These adapters should be installed by hand only, no tools should be used. The threads are cut to fit snug for increased strength. Due to manufacturing variances some tubes fit tighter than others. For safety reasons these adapters are designed to fit well with looser-fitting tubes. Some tubes will be a tight fit but should go in hand-tight. The adapter should be well lubricated, the threads on the adapter and in the tube should be very clean, otherwise the adapter may bind. On rare occasions a tube will be extremely tight and simply won't fit. In that case you can ship the tube to me to have the threads reworked. The other option is to buy a tube and adapter together so I can make sure they will fit together.
These adapters fit all Crosman / Benjamin 7/8" OD tubes. This includes Discovery, Maximus, Challenger, Marauder Pistol, 1701P, 1720T, 2240, 2250, 2260, 2300T, 2300S, 2300KT, 2400KT and similar models with tubes threaded inside the front. Models that operate on 12 gram CO2 cartridges require additional upgrades to convert to bulk-fill or PCP.
These are available in several versions. All include thread-on caps. The version with a pressure gauge is listed with 3K and 5K burst disks. The 3K version has a black-faced 1200 PSI gauge, the disc will blow as pressure nears 3000 PSI. This is recommended for bulk-fill CO2. The 5K version has a 4000 PSI gauge and the disc will blow as pressure nears 5000 PSI. This is recommended for PCP built to operate up to 3200 PSI. Both versions are listed in plain 6061-T6 aluminum and black anodized 6061-T6 aluminum. The version with a gauge should be threaded in all the way, then backed off until it's in the position you prefer. It does not need to be tightened against the front of the tube. It's actually stronger when not tightened all the way, there's less overall stress on the threads.
3200 PSI is the absolute safe limit for Crosman / Benjamin 7/8" OD tube assemblies. This is only safe if you have adequately upgraded the necessary parts to handle that much pressure. Otherwise you should only use the pressure the manufacturer suggests.