1-1/4 BSW thread: 31.75 mm × 7 TPI

Threads › 1-1/4 BSW

1-1/4 BSW is a BSW Whitworth thread: 31.75 mm major diameter, 7 TPI (3.629 mm pitch), 55° form.

Standard	BSW Whitworth
Major diameter	31.75 mm (1.25″)
Pitch	7 TPI (3.629 mm)
Thread angle	55°
Minor diameter, bolt	27.1 mm
Minor diameter, nut	27.1 mm
Tap drill	Not listed. Check your tap maker's chart.

Minor diameters worked out from the basic profile: major Ø − 1.28065 × pitch.

Open 1-1/4 BSW in Benchsmith Add to order list

Measure it

Bolt: Measure across the crests of the bolt with the caliper jaws. Worn bolts read 0.1 to 0.3 under.

Nut or tapped hole: Measure across the tops of the threads inside the nut or hole with the inside jaws. That is the minor Ø. Worn or oversize holes read a little over.

Same diameter, other pitch

1-1/4-7 UNC7 TPI (3.629 mm pitch)
1-1/4-12 UNF12 TPI (2.117 mm pitch)

Close in size, won't mate

1-1/4-5 ACMEØ31.75 mm, 5 TPI
Tr32×6Ø32 mm, 6 mm pitch

One size up or down

1 BSWØ25.4 mm (down)
1-1/2 BSWØ38.1 mm (up)

Got a bolt and no idea what it is? Measure the outside Ø and type it into Benchsmith → Find → Thread.