I am going to build a long casting, 13' "6-8 and bait" rod for a casting conventional reel. (like an abu 6500). I will have a mono top shot (30lb) or a shock leader (50 lb) on this set up (with braid underneath).
I am eyeballing the microwave 18/8 MW18 set. I really am amazed at how microwaves help in a spinning set up.
however, i have never used a ring size smaller than 10 on a surf rod.
Is 8mm too small for the rig and purpose i describe? What is the "rule of thumb" on the smallest ring size one should use for various line diameters and applications?

Thanks in advance!
Jim

You'll have to cast it with those size rings and see they easily clear any shock leader knot you use.

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The rule of thumb is to use the absolute smallest guides you can get away with that will still effectively pass the knots & leader sizes you intend to throw .

When i started i used size 8 runners and it worked OK but i knew i could increase rod performance by going smaller , went down to size 7 and then size 6 and i have settled on size 5 runners for the two surf rods i'm building now . I kept hearing about guys going with these super small diameter runners on their freshwater and inshore rods like size 4.5 .

I decided to order some in the size 5 and 5.5 sizes for my two inshore saltwater rods and i went with the Fuji KR concept since i wanted to build the rods tuned specifically for two specific reels and the Microwaves didn't allow me to do that .

Well all i could say was "Wow" , i was floored at just how big of a difference the much smaller guides made in the overall feel and castability of my 8 and 9 foot inshore rods . I was so amazed that both of my 11 foot surf rods have size 5 runners & they both cast into other time zones .

Thank yall. THat is very interesting. I am sure i can pass a braid to FG mono knot thru an 8. I appreciate the feedback, and would love to hear from anyone else with a pro or con view.

I am currently using BK guides and New Concept spacing on ALL the surf rods I build. The smallest that I use on drum rods in an #8. The reason I haven,t switched to #6 or smaller is, think about the shock knot, a small amount of grass and fishing 3 people deep on Cape Point. You are trying to wind in your line with grass or even after your get the debris off the shock knot with just a small amount of grass still attached, will it go thru such small guides.? Think about the fact that you are throwing "bait and 8", that will helicopter and also slow down your cast. Compromise...