Thread inspection in scuba diving: G5/8" put to the test

Just before the dive. The cylinder is ready, and the regulator is being connected. The thread engages, and the connection screws in without any difficulty. Everything is seated as it should be. At least, that is how it seems. But what if the thread still engages even though it no longer meets its specified limit dimensions?

Underwater, this connection must function reliably, because it is part of the breathing gas supply. Damage, wear or dimensional deviations, however, are not always visible at first glance. And just because two components can be screwed together without difficulty does not mean that their threads are dimensionally accurate.

What cannot be reliably detected when connecting the equipment can be checked specifically in the workshop. This is exactly what S.G. Scuba Gaskets and the Association of Scuba Service Engineers and Technicians (ASSET) demonstrate in a practical application. Using VÖLKEL limit thread gauges, the technicians check both sides of the connection: the internal thread of the cylinder valve and the external thread on the regulator. Not by feel, but against defined limit dimensions.

Where is the G5/8" thread located in scuba equipment?

Two threads meet at the DIN connection: the G5/8" external thread on the regulator’s first stage and the matching internal thread in the cylinder valve outlet. When screwed together, the two engage and form the mechanical connection.

An interesting point: the thread itself does not provide the seal. That job is performed by a separate O-ring. The thread holds the components together, while the O-ring seals the connection. These are two different functions that must also be assessed separately during a technical inspection.

DIN connections are available in different versions, including those for 232 and 300 bar. Matching thread designations alone are not enough to determine whether the complete connection is compatible. The other connection dimensions specified in DIN EN ISO 12209 must also be correct. This connection should not be confused with the M25 × 2 cylinder neck thread, which is used on many European scuba cylinders to attach the cylinder valve.

Why is thread inspection important in scuba diving?

Before and after dives, a regulator is regularly connected to and disconnected from the cylinder. This exposes the threads to mechanical loads. Over time, wear, corrosion or damage may occur. A connection screwed in at an angle or forced into place can also deform the threads and damage the thread flanks.

More significant damage can often be seen with the naked eye. But what about deviations that are barely noticeable from the outside? A thread may look undamaged, screw together without difficulty and still be outside its permitted limit dimensions. This is precisely where first impressions are no longer enough. Limit thread gauges allow a targeted check of dimensional accuracy and therefore complement visual inspection.

For Joey Ridge, scuba technician and instructor at ASSET, thread inspection is therefore an important part of the inspection process. He explains:

“Just because a regulator can still be screwed into the cylinder valve does not automatically mean that the threads are within the specified tolerances. With GO/NO-GO thread gauges, we can check both sides of the connection against defined limit dimensions. This is an important part of the inspection, alongside visual inspection and checking the sealing surfaces.”

The footage from the ASSET workshop shows how this inspection works.

How is the G5/8" internal thread on the cylinder valve checked?

In the ASSET workshop, the inspection begins at the cylinder valve. The technician inserts a VÖLKEL G5/8" × 14 limit thread plug gauge into the internal thread. The GO side is used first: it must be possible to screw it in by hand, without excessive force, over the specified thread length. If it stops before that, the thread may be too tight or damaged.

The countercheck follows. The technician applies the red-marked NO-GO side. Unlike the GO side, it may engage only to a limited extent: in this inspection, no more than two turns. If it can be screwed in farther, the thread is outside the limit conditions being checked. These two inspection steps establish whether the thread characteristics being checked are within their permitted limits.

It is important that the gauges are clean and undamaged and are used without force. Even contamination can affect the inspection result. The tool used is the VÖLKEL precision GO/NO-GO thread plug gauge G5/8" × 14, item 16132.

How is the G5/8" external thread on the regulator checked?

The internal thread is only one side of the connection. Next comes its counterpart on the regulator. Dimensional accuracy must also be checked here, but with different inspection tools: instead of a plug gauge, two separate limit thread ring gauges are used.

First, the GO thread ring gauge is screwed onto the external thread. It must screw on by hand over the entire thread length to be checked. If it stops before that, damage or a dimensional deviation may be present. Next comes the NO-GO thread ring gauge. In this inspection, it may screw on by no more than two turns. Only when both checks meet the specified conditions are the checked thread limits met.

For this, the ASSET workshop uses the VÖLKEL precision GO thread ring gauge G5/8" × 14, item 16322 and the precision NO-GO thread ring gauge G5/8" × 14, item 16466. The workshop photographs show the practical application on a DIN connection for 300 bar.

What thread gauges can check, and what they cannot

After inspection, one crucial question has been answered: do the threads meet the checked limit dimensions? If both threaded sides pass the GO/NO-GO inspection, their dimensional accuracy is confirmed with respect to the characteristics checked. This makes it possible to assess a potential source of error specifically, rather than relying solely on appearance or how the connection feels when screwed together.

However, other factors are also crucial to the safety of the entire connection. Thread gauges can assess neither leak-tightness nor the condition of the O-ring. According to Scuba Gaskets, a size 111 O-ring is used in the DIN connection shown. The company offers matching seals in different materials, whose suitability depends on the equipment manufacturer’s specifications. This is where the areas of expertise come together: Scuba Gaskets supplies seals and service components, ASSET contributes its maintenance and technical inspection experience, and VÖLKEL provides the appropriate limit thread gauges.

Back at the dive site: the regulator can be screwed into the cylinder valve without difficulty. What initially simply looked good can now be assessed for thread dimensional accuracy through a defined inspection. Because whether a thread can be screwed together is one thing. Whether it meets the required limit dimensions is another.

Anyone who would like to understand the inspection principle in more detail can find further information on using limit thread gauges in our blog article “Thread inspection: GO and NO-GO explained simply”.