What is the tap drill size for G 2-1/4?

G 2-1/4 is larger than the biggest catalogued hand tap, so there is no tap drill. The hole is bored to the minor diameter of 62.753 mm and the thread is single-point turned or cut with a die head.

What is the actual diameter of a G 2-1/4 thread?

G 2-1/4 has a major diameter of 65.710 mm (2.5870 in), not 2-1/4 inch. G sizes are named by nominal bore, not by measured diameter.

Does G 2-1/4 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 2-1/4 (BSPP) Thread Dimensions

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

Bore diameter (no hand tap this size) 62.753 mm Major 65.710 mm · pitch dia 64.231 mm · minor 62.753 mm

G 2-1/4 does not measure 2-1/4″. It measures 65.710 mm (2.5870″) 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)

65.710 mm

Pitch diameter (d₂)

64.231 mm

Minor diameter (d₁)

62.753 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.

There is no tap drill for G 2-1/4

G 2-1/4 is past the largest catalogued hand tap, which is roughly G 2. A hole this size is bored rather than drilled, and the thread is single-point turned on a lathe or cut with a die head. Bore to the minor diameter, 62.753 mm, and cut the form from there.

This is worth stating plainly because several widely-copied BSPP tables carry a tap drill column all the way to G 6 and fill it with whatever their database returned. One well-known chart prints -999.00 for every size above G 2.

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 2-1/4 against the taper threads

G 2-1/4 shares its 55° Whitworth form with 2-1/4″ BSPT (R2-1/4 / Rc2-1/4), 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.

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