I think, as a rule of thumb, that the first two digits represents the outside diameter in 64ths of an inch and the last two digits are the wall thickness in thousandths of an inch. Logically, all you would have to do is subtract two times the wall thickness from the outside diameter to get the inside diameter. If my math serves me correctly, this is how it should come out, or at least close enough for government work:
26/64" = .40625
0.40625
- 0.026 (2X.013)
----------
0.38025" - 2613 inside diameter
I pulled out the calipers on some 2219's and it was fairly close. Hope this helps.
Wilecoyote76
