Home › 1/4" BSPT (R1/4)
1/4" BSPT (R1/4) Thread Dimensions
ISO 7/1 (BS 21) · 55° thread angle · tapered 3/4″ per foot on diameter (1:16), which is 1°47′ of included taper
Tap drill
11.5 mm (0.453")
Major diameter 0.518″ (13.16 mm) ·
19 TPI · hand-tight 0.209″ (~4.0 turns)
A 1/4″ pipe thread is not
1/4″ across. It measures 0.518″
(13.16 mm) at the large end. Pipe sizes are named by nominal
bore, a convention inherited from wrought iron pipe where the number described
the rough inside diameter. If you have measured a fitting and the number doesn't
match, that is why — measure the outside diameter and match it to the table
below, don't trust the name.
Dimensions
Major diameter
0.518 in
Major diameter
13.16 mm
Threads per inch
19
Pitch
1.337 mm
Pitch diameter (E1)
0.4868 in
Minor diameter
0.46 in
Thread angle
55°
Hand-tight (L1)
0.209 in
Pitch diameter is quoted at the gauge plane — the reference point at the end of hand-tight engagement. Because the thread is tapered, quoting a single pitch diameter only means anything if you also say where along the thread it was measured.
Tapping a 1/4" BSPT (R1/4) hole
| Step | Detail |
|---|---|
| Drill | 11.5 mm (0.453") |
| Tap | 1/4" BSPT (R1/4) taper pipe tap |
| Depth | Allow at least 0.52″ of usable thread, plus room for the tap's vanish threads |
| Assembly | ~4.0 turns hand-tight, then 2–3 turns with a wrench |
BSPT tap drill sizes vary between tap makers more than NPT ones do, and published tables disagree by a few tenths of a millimetre. Treat this as a starting point and check the recommendation that came with your tap.
The taper is what seals, not the sealant — the sealant fills the
spiral leak path along the thread crest. Over-tightening a tapered thread splits female
fittings, especially cast iron and brass, because every extra turn wedges the joint
harder. If it is still weeping after three wrench turns, back it out and re-tape rather
than pulling harder.
Is this NPT or NPT?
1/4″ NPT and 1/4″ BSPT have different thread pitches. They will thread together far enough to feel right, then leak, because the thread angles differ — 60° for NPT against 55° for BSPT. How to tell them apart →