Well let me see if I can explain this.
A non-vented cap is flat on the inside. Nothing to it.
A vented cap has a round metal piece that measure's about 2" with a hole in the middle of it. Basically there is much more to it then a non-vented cap.
If you ever look at caps on E-bay, you can see the difference.
Wish I was a little better at explainig it, hope this helps.
