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As it says in our custom fill adapter listings, Crosman tube threads can vary in fit by up to around .010". This can cause a fill adapter to fit well in one tube and too loose in another. For safety we make the threads on our custom adapters a little over-sized. This allows them to fit snug in most tubes, tight in some and on rare occasions not fit at all in an exceptionally tight tube. 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 well together 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 tubes dangerously loose for PCP purposes.
The cause of the large thread variance is the tooling Crosman uses. It's in different states of wear from one tube or batch to the next. The thread majors tend to be rounded-over and the minors rounded-out to varying degrees which affects the fit 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. Our custom fill adapters made of 7075 aluminum are the safest option. Crosman/Benjamin fill adapters are generally safe but we have found examples that fit too loose on rare occasions. If any adapter fits your tube loose don't use it. It's ok for it to thread in easily but should not be able to wiggle once it's about half-way to two-thirds in. Test-fitting should be done with the o-ring off to allow it to wiggle if loose.
There are variances in wall thickness and threading that can cause some Crosman 22XX tubes to be unsafe for bulk-fill and PCP conversions. Before making such conversions the tube needs to be checked to make sure it's safe.
One issue is some tubes are finished with a ground texture. Those tubes have a rough outer surface. The grinding process used to create that finish removes varying amounts of metal producing a slightly thinner wall. This type of finish is common with Discovery and Maximus tubes but doesn't generally remove enough material to affect the wall thickness significantly. We recently received a 2260 MB tube from Crosman that had been ground too much and had a dangerously thin wall. All previous 2260 MB tubes we've seen were non-ground with full wall thickness. 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. Tubes should measure a back wall thickness of around .060" to .065", front wall thickness at the threaded section of .045" to .052" and outside diameter of .865" - .875". The over-ground 2260 MB tube we received that was too thin measured around .0575" at the thinnest section of the back wall, .044" at the thinnest section of the threaded front wall, the outside diameter averaged around .864 at the back and .858" at the front. The wall thickness was inconsistent so the measurements were slightly different from one spot to the next, this causes the math to work out inconsistently. That tube would not be safe for bulk-fill, HPA or PCP conversion due to the varying wall thickness and thin measurements.
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