PCP

Guns, threads and chinese tolerances.

One of my hobbies is going to the shooting-range and punching holes into paper. I find there is nothing quite like it to put your mind at ease and I really enjoy focusing on just one thing at a time. But don’t get me wrong, I’m not a gun nut. While I’m sometimes invited to send a few .22 rounds or larger down range, I tend to stick to my plinker toy most of the time. That’s because I enjoy the challenge of hitting a target in not ideal conditions. In my case it’s a cheap PCP air rifle chambered in .177 with just under 7.5 J of muzzle energy. At a distance of 50 meters, the gun is way past it’s designed range for accurate shots and needs a refill when ever the pressure drops below 160 bars. Filling the rifle from the 200 bar scuba tank at the club will only give me around 40 to 60 shots, but the air cylinder in my gun can actually go to 300 bar. A good friends gave me this hand pump so that I can fill up my gun for an entire shooting session.

Unfortunately, my gun has a removable air cylinder using the coarse Walther thread and the only fill probe I could find is the one from best fittings with a 1/8 BSP thread at the other end. The Chinese pump makes use of an undocumented thread size and the quick couplers that came with the pump don’t connect to the best fittings coupling system. In order to connect both systems, I’d have to figure out the size of threads used by the Chinese pump and then find an adapter. My dad had the unknown Chinese thread measured at work using a computer microscope and that strongly hinted at a metric fine thread size 10, but the numbers are quite a bit off from the M10x1 standard. A walk to a more specialized hardware store confirmed that the thread in question is indeed the M10x1, but with crazy huge tolerances.

Microscopic view of a Chinese M10x1 male thread with 52° instead of 60° angle and wrong diameters.

With this out of the way, it’s finally time to design the requirements for a new refill system. The entire setup needs to be flexible enough so that I can easily adapt it to future needs but simple enough so that it isn’t a pain to use. To achieve this, I need the hose to have a coupling socket at either end and an integrated bleed valve too. This lets me transfer compressed air between any containment unit which may be a pump, a scuba tank or a fill probe. After a transfer has finished, the entire system can be depressurized with the in-line bleed valve, so the coupling mechanisms can be released easily. I also want to have another hose that can extend the first one using a coupling plug on one end and a socket on the other. The extension hose is also a good place to put a moisture and oil filter as an optional component in the system. This lets me add the filter only in case a greasy or moist source of compressed air is used.

The full refill setup with best fittings couplers. A 1000 mm transfer hose with an in-line bleed valve. A 400 mm extension hose with a moisture and oil filter. A fill probe for the Walther air cylinder. An adapter for 300 bar DIN connections on scuba tanks. A hand pump that can deliver compressed air at 300 bar.

Finding an in-line bleed valve that also fits into the system wasn’t that easy but Sam Bingham from best fittings was super helpful and offered to modify an existing valve to suite my needs. Thanks again, that’s true customer support. I finally have the refill system that I always had in mind. It also uses the more common 1/8 BSP threads and the higher quality coupling system along with all the extensibility one could wish for. I should note that I don’t get paid to mention the company best fittings in any way. It’s simply a personal choice of products that are of a high quality and are widely available. The brass couplers that came with the Chinese hand pump are not rugged enough and received dents and marks after a first use. I simply don’t want to compromise on quality and safety when working with pressures as high as 300 bar.

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