Mark,
I have repaired many many rods and always use the same technique.
If possible I use a fiberglass inner plug and a fiberglass outer sleeve.
In the case where the rod has been crushed and has a split going back a ways from the apparent break, I will make the plug and sleeve at least an inch longer than the sign of any visible defect.
I always make the plug and sleeve as thin as possible to minimize weight and stiffness of the repair.
For a clean break, I generally have a plug and over sleeve 2-3 inches long. I make a point to never cut either side of a break. Rather, I make a policy to insert all of the broken edges together to add both more gluing area as well as to guarantee the alignment of the rod.
As others have said, I chamfer the edges and minimize the outside of the diameter of the outer sleeve to keep the bulk of the strength of the sleeve, but minimize the weight build up.
I have always used in inner plug on the belief that I do not want the ends of the break crushing together and deforming under a fully flexed and stressed rod.
I will always do a thread wrap of about 3/8th of an inch long at each end of the outer sleeve with a coat of finish on the thread wrap to seal it together.
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I approach the repair of a rod blank in the same way that a carpenter would repair a broken beam in a structure. There needs to be sufficient in the repair to essentially replicate the original strength of the bearm.
After all, that is essentially what is being done - i.e. repairing a structure to have strength at least equal to the original build.
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In the case of a tip break of a very thin tip, I will generally use an appropriate size piano wire to serve as an inner plug. Again, I use the inner plug for the same reason as I use it for any other part of a rod break. i.e. I want an inner support to avoid future crushing damage from a fully stressed rod after the repair.
But, by all means, use the system that works for you, and if repairing for some one else - for the final fishing needs of that particular person.
Note:
To thin an inner plug and or an outer sleeve, I will make the repair pieces 2-4 inches longer than the final pieces. Then, I chuck the pieces into a variable speed drill and hold the spinning pieces against a spinning bench sander. First I thin one end of the repair piece and then, reverse the piece in the drill and reduce the outer diameter of the other end of the piece. