The custom valve screws I sell fit the holes in the tube perfectly leaving only enough extra room to account for manufacturing variances. This increases the size of the area force is applied to on the tube. As a result the tube can handle higher pressure before the holes start to elongate. The screws have a failure pressure much higher than the tube. This combination of improvements makes these custom manufactured screws the safest way to upgrade your valve for higher pressure. They are 410 stainless steel heat treated to meet metric class 12.9 with a specified hardness of HRc 39-43. The approximate tensile strength is 185,000 PSI.
The following information was updated after new tests were performed in February 2017.When my custom Discovery screws are tested individually, a side hole in the tube starts to elongate slightly as pressure nears 3600 PSI. With two side screws elongation starts as pressure nears 7200 PSI. The bottom hole starts to elongate as pressure exceeds 2800 PSI with obvious elongation by 3000. The lower failure pressure of the bottom hole is due to another hole close behind it which makes the tube weaker. 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 tubing as the Discovery, Challenger and Maximus. They don't have the extra hole in the bottom of the tube. Those tubes can handle slightly higher pressure but the other components are still limited the same as the lower-pressure models. Maximus models only have the two side screws, those tubes start to fail as pressure exceeds 7200 PSI. 22XX tubes with one custom valve screw 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 stress or failure at this pressure, the gap to the right of the screw has not enlarged at this point. The third photo is a close-up at 3600 PSI where elongation started to become noticeable. After releasing the pressure and removing the screw the hole measured .004" longer. The third 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".
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. Due to strength and superior fit my custom screws allow higher safe operating pressures than all of them.
All of the custom valve screws I sell 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 diameter head that is taller. All other PCP models with 7/8 OD tubes require the smaller head. The 22XX CO2 models fit the small head but the valve needs to be drilled and tapped to accept the larger thread size.
The Marauder Rifles have larger diameter tubes and valves that require 10-32 x 1/4 valve screws. None of the screws I sell fit those models.
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SAFE PRESSURE LIMITS
Below is a hydraulic pump I set up for pressure testing parts. I don't have it any more but it produced a little over 9500 PSI. The connector on the front was a modified Discovery fill adapter that threaded directly into 7/8" OD tubes. Hydraulic fluid does not compress as much as air so it doesn't explode as violently. It is much safer to use for this type of testing. It's also a lot messier which is why I sold it. I only test with air now.
All Crosman PCP models with 7/8" OD tubes are made from the same 1020 DOM carbon steel tubing. Except for the valve screws, all of the pressure-containing components will safely handle up to 3200 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 comfortably within the 3:1 margin, 2400 is the absolute limit. For 3000 PSI models the limit is 3200.
All the 22XX CO2 models are made of the same tubing used for 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 bulk-fill, PCP and HPA and handle the same operating pressures. The general rule is one high-strength valve screw should be used for each 1000 PSI.
There is more information to support the claims made on this page available at the links below.
The first link is to postings on the GTA forum where this subject is being explained with additional info from myself and others. The second link is to an article about the Discovery by Jim Chapman at the American Airgun Hunter website. About 1/3 of the way down that page there is information about the pressures Crosman tested the PCP tube assembly to and what the tube is made of. That article contains a lot of other interesting info about that model.
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. Because I used hydraulic fluid instead of air the valve did not shoot out of the tube, it moved forward about 1/2".
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.