I have been looking into the "O" rings on a senco, and was wondering how big the diamiter was on them and if its even worth it?

Too expensive. Do a search on here for Senko rigging. You will find my post with regards to "Goodys" and the Senko. I'm sure you will be much more pleased with the results. If you are using the Yama Senko, you MUST use some kind of banding, otherwise you will tear up too many baits. Some of the other Senko knock-offs are less fragile. But in my opinion, they produce less fish as a result. Again....just my opinion.

Use a little bit of black shrink wrap. Works just as well and is cheap. Pre-shrink a little strip of shrink wrap onto 4-5 senkos before you go out and that should be plenty.

Do it because Guido says so...

The correct o ring size is 3/8" o.d. (outside diameter) and 1/4" i.d. ( inside diameter)

The shrink tubing is by far the better way to go, no doubt.

Cannot believe a Senco thread has lasted this long.