Here is where our customers see their minds boggle! So we hope to see this page being a popular go-to for those who's heads have rolled off trying to figure out the sizes of their fittings! So go locate your head and knock your self out reading through all of the below answers to your problems!

You may have already purchased the wrong sized fitting before now, which has just become a huge mystery on where the sizing comes from! On the other hand, you might be reading this after blowing your mind at whether you need a 1/4” or a 3/8”!

WE KNOW! It’s utter sorcery! It’s time someone laid it out straight!



So, first thing to explain is that the systems we retail are commonly created in the USA, which are all manufactured using the American national Imperial units system. Here in the UK, we use the Metric system. Therefore, we use meters and grams in the UK, whereas the USA use inches, pounds and feet. This is where the units for the fittings we supply originate from. All in inches and feet!

Some fittings use measurements taken from an inner diameter and some from an outer diameter, which vary by eye too. So for what will have the same inches size, may seem different to eye. So, for example, if you have a 1/4” airline, this may not match the size of a 1/4” thread, as these are measured differently, using inner diameter for airline and outer diameter for thread measurements. Same for when you order a “equal” ended fitting (3/8”PTC to 3/8”NPT for example) and you think one end is larger than the other, it’s OK, it’s just how the PTC and NPT measure up with Imperial units, using the inner/outer diameters!

Another feature of the fittings range is that most of the Imperial threads (internal or external) are slightly tapered too. So, there are no metric threads to suit the imperial range. It just will not create that air tight lock the systems need from the connections!

If you are ever unsure, feel free to send over some photos to our sales team and we can always help confirm that sizing for you. See our Contact Page on how to get in touch. 



This depends. Some fittings come with a thin layer of PTFE on the threads and others (such as our Nickle range) come with a rubber O-ring at the base of the threads. The O-ring eliminates any need for any thread lock, although we've known installers use it just for peace of mind. The O-ring will create the air-tight seal you need on its own! 

The one thing we don't recommend as a overall rule is DO NOT USE PTFE TAPE - This is known to lose fibres or threads when screwed in or out of ports, allowing debris to get trapped in the manifold or air suspension system. It causes valves to stick open and all sorts of other issues that will ultimately kill your system! A very costly repair for the sake of a few quid on PTFE! So avoid this stuff like the plague! 

Unbelievably, some kits come with PTFE tape including in a fitting pack. But this doesn't mean you should use it! Invest in some paste and throw that nasty-ass tape out! 

We fully recommend using Thread Lock Paste or PTFE Paste, which you can find on our website here or in any reputable DIY store.



This is a common question from our customers and we see over tightened fitting issues all the time! 

The recommended torque settings for your fittings and connections are to wind the threads as much as you can by hand, until you feel it starting to "nip". Take a spanner and wind the fitting in one full turn up to a turn and a half, but no more than that. 

If you prefer to use thread lock paste or PTFE, that and the above advice will be perfect to create the seal you need in the connection. 

Never expect the fittings to thread 100% of the way in to the point where you cannot see any threads, this is a clear sign of over tightening and will likely be creating a weak point in the system or eventually snap the threads off completely. 

It is common throughout the system to still be able to see threads or connections looking like it's not threaded in enough, but this is normal!  

The reason we see crossed threads or snapped fittings is due to over tightening in the tapered NPT threads.

The NPT threads are tapered to create a better join. However, when fittings are over tightened, the taper applies pressure to the wider thread and this pressure creates damage or results in snapping the fitting altogether.

So only apply the right torque to your fittings and trust our advice! :) 



There are many different styles of fittings on our website and some have multi-uses. See below for a detailed outline on each style and their available sizing combinations: 

Male Straight – This fitting is usually a straight design with a PTC port on one end and an external NPT thread on the other. These come in a variety of finishes and sizing combinations from 1/8” up to 1/2". Commonly used on bag to airline joins, tanks and water traps. See below example:

Push To Connect PTC - Male Straight


Female Straight – This is the same design as a Male Straight, but with an internal NPT thread on the opposite end to the PTC port. Again, these fittings are available in a variety of finishes and sizing combinations from 1/8” to 3/8”. Commonly used on compressors and braided lines. See below example:

