I am fairly new to rod building and am going to be doing my first rod with an exposed blank and was wondering how you know which id to get without having the blank in your hand. I will be ordering them at the same time. The blank is 7 foot med/heavy with a butt diameter of .670. That is 17.018 millimeters. The seat I am getting comes in half mm sizes. I was thinking I would need 16.5 but don't know how to know for sure. And I am doing a split handle.

Tell the folks you are ordering from and they should be able to fit the seat for you.

Read Fuji post below.

You don't size it off the butt, you do it off the location on the blank where you want it positioned

Ron Weber

Some places provide a chart with the diameter of the blank per inch from the butt. This is a starting point. Agree with above ... call your provider. I watched a pretty good YouTube video from Mudhole ... type in ECSM reel seat installation. They actually used one size smaller, then hand filed the inside of the seat to fit. You can just pick the one that slides all the way down to your cork, but there will be a very small gap around the blank. I have had better luck picking one size smaller, then taking a reamer and barely taking out material so that its a snug fit around the exposed part. Most of the 7.0 and 7.3 Mudhole Mag Taper bass rods I have been building are taking the 13.0 and 13.5 size using 10" and 10 3/4" rear grips.