As it says in our listings for custom fill adapters, Crosman tube threads vary in fit by as much as .010" which causes fill adapters to fit well in some tubes and loose in others. For safety we make the threads on our custom adapters a little over-sized. This causes our adapters to fit snug in most tubes, very tight in others and in rare cases too tight to fit at all. The threads in many 22XX tubes like 2240, 2300KT, 2400KT 2250 and 2260 often fit as tight or tighter than with Discovery, Maximus, Marauder Pistol and other PCP models. Crosman/Benjamin tubes and adapters generally fit pretty well but can occasionally be loose enough to be a safety concern. Many aftermarket adapters are threaded with a die or machined to standard 13/16-28 thread assuming they will fit safely. Adapters with this standard thread size can fit some Crosman tubes dangerously loose for PCP purposes.

 The cause of the thread variance is the tooling Crosman uses to thread the tubes. It's in different states of wear from one production run to the next  which affects the thread size and contours. The thread majors and minors can end up sharp, rounded or flat and this can affect the fit of a fill adapter dramatically.

​  The bottom line is, as long as your tube and fill adapter fit snug together, the tube has it's full wall thickness (not ground thinner) and you secure the valve correctly with high-strength valve screws you can safely convert your gun to bulk-fill, HPA, PCP or use higher pressures as explained in our custom valve-screw listings. We only suggest using our custom fill adapters made of 7075 aluminum. though as long as they fit safely Crosman fill adapters are usually safe also. If any adapter fits your tube too loose don't use it. It can thread in easily but should not be able to wiggle at all once it's about half-way in.


 There are variances in the wall thickness and fit of the threads that can cause some crosman/Benjamin tubes to be unsafe for bulk-fill and PCP conversions. Before making such conversions or using pressures higher than recommended by Crosman the tube needs to be checked to make sure it's safe.

 One recently discovered issue is some tubes are finished with a ground texture. Those tubes have a rougher outer surface than most.  The grinding process used to finish those tubes removes varying amounts of metal producing a slightly thinner wall thickness. This type of ground texture is common with Discovery and Maximus tubes but doesn't generally remove enough material to affect the wall thickness significantly. We recently found a 2260 MB tube that had been ground too much and had a dangerously thin wall. All previous 2260 MB tubes we've seen were non-ground with a full wall thickness. 2260 MB tubes with a ground finish may not be safe for bulk-fill or PCP conversions, the one we found certainly wasn't. We were unaware these over-ground 2260 MB tubes existed until we randomly received one from Crosman. If you have a 22XX tube with a ground finish don't use it for PCP, HPA or bulk-fill conversions until you've checked the wall thickness and know it's safe. The ground tube measured in the photos below wouldn't be safe for bulk-fill, it' has a wall thickness far too thin, especially at the threaded section in the front.  Tubes should generally measure a back wall thickness of .0625" to .065", front wall thickness at the threaded section of .049"-.051" and an outside tube diameter of .875". The over-ground tube in the photos below is only .0575" at the back wall, .044 at the threaded front wall and the outside diameter is only .865" at the back and even thinner at the front at only .858".