1 BSW thread: 25.4 mm × 8 TPI

1 BSW is a BSW Whitworth thread: 25.4 mm major diameter, 8 TPI (3.175 mm pitch), 55° form. Tap drill 22 mm.

Standard	BSW Whitworth
Major diameter	25.4 mm (1″)
Pitch	8 TPI (3.175 mm)
Thread angle	55°
Minor diameter, bolt	21.33 mm
Minor diameter, nut	21.33 mm
Tap drill	22 mm

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

Open 1 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

1-8 UNC8 TPI (3.175 mm pitch)
1-12 UNF12 TPI (2.117 mm pitch)

Close in size, won't mate

1-5 ACMEØ25.4 mm, 5 TPI
Tr26×5Ø26 mm, 5 mm pitch

One size up or down

7/8 BSWØ22.23 mm (down)
1-1/4 BSWØ31.75 mm (up)

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