HPA Fittings Warning – If You Have A PCP You Need To Read This!

This is a HPA fittings warning for anyone who either has – or is thinking of getting – a PCP airgun. It’s been a voyage of discovery for me…

The important points are these:

1. Contrary to what “everyone” says, there is no standard for these quick disconnect fittings.

2. You need to inspect your fittings TODAY as they can become damaged with use. I found one and it’s frightened me!

Please note. This investigation has been completed with two additional HAM posts. You may want to read these also…

Part Two of this analysis “What is a Foster Fitting?” can be read here.

Part Three is “Why Some HPA Fittings Don’t Fit”.

Below. Some female quick disconnects. All slightly different – even externally!

HPA Fittings Warning. Standard? What Standard?

Firstly, let’s give a word of praise to the AirgunGuild Forum. Alan and Steve – the owners – are concerned about HPA fittings compatibility, particularly as PCP fill pressures rise. They’ve started to tackle the issue with a public discussion.

A few years back, 3,000 PSI was pretty-much the standard for PCPs, now it’s up to 4,500 PSI in a growing number of cases. As pressures increase, the importance of HPA fittings compatibility and the risk of dangerous failures increase too.

Should an HPA connection fail under pressure, the results could be catastrophic, possibly a whiplash from the fill hose that could take out your eye. Or worse!

As a “live coward”, this concerns me. In fact, it concerns me even more due to something I’ve found during the preparation of this article.

The overwhelming majority of HPA quick disconnects use a male/female system that’s also been used for paintball guns. HAM usually calls these “1/8-Inch quick disconnects” based on the National Pipe Thread size they use. But they’re often called “Foster fittings” due to the name – Foster – of a manufacturer.

In fact, high quality quick disconnect fittings are manufactured by a number of US companies, including Foster, Hansen and Parker. There’s also plenty from Chinese companies too, as you would expect. But – so far as I can find – there is no published standard for these fittings. They vary by manufacturer, by application (gas, air, water etc) and designed working pressure.

Male Fitting Measurements

When trying to understand an issue, I like to measure things. Preferably many samples of “the same part”. This gives us a good idea of manufacturing aims and tolerances for production parts.

HPA Fittings Warning - If You Have A PCP You Need To Read This!

So I measured the diameter of no less than 107 male quick disconnects from Crosman. Why Crosman? Well, I have lots of them. Also, I know from long experience that their fittings are pretty consistent and work well. I’d say that Crosman has a good handle on these parts.

So here’s what I found…

HPA Fittings Warning - If You Have A PCP You Need To Read This!

As you can see from the chart above, the diameter of these fittings varied between 303 Thousandths of an Inch (Thou) to 309 Thou. No less than 96 of them were in the range between 304 and 307 Thou. That’s 90%, showing close manufacturing quality control.

Based on this, we can conclude that the manufacturing specification for these parts is 305 or 306 Thou.

But this is clearly not the diameter used by all manufacturers. In particular, I’ve often found that Chinese-manufactured male quick disconnects seem to be larger. Currently I only have one here. It’s from Snowpeak and it’s 313 Thou.

HPA Fittings Warning - If You Have A PCP You Need To Read This!

Yes, I understand that this is a sample of one. But it matches with my previous experience. When I tried to connect it with any of the female fittings I have here it did not fit. You can see the locking balls in the female exposed in the photograph below.

HPA Fittings Warning - If You Have A PCP You Need To Read This!

The diameter is so large that the locking balls in the female connector will not close over the male. Don’t even think of trying to feed High Pressure Air through this combination! It’s not going to hold and could be very dangerous!!!

Now this is NOT a slam on Snowpeak. As there’s no published standard, I don’t doubt that this male will fit the Chinese-manufactured females that they use for testing and product development. Snowpeak is a major airgun manufacturer. They’re not going to do anything that they understand to be potentially dangerous.

Female Fitting Types

Now let’s look closely at some female fittings. They should all be pretty similar, right? Well actually, no!

HPA quick disconnects may be manufactured from brass or steel. That’s pretty easy to spot, although sometimes fittings are brass internally with a steel ring to retain the locking balls. All brass fittings are suitable for lower pressures only.

But female fittings also have different numbers of locking balls. We’ve found 4, 5 or 6 balls being used in the female quick disconnects examined by HAM. I’m going to guess that more locking balls is better and that anything less than 6 is unsuitable for HPA use with higher pressure ranges.

First, here’s a female. It’s a high quality fitting but manufactured from brass and with only 4 locking balls. The manufacturer – Parker – specifies it for a maximum working pressure of 2,500 PSI. Should we be using anything like this for our PCPs? Obviously no!

Next we’re looking at a steel female connector with 5 locking balls.

Then we reach 6 locking balls within a steel body. Until anyone gives me documentary proof to the contrary, this is what I believe we need to be using for our PCPs!

Note that some airgun manufacturers – notably Brocock, Daystate and FX – use extra-long female connectors for the recessed males in their designs. All these that I have seen use 6 balls. That’s probably a good guide that quality manufacturers think this is correct, too.

The photograph below shows a Best Fittings female as supplied with Daystate airguns. (The black part is an adapter).

One other point. It looks as if some females use locking balls of a smaller diameter than others. But I’ve not been able to measure this. If this is not just an optical illusion, I’d have to say that – intuitively – larger locking balls are likely to be better…

Potential Danger Points

OK, this is where I frightened myself!

When you look carefully inside a female quick disconnect, you can see a rubber O ring deep inside. This obviously provides sealing when the connector is in use. I imagined that extensive use could damage this O ring and result in leaks.

It’s easiest to see this O ring in the photograph below.

HPA Fittings Warning - If You Have A PCP You Need To Read This!

But I did not expect to find this…

One fitting I have used repeatedly shows damage to the locking balls. Not only that but two of the 6 locking balls in this female quick disconnect are missing entirely. That was a HUGE HPA fittings warning for me!!!

HPA Fittings Warning - If You Have A PCP You Need To Read This!

How did the locking balls escape? I have no idea. But I can tell you that this connector will never be used again and has already left the building with other junk.


So please heed my HPA fittings warning. Check all the HPA quick disconnects you have in use. Make sure that they are appropriate for the pressure and ensure that they’re not damaged. Safety has to be the top priority for all of us!