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Description
Technical data for the G 1 1/4 inch left-hand hand tap
| Technical data | ||
|---|---|---|
| Nominal diameter d1 inches/mm | G 1 1/4 inch | 41.91 mm |
| Thread pitch P inches/mm | 11 threads per inch | 2.31 mm |
| Overall length l1 | 125 mm | |
| Flute length l2 | 36 mm | |
| Shank diameter d2 | 32 mm | |
| Square drive size a | 24 mm | |
| Tap drill diameter | 39.5 mm | |
| Form C chamfer | No. 1: 5P, No. 2: 2P | |
| Thread angle G | 55° | |
| Thread standard | BSP DIN ISO 228 | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The G 1 1/4 LH thread is cut into the wall of a hole with a diameter of 39.5 mm. The thread groove extends 1.21 mm into the wall. A single tap would have to cut the entire groove at once – by hand, this means high resistance and a risk of breakage. The set divides this work into two passes.
The drawing shows the entire set in operation. Both taps have the same shank and the same length. They differ in their chamfer, the tapered tip: No. 1 has a chamfer spanning 5 threads, while No. 2 spans only 2 threads. The detail windows at the hole wall show how the thread groove deepens after each tap.
| Tap | Chamfer | Chamfer angle | Material removed | How it works in the hole |
|---|---|---|---|---|
| No. 1 – roughing tap | long, 5 threads | 7° | the main portion of the groove | enters smoothly and establishes thread alignment |
| No. 2 – finishing tap | short, 2 threads | 20° | the remainder, to the full profile | screws into the previously cut threads and produces the full thread profile |
How to distinguish the taps in the set
The chamfer on No. 1 is 2.5 times as long as the chamfer on No. 2 – the difference is immediately visible when you place them side by side. No. 1 has an incomplete thread profile; only No. 2 has the full profile. The number is also indicated on the shank: No. 1 has one ring, No. 2 has none. Keep the set together and always start with the roughing tap.
How to tap a G 1 1/4 LH hole – sequence 1, 2
Hand tapping step by step
Thread G 1 1/4 LH in four steps
ØDrilling
Drill bit 39.5 mm
1Roughing tap
Chamfer 5 threads
2Finishing tap
Chamfer 2 threads
✓Finished thread
Thread G 1 1/4 LH
Turning sequence: half a turn counterclockwise, a quarter turn clockwise | Between taps: remove chips from the hole
Start with the hole. The G thread has a Whitworth profile, with a thread height of 0.64 times the pitch. The minor diameter of the internal thread is the 41.91 mm diameter minus twice the thread height, or 1.28 × the 2.309 mm pitch: the result is 38.952 mm. ISO 228-1 permits a hole diameter from 38.952 to 39.592 mm, and a 39.5 mm drill falls within this range. An undersized hole is a common cause of taps twisting off in the material.
Through hole or blind hole – how deep to drill
The set taps both types of holes. In a through hole, the thread extends through the full material thickness. In a blind hole, cut the thread to match the thread length of the fitting being installed. Drill the hole deeper than the thread. The finishing tap has a 2-thread chamfer, so approximately 2 incomplete threads remain near the bottom. The extra depth below the thread accommodates these along with the chips.
No. 1 – the roughing tap establishes thread alignment
Fit the roughing tap into a tap wrench and position it perpendicular to the surface. Check the angle from two sides, for example with a square. This is a left-hand thread, so turn in the opposite direction to usual: half a turn counterclockwise, then a quarter turn clockwise. Reversing breaks the chip into short pieces — a slight crunch is normal. The first threads determine whether the thread will be straight.
No. 2 – the finishing tap produces the full profile
Unscrew the roughing tap and remove the chips from the hole. Screw the finishing tap into the cut threads by hand – it should engage them naturally. Continue turning at the same rhythm as with the roughing tap. The finishing tap removes the remaining material and cuts the groove to its full profile. When it screws in smoothly all the way, the thread is ready – check it with a G 1 1/4 LH thread plug gauge: the GO end screws in by hand over the full length, while the NO-GO end enters no more than 2 turns.
1 1/4 inch left-hand pipe hand tap
Hand Tap 1 1/4 inch left-hand G DIN 5157 HSS is a precision 2-piece set (roughing tap + finishing tap), designed for manually cutting G parallel pipe threads (BSP – British Standard Pipe) o left-hand thread direction and a thread angle of 55°. The tool is made of high-grade high-speed steel (HSS), ensuring durability, wear resistance, and precise guidance in the hole.
The G 1 1/4 inch thread has an external diameter of 41.910 mm, in accordance with ISO 228-1. Left-hand threads are used in applications where the threaded connection is subject to self-loosening – for example, in hydraulic, gas, and rotary industrial systems.
G 1 1/4 inch left-hand pipe tap meets the requirements of standard DIN 5157, which means the set contains two tools – a roughing tap and a finishing tap – enabling safe, staged cutting of a precise thread compliant with the BSP standard.
- 2-piece set (DIN 5157) – roughing and finishing taps for manually cutting left-hand threads.
- G 1 1/4 inch left-hand thread – major diameter 41.910 mm (ISO 228-1).
- 55° thread profile – typical of BSP pipe connections, ensuring leak-tightness and durability.
- HSS high-speed steel – abrasion-resistant, designed for use in steel, brass, and cast iron.
- Specialized applications – in installations where a left-hand BSP thread is required.
Hand Tap 1 1/4 inch left-hand G DIN 5157 HSS is a reliable tool for professionals who need a durable, precise solution for manually cutting left-hand threads in BSP piping systems.
Stages of cutting a G 1 1/4 inch thread with a 2-piece set: roughing and finishing taps
Frequently asked questions
The question “how many mm is G 1 1/4 inch” or “how many cm is G 1 1/4 inch” often arises when selecting pipe tapping tools. Contrary to what one might expect, the value 1 1/4 inch (equivalent to 41.91 mm or 4.19 cm) does not indicate the thread diameter, but the nominal size of the pipe that the thread fits. In practice, the 1 1/4 inch G tap has an outside diameter of 41.91 mm, because G (BSP) pipe threads are designed for thick-walled pipes.
For this reason, always check the thread diameter in the technical data instead of converting inches directly to millimeters. In the BSP system, thread size is determined by the design of the entire installation, not just the nominal dimension.
No. A G (BSP) thread is a parallel pipe thread, meaning its diameter is constant along its entire length. Unlike taper threads (designated R or BSPT), G threaded connections are not self-sealing. This means that a G thread does not provide a seal on its own – it requires an additional sealing element, e.g. a flat gasket, O-ring or plastic washer. This sealing method is commonly used in hydraulic, pneumatic and industrial installations.
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