Metric Thread Pitch Chart (Coarse and Fine) and BSW Threads

Thread pitch is the distance from one thread crest to the next, in mm. Each metric size has one normal coarse pitch, and most have one or more fine pitches. BSW (Whitworth) threads are given in threads per inch (TPI). This chart covers metric threads from M3 to M36 and BSW threads from 1/4″ to 1-1/4″.

How to Read a Metric Thread Size

M12 x 1.75 means an ISO metric thread of 12 mm nominal diameter with a 1.75 mm pitch. This is the coarse pitch for M12, so it is usually left out: M12 alone means M12 x 1.75. M12 x 1.25 has the same diameter with a fine 1.25 mm pitch, and a fine pitch must always be written. On a bolt order, M12 x 50 is a coarse M12 bolt 50 mm long, and a fine one is written M12 x 1.25 x 50.

Metric Thread Pitch and Tapping Drill Chart, M3 to M36

How to read the chart: all sizes in mm. Coarse pitches are from ISO 261 and fine pitches are the ISO 262 selection for bolts, screws and nuts. A dash means ISO 262 selects no fine pitch in that size.

Tapping drill rule: as a guide, drill size = nominal diameter minus pitch, so M10 takes 10 minus 1.5 = 8.5 mm. For M12 the rule gives 10.25 mm and the standard drill is 10.2 mm. Fine threads follow the same rule: M12 x 1.5 takes a 10.5 mm drill.

BSW (Whitworth) Thread Chart, 1/4″ to 1-1/4″

BSW is the coarse Whitworth series in BS 84. The pitch in mm is 25.4 divided by the TPI. BSW threads have a 55 degree thread angle with rounded crests and roots. ISO metric and UNC threads have a 60 degree angle.

How to Measure Thread Pitch

• Pitch gauge: try the gauge leaves on the thread until one sits fully with no light showing, then read the pitch or TPI marked on it. Use a gauge with metric (60 degree) and Whitworth (55 degree) leaves.
• Count over 10 mm: count the threads in 10 mm along a steel rule. 10 divided by the count is the pitch: 8 threads means 1.25 mm, as on an M8 coarse or an M10 x 1.25 bolt. For BSW, count the threads in 25.4 mm to get the TPI.
• Diameter: measure the outside of the thread with a vernier. It reads slightly under the nominal size.

Coarse or Fine Thread: Which to Use

Coarse thread is the standard choice for general work. It is quicker to assemble, less likely to cross thread, and less affected by dirt, small damage and thick coatings. It also strips less easily in soft metals such as aluminium and cast iron. Hot dip galvanizing to ISO 10684 covers coarse threads from M8 to M64, so galvanized bolts are normally coarse pitch.

Fine thread has a larger stress area, so it is a little stronger at the same size: 88.1 mm² for M12 x 1.5 against 84.3 mm² for M12 coarse, from ISO 898-1. It gives finer adjustment and suits thin walled parts, automobile parts and hydraulic fittings. It needs clean threads and careful assembly.

JMIKS bolts, nuts and studs are normally supplied in coarse pitch. For fine pitch, send the full size, such as M12 x 1.25.

Bolts, Nuts and Studs from JMIKS Fasteners

JMIKS Fasteners supplies hex bolts, hex nuts, B7 and B8 stud bolts and threaded rods in metric sizes up to M36 and in 1/4″ to 1-1/4″ BSW. Grades are SS 304, SS 316 and SS 202, mild steel 4.6 and 5.6, high tensile 8.8, 10.9 and 12.9, and ASTM A193 B7, B8 and B8M for studs. Mild steel and high tensile parts come self colour, zinc plated or hot dip galvanized, and 12.9 is normally self colour. See the spanner size chart, the bolt torque chart and the bolt grade chart.

Thread Pitch FAQs

What is the pitch of an M10 bolt?

1.5 mm, the ISO coarse pitch for M10. Fine M10 bolts have a 1.25 mm or 1 mm pitch, which is always written in the size, such as M10 x 1.25.

What does M12 x 1.25 mean?

It is a metric thread of 12 mm nominal diameter with a fine pitch of 1.25 mm. The normal M12 coarse pitch is 1.75 mm, so an M12 x 1.25 nut will not fit a standard M12 bolt.

Is BSW the same as UNC?

No. BSW has a 55 degree thread angle with rounded crests and roots, and UNC has a 60 degree angle with flat crests. Most sizes from 1/4″ to 1-1/4″ have the same threads per inch, but 1/2″ is 12 in BSW and 13 in UNC. Even where the count matches, the flanks do not fit fully and the joint is weaker, so do not mix them.

What drill size do I use for an M8 tap?

6.8 mm for the coarse M8 x 1.25 thread, which is 8 minus 1.25. For a fine M8 x 1 thread, use a 7 mm drill.

How can I tell an M12 bolt from a 1/2″ BSW bolt?

Check the diameter and the pitch. M12 is 12 mm with a 1.75 mm pitch. 1/2″ BSW is 12.70 mm with 12 threads per inch, a pitch of about 2.12 mm. A pitch gauge shows the difference at once.

Get a Quote for Bolts, Nuts and Studs

JMIKS Fasteners is a fastener wholesaler and supplier in Nava Odhav, Ahmedabad. Send the size, pitch, grade, finish and quantity on WhatsApp (+91 90333 11425) or through the enquiry form, or get our catalogue from the catalogue page. We reply within one working day.