CompareComparing...

So What Is A Foster Fitting?

The PCP airgun world uses 1/8-Inch NPT quick disconnects to fill with High Pressure Air. These are often called “Foster fittings”. But what is a Foster fitting?

Well the name Foster comes from a manufacturer of these – and many other types of – quality high pressure connectors.

In this HAM post, we’re looking at female fittings that are actually branded by the Foster Manufacturing Company of Springfield, Montana. The specific Foster female 1/8-Inch NPT quick disconnect in question is the model 12 FS. It’s described by Foster as a “FST, ST Straight Thru Socket, 1/8″ FPT Ball Lock, 303 Stainless Steel”. Here’s the link to the manufacturer’s website.

These fit with the Foster 12 MPS male. This is a 303 Stainless “FST Series”, straight-through plug. Again, here’s a link to the manufacturer’s website for this part.

The Foster catalog states clearly that when a stainless 12 FS and a stainless 12 MPS fitting are combined together that they are suitable for a maximum pressure of 4,200 PSI. (Not 4,500 PSI, you’ll note!).

However, there’s many other 1/8-Inch NPT quick disconnects out there in the marketplace that are “more or less” similar. We’ll look at these in more detail in another post. Today, we’re concentrating on genuine Foster female fittings.

We can tell that they’re genuine female Foster parts because they have the maker’s name on them. It’s not on the males, so that makes them more difficult to distinguish.

In order to characterize these female quick disconnects, we need to measure something! First, we can see that they have 6 locking balls.

So What Is A Foster Fitting?

Next, let’s look at the internal diameters of the female connectors…

Of course, everyone can measure the outside diameters of the male connectors – we all have digital calipers. However, in order to accurately measure internal diameters, the best tool to use is a set of pin gauges. Fewer of us are familiar with – or have a set – of these.

Pin gauges are sets of precision-ground steel rods that are supplied in sets. Each gauge is 1 Thousandths of an Inch larger (or smaller) than the next one. We can use these pin gauges to measure our female quick disconnects as “go-no go” gauges with an accuracy of 1 thousandths of an Inch.

Here’s what the set looks like…

For this investigation, I was able to round-up 16 genuine Foster 12 FS fittings. Sure, that’s less than I would have liked, but it’s what I could find and it’s probably a sufficiently-large sample size to draw some valid overall conclusions.

So I measured the female connectors in two conditions. With the collar “out” (the normal, clamping, situation) and with the collar “in” – in other words pulled back in the normal operation fashion when connecting a male.

As you can see, larger pin gauges will fit into the female Foster fitting when the collar is pulled back.

Of course, we expect to find a variation in measurements even with high-quality volume-manufactured products like those from Foster. Here’s how the measurements stacked-up…

So What Is A Foster Fitting?

Clearly the majority of parts have an Inside Diameter of between 257 and 259 Thou between the balls, with the collar up. Extremes measured were 256 Thou on the small side and 260 Thou as largest. Probably the design spec is 258 Thou – or thereabouts.

So What Is A Foster Fitting?

With the collar down, the design spec looks to be 312 or 313 Thou. Extreme sizes are 311 and 315 Thou.


So now we can answer the question: what is a Foster fitting?

– A model 12 FS female connector with 6 balls, manufactured by the Foster Manufacturing Company. Design specification between balls probably 258 Thou I/D with collar up, 312 or 313 Thou with the collar down. (Foster does not publish detailed design specs.)

– It matches the Foster model 12 MPS male connector.

– With both male and female fittings manufactured from 303 Stainless Steel, the maximum rated pressure is 4,200 PSI.


This is Part Two of the HAM series on male/female quick disconnects for PCP airguns. Part One can be found here. The final part and conclusions can be found from this link.