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Which Hose Do I Need

Replacing old hoses on your regulator can feel like a minefield with all of the different size threads available and fittings.  This blog will help you understand the different threads used on SCUBA regulators today.  At the bottom I have compiled a list of first stages to try and help you choose which threads you need when replacing old ones.

Regulator manufacturers use different sized screw threads when connecting all of the hoses to their first stages so that nobody can accidentally attach a low pressure hose to a high pressure port and damage their regulators.  As many regulator parts are made out of marine grade brass, which is quite a soft metal, attempting to screw two different threads together can damage the threads and be very expensive to fix, or in most cases replace.  Hoses and blanking plugs only need to be screwed in with 5Nm of Torque, which is just over hand tight, too much force and you can strip the threads.



All Regulator first stages will have:

  • at least one high pressure port usually marked with HP stamped next to it
  • multiple 3/8″ low pressure ports for second stages and inflator hoses

Some Regulator first stages will have:

  • one 1/2″ low pressure port for a primary second stage and multiple 3/8″ for other low pressure hoses


Choosing hoses can be quite confusing when trying to replace existing hoses with new ones.  
These are measured by the maximum diameter of the thread (which is the number part) and the thread quality and standard (the letters before or after) There are three commonly used screw threads for low pressure hoses connecting to a first stage and your second stage. 
On the first stage itself you will find:

  • 3/8”UNF (Outlined in RED)
  • 1/2”UNF (Outlined in GREEN)
  • 7/16”UNF (Outlined in BLUE)


High pressure and low pressure hoses can be easily told apart by looking at the male end of the hose. A low pressure hose will have a fairly large bore hole for improved airflow compared to the high pressure’s pin hole sized bore.  The small bore of a high pressure hose is due to the high pressures they deal with where the internal volume needs to be kept to a minimum.




(Red and Green in diagrams)

Both the 3/8” and 1/2” sized threads are used for low pressure hoses, 3/8” is by far the most common but there are some exceptions that use the 1/2” listed at the bottom. 1/2” ports can usually be easily identified on the first stage by a large bore tube in the middle of the first stage port (GREEN in first picture) and are usually only used for the primary second stage.

On the other end of the hose you will either find a BCD / drysuit inflator or a female 9/16” that will fit most standard second stages. Read my blog on the different types of BCD and drysuit inflator hoses .




(Blue in diagrams)

The high pressure port on your first stage will have a 7/16” UNF thread. High pressure hoses will have a 7/16″ male fitting on one end and a 7/16″ female thread on the other that should fit all high pressure gauges or transmitters.

At the Male end of the hose you’ll find a tiny opening but the female end is where all the interesting stuff happens. First off the female end will swivel and requires an O-Ring to seal so you need a swivel pin that will sit inside the hose and create a seal between the hose and SPG or transmitter.




Hoses are available in a huge range of lengths depending on their intended usage.  Longer hoses can reduce strain on your neck and jaw but too long will kink, drag and create a snag hazard, shorter hoses can neaten hose routing but limit flexibility.  As an instructor I prefer to have a much longer hose on my alternate air stage for added flexibility in an emergency, so I tend to fit a 150-210cm hose for my octo.

  • Most primary second stages come with a standard 75cm hose
  • Standard alternate air sources usually come with a 90-100cm hose
  • BCD inflator hoses tend to be around 75cm but are dependent on your BCD’s corrugated hose
  • Pressure gauges have a 90cm high pressure hose that are sometimes much thinner than other hoses




(Yellow in diagrams) If all else fails you can take a ruler and measure the internal diameter of a port from the root of the thread across to the opposing root and the external diameter of a hose from crest to crest to see what size it is as a fraction of an inch.


Regulator First Stages that use 3/8″UNF Low Pressure Ports

  • All current Apeks
  • All current AquaLung
  • All Cressi
  • All Dive Rite
  • All Oceanic
  • All ScubaPro
  • All Seac Sub
  • Mares 2S, 12S & 52 1st Stages

Regulator First Stages that use a 1/2″UNF Low Pressure Port

  • Some Older Apeks
  • Mares 22 1st Stage



Still confused or need some advice? Check out our video guide below, or feel free to give us a call on 0844 567 70 73 or email us with details here.