I've built many fly rods in the past but never a spinning rod. I'm building a spinning rod for my 5 year old son and need help with guide sizing. I have the issue of rodmaker that covers the new concept system, and I understand it well. The diameter of the spool is 23mm, so I'm thinking the first guide should be a 25. After that I'm a little confused. Should I just go 25, 20, 16, 12, 10 (if need be) until I hit the choke guide?
Thanks for your help.

Couple of comments: (1) Are you sure of that 23 mm diameter, or is that maybe the radius? Even the smallest 500 size spinning reel I have has a 33.5 mm spool diameter. (2) In general, you want the first butt guide diameter to be smaller, not the same size as, the spool diameter. For example, for my 2000 size reels, with a spool diameter (measured at the outer lip) of 41 mm, a 25 mm butt guide works well. (3) You choose the sizes of the other guides so as to give a straight-line drop for the butt guide to the choke guide. What works for me most of the time is 25 (butt), 16, 10, 8L (choke).

Sorry, you're right. It's 46mm diameter, the 23mm is half the diameter.

Then with a 46 mm spool, a 25 mm butt guide should work well. Some might use a 30, but not me. As always, test cast.

Mike,

A couple other considerations are guide height above the rod and distance from the reel to the first (butt) guide. If the guide frame is too short or too tall, there will be additional friction. Similarly, if the butt guide is sized or located incorrectly, the loops of line coming off of the reel will actually slap against the blank, shortening the cast, wearing the line, and over time, damaging the blank (for heavier "wonder" lines)

When setting up the guides, you ideally want a conical path through all of them, from the reel face to the tip, with each ring size progressively smaller than the previous one.

And . . . . . like Tom says "ALWAYS TEST CAST", watching for line "slapping".

Skip

Skip describes one way of doing it, but the new concept layout does not use a conical path. Instead, it drops the line down to the blank in a straight line from the spool to the intercept point, then hugs the blank out to the tip. See the Library article on this.