I am getting a little confused. I have up the drawing for a BR 4" flange and the diameter that is on the drawing is 100.4 mm for the diameter that has the O-Ring races cut into it. What are you measuring?
Heck I am confused at the drawings the BR has online for the AL O-Ring Flange and the AL Tube
The Tube drawing has the ID being 3.962" … I am using English measurements since I happen to have that O-Ring book and for the denoted O-Ring size of “238”, the ID on the tube should be 3.762. Either someone has a typo or the tube and the end caps are cut to some other calculation.
According to the Parker O-Ring handbook for a 238 size O-Ring I would have the ID on the tube at 3.762" and the Flange would have an OD of 3.747 with an O-ring race depth of .111 to .113"
Again, I am confused as all heck! It is not my design so I can’t speak to the calculations that the parts are cut to. The maximum tested depth on the AL tube is 2000 feet so I would assume that everything is supposed to be using a Industrial Static O-Ring race design.