G threads are also called Whitworth parallel threads. They are cylindrical on the inside and outside. They are suitable for pressure-tight connections which are not sealed in/by the thread - therefore they are not metallically sealing.
Eyeballing the Difference Between NPT and G Connections. When placed side by side, the difference between the two connection types is clear. The NPT thread is slightly tapered, while the G connection is straight. Another obvious visual: The G connector ends with a small protrusion (nipple), used to center the gasket.
G (supposedly from German/English “Gas”) are the cylindrical pipe threads with 55°apex angle used on originally inch based pipes. BSPP thread is often referred to as “G” threads, the two types of thread are the same parallel straight, the threads are measured in threads per inch. Therefore, BSPP is the same as G.
For example, G threads are not compatible NPT threads because of their differing angles, shapes, and thread pitches (threads per inch).
G or R (Whitworth) or BSP (British Standard Pipe) Thread
Please note for the final determination whether you have a fitting with G or R-threads: G-threads, male as well as female, are cylindrical whereas you have to distinguish that male R-threads have a conical form and its female counterpart is cylindrical.
Thread Cheat Sheet and Notes:
A Rc thread male will go into a G thread female and seal correctly. A G thread male will NOT go into a Rc female.
Tolerance letter designation
Internal threads only use G and H, and external threads use a-h. As seen in Figure 2, H describes tolerance with zero deviation from the basic profile, and G describes tolerance with deviation above the basic profile.
There are three standard thread series in the Unified screw thread system that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).
G & R Thread Types
G is the international standard shorthand for BSPP - BSP Parallel threads. R is the standard for BSPT - BSP Tapered.
Two types of threads are used: Parallel (straight) threads, British Standard Pipe Parallel thread (BSPP). These are sometimes denoted by the letter G, for example G 1/2" means a parallel 1/2 inch BSP thread. Taper threads, British Standard Pipe Taper thread (BSPT).
Meanwhile G threads are taken from BS2779:1986 pipe threads for tubes and fittings where pressure-tight joints are usually made on a washer or 'O' ring. Using a ruler or vernier, measure across the outside diameter of a male or hose tailpiece thread or measure the inside diameter of a female thread.
One-G's small amount of stretch helps you feel like you're "locking" your beads into place to achieve perfect thread tension throughout your project. Don't fret that this stretch will lead to loose beadwork overtime; instead, the thread quickly returns to its original form to secure the beads.
G is a kind of thread of 55 degree non-threaded sealed pipe,belonging to the WHITW thread. G stands for cylindrical thread. G is the general name of the pipe thread (Guan),the division of 55, 60 degrees is functional, commonly called the pipe circle.
NPT has a tapered male and female thread which seals with Teflon tape or jointing compound. Pipe threads used in hydraulic circuits can be divided into two types: Jointing threads are pipe threads for joints made pressure tight by sealing on the threads and are taper external and parallel or taper internal threads.
To determine the NPT size, with a caliper, measure the diameter of the 2nd or 3rd thread on your pipe, fitting or valve as shown below. Find the caliper measurement in the chart and cross reference the nominal NPT pipe size. Don't get confused, the actual measurement is not the same as the NPT pipe size.
BSPP, BSPF (PF) and G threads are all BS EN ISO-228-1 standard and can be compatible. All four are imperial threads.
Two types of threads are distinguished: Parallel (straight) threads, British Standard Pipe Parallel thread (BSPP; originally also known as British Standard Pipe Fitting thread/BSPF and British Standard Pipe Mechanical thread/BSPM), which have a constant diameter; denoted by the letter G.
NPT and BSP threads are generally not compatible due to the differences in their thread forms. NPT threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
Kevlar(TM) thread is one of the strongest and most fire retardant commercially available threads. It is about 2.5 times stronger than nylon or polyester, has almost no stretch, does not melt, and decomposes at 800F.
UNC. UNC stands for 'Unified Coarse' and is the most commonly used thread type in the United States of America. UNC fixings are often used within the agricultural industry because of their strong coarse threads.
Thor Thread, also known as PBO Fiber, has insane tensile strength. It beats steel, kevler, and carbon fiber. Thor Thread is literally the strongest purchasable material on Earth, by weight.
Size for size, a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter). Because of the smaller pitch, they allow finer adjustments in applications that need such a feature.
A 1/4 BSP (G1/4) thread has a 13.157mm outside diameter and a 1/2 BSP (G1/2) thread has a 20.955mm outside diameter.
V Threads at Work
Metric ISO threaded screws are V-shaped at the outer edges, and are self-locking so do they not need a screw lock. The flank angle of a V thread is 60° and it is commonly used for all places where fine adjustments with leads as fine as 0.0125inch (0.3175mm) are critical such as microscope stages.