Cleaning out my old rods and reels from yard sales and flea markets. I found a old 6'6 casting rod in great shape, except the real seat trigger was snapped off. I got the grip and reel seat off and was going to get a replacement off of mud hole, but don't know what size to get? Thanks. 

Size 16 seat and cork grip to your liking.  The standard ones will fit the 16 seat. 

 

You will have to ream the cork to fit the blank, and since you are going to put the grip on from the rear, you have the taper of the blank to contend with.  What you do is to construct a cylindrical surface of masking tape wrapped onto the blank so that the diameter of the series of tape wraps is constant and such that the grip can be slipped over the tape wraps.   At the very end you will have no tape, just the diameter of the blank plus a little.  Every two inches or so wrap tape to match the diameter of the butt of the blank.  You are constructing a cylindler on which to mount the grip.  Dry fit all parts, seat, grip, whatever, to make sure everything fits before you mix your epoxy.  

 

Using a good paste epoxy, mix it , and apply it liberally to all the tape wraps to totally encapsulate them.  This will prevent them from deteriorating if water gets in.  Slide the cork grip onto the blank, and into the seat.  If any epoxy leaks out, clean it off with 90% alcohol, liberally used on a paper towel, before it sets.  Once it sets, you cannot get it off.  

 

If this is not clear, please respond.

45 minutes ago, MickD said:

You will have to ream the cork to fit the blank.

Thanks for your help! 

Just wondering if you have advice for reaming? I'm familiar with it from watching some rod building videos, but I don't really know what I'm doing.

Batson and Mudhole have reaming tools, which may not make sense if this is your only need for one, but they work very well.  Maybe you can find some craftsman in your neighborhood who can ream the cork to the diameter you need.  

 

Some builders use taperd files for reaming.  The challenge is in keeping the bore you end up with concentric.  Not a piece of cake, but there are ways to get it done.  

 

I use the Batson Dream Reamer in  a drill/driver (make sure it's turning the right way-do it reverse of that and the sandpaper tape can be damaged), leather gloves, and I allow the work to turn in my hand as I'm reaming.  The rotation helps to maintain concentricity.  Take your time, check it often.