Measuring Threads

Here at AirRide we use imperial fittings. Why is that? Surely they are out of date and metric is the way to go?

Well there may be some truth to that but all of our management systems come from the US where imperial fittings are very common. Therefore they tend to be the standard for aftermarket suspension systems because if your manifold, MPC controller, solenoid, tank or bag is imperial it makes no sense to switch over and use a mixture of imperial and metric.

One of the oldest bag manufacturers is Firestone and they are predominantly imperial. A large number of bags or air springs are made to the PCD and dimensions of the original Firestone bags so as a result most bags are also imperial. The actual thread used in known as NPT but this in itself can be confusing.

American National Standard Pipe Thread are standards used to define the thread pitch, shape, diameter and angle of the taper where necessary but are often called just National Pipe Thread for short. They include both tapered and straight thread series, but in the air suspension industry we tend to use the tapered thread version which is known as NPT – (National Pipe  Taper, not National Pipe Thread). This is used because the taper has greater sealing properties and in theory they can be assembled without thread seal. In practice this doesn’t work, so don’t try it!!

NOTE:- NPT looks very similar to BSP (British Standard Pipe) and whilst they are able to be threaded into each they wont seal reliably, especially as is commonly the case where BSP is not tapered. Dimensionally BSP is different and they should not be used together.

To correctly measure the thread size you need some specialist (although not expensive) tools. You would need a thread pitch gauge, thread caliper, and depth gauge. In practice if you have AirLift Company, AirLift Performance , Viair, Universal Air Suspension, Slam Specialities, Firestone, or our own AirRide components then you will almost certainly have NPT threads, and you don’t need specialist equipment or a full understanding to identify your fitting.

Measuring is simple but there is a common problem that can cause some people to order the wrong thread size, so be careful, and use the guide table included here. Fortunately most systems use 1/8 NPT, 1/4 NPT, 3/8 NPT and 1/2 NPT. In practice the most common is 1/4 NPT. So really we have only 4 fittings in common use, however measuring any element of them will NOT equate to 1/8, 1/4, 3/8 or 1/2 inch so this is where care is needed.

If the OD (outside diameter) of your thread (male fitting) or ID (internal diameter) of the hole (female fitting) is around 12.5mm then its going to be 1/4 NPT. This is where the confusion lies. 12.5mm is 1/2 inch so shouldn’t that be 1/2 NPT? No. It relates to nominal bore (internal diameter of the pipe and water flow,,,) which is beyond this note, and not needed for identification. The measured size is always higher than the named size.

The attached table will help you – there are a lot of different threads and this is meant to assist only for the NPT threads on our website, and the dimensions are approximate for ruler measurement (and not technically correct).

Fittings come in male and female varieties, which are self explanatory, and in straight or 90 degree, which are also self explanatory. All our 90’s are full 360 swivels. Fixed 90’s are useless for air suspension as you cannot control where they end up when they are tight. 360’s can still be turned even when tight.

Our standard fittings are for 1/4 Push To Connect (PTC) hose, although we also have fittings for 1/8, 3/8 and 1/2 PTC pipe, hose, line at the other end of the fitting to the thread. We use only the highest quality fittings which simply don't leak. We have tried lower cost fittings (we get offered fittings from new suppliers almost every day) and they often just don't seal. Our policy for new product ranges is that first we use it on our own vehicles, often for up to a year, then we install only in our workshop and only if we are totally satisfied will we start to supply products to customers knowing they have been tested and checked in the real world.

If in doubt please send us a picture of your fitting, ideally next to a ruler and we can likely help. If you need help identifying your requirements please call us on 0800 772 0315 or send an email to  ENQUIRIES@AIRRIDE.UK

Graphic showing sizes and explanations for air suspension fittings