The custom valve screws we offer fit the screw-holes in the tube perfectly. They leave just enough tolerance to account for manufacturing variances. This increases the area force is applied to the tube. As a result it can handle higher pressure before the screw-holes start to elongate. This makes them the safest screws on the market to upgrade your Discovery for higher pressure. They are 410 stainless steel, heat-treated to metric class 12.9 with a hardness of about HRc 41. The approximate tensile strength is 185,000 PSI.
When these screws are tested individually, one side-hole in the tube starts to elongate slightly as pressure nears 3400 PSI. With two side screws elongation starts as pressure exceeds 7000 PSI. The bottom hole starts to elongate as pressure exceeds 2800 PSI. The bottom hole has a lower failure pressure due to another hole close behind it. This makes the tube weaker in that screw position. Failure with all three screws installed starts as pressure exceeds 10,000 PSI.
PCP pistols, designed to operate at 3000 PSI, are made with the same size and type of tubing as the Discovery, Challenger and Maximus. They don't have the extra hole behind the bottom valve screw, they are slightly stronger in that position. This makes up for the smaller head diameter of the screws which keeps the failure pressure about the same. Maximus models only have two valve screws, one on each side. Those have the smaller head diameter which causes failure pressure to be slightly lower than two custom Discovery screws. With our custom Maximus screws failure starts as pressure nears 7000 PSI. 22XX tubes, with one of our custom valve screws in the bottom position, start to fail as pressure exceeds 2800 PSI.
The first two photos below show one custom Discovery valve screw in the right-side position at 3300 PSI. The gauge in the first photo clearly shows the pressure. There is no sign of failure at this pressure, the gap to the right of the screw has not enlarged, the second photo is a close-up at 3300 PSI. The third photo is a close-up at 3600 PSI, elongation is clearly noticeable. After releasing the pressure and removing the screw the hole measured .004" longer. The last photo shows the bottom screw position at 3050 PSI where that test was stopped. Movement started around 2800 and advanced quickly. That hole elongated by .010" due to the additional hole right behind the screw hole causing the tube toi be weaker in that blocation.
Discovery has the weakest valve screws of all the PCP models. It's the only model with screws that will fail before the tube does. All others have alloy-steel low-head cap screws that are much stronger.
All of the custom valve screws we have listed are thread-size 8-32 x 3/16. There are only two different head sizes. The Discovery screws only fit Discovery models, they have a larger head diameter that is taller. All other PCP models with 7/8 OD tubes require the smaller screw heads. The 22XX CO2 models fit the small screw heads but the valve needs to be drilled and tapped to accept the larger thread size.
The Marauder Rifles have a larger tube diameter and valves that require 10-32 x 1/4 screws. None of our custom screws fit those models.
22XX VALVE SCREW TEST
Using a hydraulic pump I tested a stock 22XX valve screw to failure. The screw head started to lift around 2100 PSI and sheared off at around 2500. Using hydraulic fluid instead of air, the valve did not shoot out of the tube. It only moved about a half-inch.
The first photo shows the valve screw at 2000 PSI. The second is right after the screw head sheared off. The last shows the remainder of the screw left in the valve.
Below is one of the hydraulic pumps I set up for pressure testing parts. It produces around 10,000 PSI. The connector on the front is a modified Discovery fill adapter that threads directly into the Crosman/Benjamin 7/8" OD tubes. Hydraulic fluid doesn't compress like air does, it doesn't explode violently in this type of testing
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UPDATED MAY 26, 2018
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SAFE PRESSURE LIMITS
All Crosman PCP models with 7/8" OD tubes are made from similar 1020 series DOM carbon steel tubing. Except for the valve screws, all of the pressure-containing components will safely handle 3000 PSI. All have aluminum valves secured with 8-32 screws. All use three screws to secure the valve except for the Maximus which only has two screws as mentioned above.
These are the safe pressure limits for the following models with my upgrade valve screws. For Discovery 3000 PSI leaves better than a 3:1 safety margin, 3200 is the absolute limit. Maximus has a 13XX valve stem that should be upgraded to a Marauder stem for higher pressure. With only two valve screws 2200 PSI is within the 3:1 margin, 2400 is the absolute limit. For 3000 PSI models the absolute limit is 3200.
All 22XX CO2 models are made of similar 1020 series DOM tubing to the PCP models. If the same PCP components are used and the valve is secured in the same fashion, these models can be converted to PCP or HPA and handle up to 3000 PSI. The general rule is one custom valve screw for each 1000 PSI.
The link below, to an article from The American Airgun Hunter, contains a lot of info about the Discovery model and it's roots. It also mentions safety testing Crosman did and the type of tubing they use.