What is the tap drill size for G 1-1/4?

39.50 mm, which gives 82% thread engagement. Published figures for BSPP sizes vary because each table silently assumes a different engagement, anywhere from about 71% to 88%. The minor diameter is 38.953 mm.

What is the actual diameter of a G 1-1/4 thread?

G 1-1/4 has a major diameter of 41.910 mm (1.6500 in), not 1-1/4 inch. G sizes are named by nominal bore, not by measured diameter.

Does G 1-1/4 seal on the thread?

No. BSPP is a parallel thread and cannot seal by thread interference. It needs a bonded seal, an O-ring or a cone seat.

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G 1-1/4 (BSPP) Thread Dimensions

ISO 228-1 · 55° Whitworth form · parallel, not tapered · 11 TPI

Tap drill (82% engagement) 39.50 mm Major 41.910 mm · pitch dia 40.431 mm · minor 38.953 mm

G 1-1/4 does not measure 1-1/4″. It measures 41.910 mm (1.6500″) across the crests. Like every pipe thread, the G number is a nominal bore inherited from wrought iron pipe, not a diameter you can put a caliper on.

Dimensions

Major diameter (d)

41.910 mm

Pitch diameter (d₂)

40.431 mm

Minor diameter (d₁)

38.953 mm

Threads per inch

Pitch

2.309 mm

Thread height (h)

1.479 mm

Crest / root radius

0.317 mm

Thread angle

55°

ISO 228-1 tabulates only the major diameter. Pitch and minor diameters follow from the basic profile — h = 0.640327 × p, d₂ = d − h, d₁ = d − 2h — which is how the figures above were produced.

Why sources disagree about the G 1-1/4 tap drill

Look this drill size up in three places and you will get three answers. They are not typos. Every table silently picks a different thread engagement — how much of the theoretical thread form the tap actually cuts — and then prints the resulting number without saying which one it chose. Across published BSPP tables the implied engagement ranges from about 71% to about 88%.

So here is every drill you can actually buy for this size, and what each one really gives you:

Drill	Thread engagement
39.00 mm	98%	Maximum strength, highest tapping torque
39.50 mm	82%	Common commercial choice ← recommended

39.50 mm is the one to use unless you have a reason not to. Going nearer the minor diameter (38.953 mm) buys thread strength at the cost of tapping torque and tap breakage — and on a parallel pipe thread that trade is rarely worth making, because the thread is not what seals the joint.

BSPP does not seal on the thread. The flanks are parallel, so tightening only pulls metal against metal and never wedges. The seal has to come from somewhere else: a bonded seal (Dowty washer) under the hex, an O-ring in a face groove per ISO 1179-2, or a 60° cone seat. PTFE tape on a BSPP thread accomplishes nothing except making the joint harder to undo.

G 1-1/4 against the taper threads

G 1-1/4 shares its 55° Whitworth form and its 11 TPI with 1-1/4″ BSPT (R1-1/4 / Rc1-1/4), which is why the two get mixed up constantly. The difference is that BSPT is tapered and BSPP is not. A BSPP male will start into a BSPT female and feel fine for a couple of turns, then either jam or never tighten.

The one pairing that is standard: ISO 7-1 allows a taper male (R) into a parallel female (Rp). Rp and G have the identical basic profile and differ only in tolerance class, which is the source of most of the surviving confusion in catalogues.

BSPP vs BSPT → Full BSPP chart → 1-1/4″ BSPT → All pipe threads →