Left-hand G pipe threads
Left-hand pipe threads are rare and correspondingly hard to source. They are used on fittings where a right-hand thread would work itself loose in service, classically on fuel gas connections. VÖLKEL makes seven machine taps for them to DIN 5156 in form C from HSSE, from G 1/8 x 28 LH to G 1" x 11 LH.
Parallel, not tapered
The G thread is parallel and does not seal on the flanks but on a sealing face or a sealing ring. The flank angle is 55° and the crests are rounded. If you need a self-sealing joint, use tapered Rc (BSPT) instead.
Why left-hand threads on gas
On fuel gases a left-hand thread is a safety measure: it prevents a fuel gas hose from being screwed onto an oxygen connection by mistake. That is why left-hand connections are widespread there and specified in the relevant standards.
While cutting
The tap turns anticlockwise, and the short reverse turn to break the chip goes clockwise. Form C has a short lead of two to three threads, so it cuts close to the bottom of the hole and also suits blind holes. Mark the workpiece and the tool clearly so nobody later starts in the wrong direction.
Right-hand versions are under G (BSP) pipe thread, further left-hand tools among the machine taps and hand taps. The differences between pipe thread types are in the article pipe thread types explained.
All sizes at a glance
7 articles in 1 series. The article number links straight to the tool.
Machine Tap DIN 5156 Form C HSSE left hand thread
| Threadnominal size | Tapping drillØ in mm | Article no. |
|---|---|---|
| G (BSP) 1/8 x 28 LH | 8.8 | 78202 |
| G (BSP) 1/4 x 19 LH | 11.8 | 78203 |
| G (BSP) 3/8 x 19 LH | 15.25 | 78204 |
| G (BSP) 1/2 x 14 LH | 19 | 78205 |
| G (BSP) 5/8 x 14 LH | 21 | 78206 |
| G (BSP) 3/4 x 14 LH | 24.5 | 78207 |
| G (BSP) 1" x 11 LH | 30.75 | 78209 |
Tapping drill: drill diameter recommended by VÖLKEL (Technical Information “Tapping drill sizes”).
