Depends on the grade of aluminum and the (axial) thread depth of engagement.... I use 7075-T6 when I can, otherwise 2024-T3.... 6061-T6 is significantly weaker.... Load depends on the ID of the tube, assuming the O-ring is inboard of the threads, so that they are only in shear.... If the O-ring is outboard of the threads (and therefore they are under pressure) the hoop strength of the tube is decreased (thinner wall because of the threading), and that will decrease the effective thread (radial) engagement if the tube expands.... Much better to have the O-ring inboard of the threads.... Bob
So a marauder fill end cap is under less than the 3000psi ?
Just so I understand, the tube is 23mm OD x 19.8 mm ID (1.6mm wall)?.... If the threads are 21mm OD, then the remaining wall thickness could only be 1mm in the threaded portion.... and that portion is pressurized?.... What is the tube material?.... At 3000 psi, I find the following....Taking a guess on the tube material as 1010 mild steel, with the above dimensions the plug threads are not the issue.... A 6061-T6 plug, with properly formed 21mm x 1mm threads with an engagement of 16mm has lots of strength in shear (~ 10:1), and the axial strength of the tube wall at the thread root is about 3.5:1, which is fine.... Using the tube dimensions above, the hoop strength of the tube where the wall is 1.6mm thick is marginal, with a yield of 2.6:1 and 3.1:1 to burst.... My concern is that the threads are pressurized, and so the wall thickness there (23mm-21mm)/2 = 1mm is not sufficient for 3000 psi, with safety margins of just 1.6:1 to yield and 1.9:1 to failure, based on that material.... Even upgrading the tube material to 4130 CrMoly, it is only 1.9:1 to yield and 2.9:1 to failure.... The plug is not the problem, IMO the O-ring must be moved to inboard of the threaded area, where the tube is full wall thickness....This is a perfect example of not responding to a simple question when all the details are required to ascertain the safety of components.... Please note, I am NOT an Engineer, these numbers are provided for reference only, and YOU are responsible for the safety of your build....Bob