Why Some HPA Fittings Don’t Fit

In order to understand HPA fittings – particularly why some HPA fittings don’t fit – the HAM Team has measured no less than 108 male and 73 female quick disconnects. By analyzing this hard data, we can draw some basic conclusions.

Previous parts of this investigation have been published in these two stories…

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

So What Is A Foster Fitting?

HPA Fittings Background Reminder

1. There are no standard, published specifications for HPA fittings.

2. HPA fittings are often described as “paintball standard”, “1/8-Inch NPT Quick Disconnects”, or “Foster fittings”. But none of these terms has a precise, specified, or documented definition. They are all just generic terms.

3. With ANY manufactured product, there WILL be variations between individual samples of “the same” part. These manufacturing tolerances mean that the actual size of any part may vary from the design specification – sometimes significantly.

4. For this analysis, HAM has measured JUST ONE DIMENSION for each of the male and female quick disconnects. This is – we believe – the most critical dimension that governs if males and females will fit together. However a full engineering analysis of all the dimensions for both connectors should be made to completely understand the issue. This is beyond the HAM Team’s capabilities. It’s an issue for the whole airgun industry…

HPA Fittings Measurements

To understand the issue, we measured the Outside Diameter (O/D) of the male fittings.

In addition, we measured the corresponding Inside Diameter (I/D) of the balls inside the female fitting – with the collar out. That’s the largest I/D the female can accommodate.

Then we plotted all the measurements onto this chart. The female measurements are in red, males are in green. Both O/Ds and I/Ds are measured in Thousandths of an Inch, the left (y) axis of the chart.

Why Some HPA Fittings Don’t Fit

Let’s now take a close-up look at the left side of the chart. Here we can see that the largest male measurements (mainly 309 Thou) are just 1 Thou less than that of the smallest female I/Ds. That’s close to being a very tight fit!

Why Some HPA Fittings Don’t Fit

Statistically, it’s likely that – if we had a larger sample size – we would find some males that have the same O/D as the smallest female I/D. These would not fit.

Indeed one male O/D was 313 Thou. It’s the red line on the far left. That’s much larger than the smallest female I/D.

In fact, as you look across the chart, you can see that it’s larger than the majority of the female I/Ds. So, it will not fit any female with an I/D of less than 314 Thou. And – relatively speaking – there’s not many of them with a large enough I/D.

So now we can see why some HPA fittings don’t fit.

“Known Good” Male And Female Measurements

First let’s look at the Crosman males we measured in Part One of this series. We know that Crosman has this issue under good control.

Why Some HPA Fittings Don’t Fit

From this, we see that the design spec for the O/D is 306 Thou. Actual production parts vary between 303 and 309 Thou. In fact, this range covered all the males HAM measured – except one. That was the Snowpeak male fill probe.

Now we can look at the Foster females. Note that we measured 16 genuine Foster parts. The additional 57 females we measured came from other manufacturers. That explains the spread of female I/Ds beyond the range of 311 – 315 Thou we measured for the Foster parts.

The design spec for the Foster female I/D looks to be 312 or 313 Thou.

Why Some HPA Fittings Don’t Fit

HAM’s conclusion from these “known good” parts is that the difference between the DESIGN SPECS for the Crosman males and Foster females is 5 or 6 Thou. (312 or 313 Thou minus 306 Thou).

Allowing for manufacturing variability between the largest Crosman O/D (309 Thou) and the smallest Foster I/D (311 Thou), that difference is just 2 Thousandths of an Inch. That’s small, although OK for our purposes.

Why Some HPA Fittings Don’t Fit

But – as we saw above – that difference came down to 1 Thou with some of the “non Foster” females. Now that’s getting tight!

And with a larger sample size, we’ll inevitably see greater manufacturing variability. Now there’s going to be situations where HPA fittings don’t fit.

What to do if your HPA fittings don’t fit?

Sadly, the only answer is to try a different combination of fittings 🙁

Based on this HAM analysis, we’d suggest you measure the O/D of the male fitting using a digital caliper. Then you can compare that measurement against the numbers in this HAM story.

Now at least you’ll have an idea of how likely you are to find a corresponding female that will fit the male you have.

As we’ve said before, the real answer lies in the hands of the industry. However, as more and more people start to use PCPs, this is likely to become an ever-larger issue.