Guys, I am branching out into the fly rod area, and have a question, I have looked at some books and I can not find any sizeing chart for flyrod reel seats? I see a bunch of numbers, but is there an explanation of these numbers some where ? Or do they go buy line weight?

They do not go by line weight. Like many of the non-composite seats, the numbers don't really refer to size insofar as ID or anything like that. You're just going to have to knuckle down and study the manufacturer's catalogs to get the size information.

Obviously, any of the regular nylon-graphite seats are sized in millimeters for inside diameter.

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It is one of the frustrations in building custom fly rods. The most important dimension is of course the seat and/or insert ID, as this must accommodate the blanks OD.

The Batson catalog does a good job of providing the specs and some others do as well.

Talking with someone like Dave Lyons at Anglers Workshop can be very helpful as you get started on fly rods.