.

For example, 2.0-8 means a 2-inch nominal major diameter and 8 threads per inch. The pitch is 1/8 inch, or 3.175 millimetres per thread. That conversion does not set the thread depth, pitch diameter, allowable clearance, class, root geometry or gaging limits.

Worked example

Suppose a single-start design calls for a 10-inch buttress thread and the selected preferred pitch is 4 TPI. The chart identifies 4 TPI as the first-preference pitch for that representative diameter. Calculate the spacing independently: 1 / 4 = 0.25 inch per thread; 25.4 / 4 = 6.35 millimetres per thread. Lead equals pitch only for a single-start thread. If the design uses multiple starts, specify the lead and number of starts separately.

Use the full drawing callout to define pull or push form, class, external or internal member, hand, root form and mating component. ASME B1.9 also covers the thread profile, tolerance, allowances, measurement and gaging. This table cannot establish dimensional acceptance or load capacity by itself.

Avoiding mistakes

Do not mix American buttress data with metric buttress, British buttress, ACME, trapezoidal or hydraulic sealing threads. Similar nominal sizes do not make their profiles interchangeable. The ASME catalog identifies B1.9 as a 7°/45° form with a basic engagement height of 0.6 pitch; use the standard and drawing to identify the intended load flank.

Also keep nominal pitch separate from fit and tolerance. A match in this chart is not proof that a screw and nut will assemble, pass inspection or carry the required load. Use class-specific dimensions and the specified gaging method from the controlling documentation. For a different thread form, see the site’s NPT thread guide; NPT dimensions do not apply to buttress threads.

The representative values cover small, mid-range and large nominal sizes so the progression is visible without reproducing every row of the standard. The size series and preferred pitch series are separate ideas: not every listed TPI applies to every diameter. Read each row as a linked combination, and use the complete B1.9 table when a required size is not among these examples.

Sources

The representative selections were checked against the independent American buttress chart and the scope listed for ASME B1.9-1973 (R2025). Pitch values are calculated using 1/TPI and 25.4/TPI. Consult the licensed standard for complete combinations, dimensional tolerances and inspection requirements.