Nickel Full Metal PTC Female Straight

Elbow - These fittings are an L shaped fitting with the same PTC port and Male NPT thread as the Male Straight. These are perfect for sending airline or hardline in or out of a component at an angle. Great for those with little wiggle room! These are available in varying angles, however most popular are either a 90° or 45° elbow. These come in a variety of finishes and sizing combinations from 1/8” up to 1/2". Commonly used on bag to airline joins, tanks and water traps. See below example:

Push to Connect PTC - Male Elbow

Nipples – Yes. Really. Nipples. These are a “threaded union” where both ends are a male NPT thread with a fixed hex nut in the middle to tighten. Great for joining two female NPT threads together! These threads can be equal or unequal to match differing sizes of female NPT. These come in a variety of finishes and sizing combinations from 1/8” up to 1/2". Commonly used on check valves, compressors, tanks and water traps. See below example:

Male NPT Nickel Hex Nipple

Plugs – Pretty much what they say on the tin! These fittings are also known as “blanks” or “bungs”. They are solid fittings that are a Male NPT and screw in to a female NPT port. Commonly used on any unused ports on a tank. This blanks off the port and keeps things air tight and flush!Available in 1/8” up to 1/2”. See below example:

Nickel Metal - Allen Key Air Tank Plugs

Reducers and Expanders – Perfect for converting something from a smaller size to a bigger size or visa-versa! Commonly used in Manifolds, joining differing sizes of airline, hardline and even NPT threads together! There are many reasons why these are needed! Available in a variety of finishes and sizing combinations from 1/8” up to 1/2". See below examples:

PTC Push To Connect - SMC Plug-In Stem ReducerPTC Push To Connect - SMC Plug-In Expander

Tees – These are “T” shaped fittings, hence the name! These help join multiple lines together and act as a 3-way union. They are also used to connect up gauge displays and other add-in features, without tearing out your lines! These are available mostly in the SMC range with plastic bodies and come in a combination of sizes from 1/8” to 3/8”. See below examples:

PTC Push To Connect - SMC Tee Equal Union

Unions – A straight fitting with dual PTC ends to act as a join. These are perfect for connecting two air lines together! These are only available in plastic and come in a range of size combinations from 1/8” to 3/8” PTC. Also available in equal or unequal sizes. See below examples:

PTC Push To Connect - SMC Unequal Reducer Union

Bulkheads – Ideal for show builds and installs, allowing you to fix hardlines through panels or false floors with flexible airline on the other side for ease of removal or access. Available in a range of metals and size combinations from 1/4” to 3/8”. Also available in PTC to PTC or PTC to NPT combinations. See below examples:

Product Link: http://www.onlychargeddubs.co.uk/bulkheads/

Nickel 1/4



PTC - PTC stands for Push To Connect. This being the term for the “airline” end of a fitting. See below example photo:

Black Plastic PTC Female Straight

NPT - NPT stands for National Pipe Thread. This can be used for external or internal thread sizes. Also known as Male (external) or Female (internal). See below example photo:

Black Plastic PTC Female Straight

AN - AN is another term used to describe a term of measurement in the fittings range. This refers to threads only and is used by American Aeronautical and Navy Engineers, hence the AN abbreviation! This is more common among fittings meant for the Air Zenith Compressors or hard line set ups, line the one below:

AN8 to NPT OB2 Compressor Adapter




This is the most popular question out of all we receive! The answer, as always, isn't that simple. As most products and various manufacturers create kits with the options of different sizes and/or this then differs throughout the kits. So it really depends on what you have. 

The most common sizing it either 1/4" or 3/8" in both PTC and NPT. But as detailed further up this page, the Imperial measurements do not relate to anything on a Metric ruler, so please do not order based on a Metric measurement of your imperial fitting! This will not interpret over well and will 100% be the wrong sized fitting when it reaches you. 

For example. If I got a 1/4" NPT fitting and measured this with my tape measure that is metric, the width or diameter of the 1/4" fitting against that tape measure will not be one quarter of our metric inch. It will look more like a third of a metric inch! 

Instead, look over the example photos below and this should hopefully help you suss out which size you have:

NPT Examples:


PTC Examples:

(To be uploaded)


Once you see the differences and have the comparison in your head, you'll be knowing the sizes by eye before you know it! 

However, if you're head is still rolling off after all that, drop us an email or include photos and we'll look to help and identify what you need! :)