G Thread
Is a G thread tapered?
No. G threads under ISO 228 are parallel, not tapered. The sealing mechanism relies on a washer, O-ring, bonded seal, or mating face rather than thread wedging.
What standard is behind G thread?
The G thread designation is defined by ISO 228-1 and ISO 228-2, which cover BSPP parallel pipe threads with a 55° Whitworth thread form.
On a CNC lathe, what code cuts a thread?
Threading cycles are commonly programmed with G32, G33, or G76 depending on the control. These are machining commands and are separate from the G thread designation in a pipe fitting callout.
G thread commonly refers to BSPP parallel pipe thread per ISO 228-1/228-2, featuring 55° Whitworth form, no taper, and sealing via washer, O-ring, bonded seal, or mating face, not the thread flank. Machinists cut it on CNC lathes with G32, G33, or G76 cycles, verify with gauges, and deburr crest and root for clean assembly.
On the shop floor, a G thread callout on a drawing tells the machinist to produce a straight BSPP thread, not a tapered one. The usual approach is to turn the boss or bore to the specified limits, cut the thread with a lathe cycle such as G76, and then deburr the crest and root carefully. Because the thread itself does not seal, the critical part is the mating face or washer seat. A machinist must protect that surface from knocks and burrs; otherwise the fitting can leak even if the thread gauge passes. Deburring is especially important when the part is a manifold or hydraulic block, since the thread only retains the fitting while the seal happens at the face. Checking pitch, pitch diameter, and thread form with a proper gauge is standard, but final assembly success depends on the sealing face condition just as much as the thread dimensions.
Wrong taper assumption: A G thread is parallel, but if it is cut like an NPT taper thread, the fitting tightens early or bottoms out, creating leakage because the sealing mechanism is completely different. The thread form and taper must match the BSPP callout exactly.
Chasing the wrong diameter: Operators sometimes open up the major or minor diameter to force a tight fit, which ruins pitch diameter control. The result is weak flank contact and a thread that fails gauge inspection or leaks under pressure.
Damaged seal surface: A BSPP assembly seals on the face or washer seat, not on the thread flanks. A nick, burr, or poor finish at the sealing land causes leaks even when the thread gauge passes, so protect the face and deburr thoroughly.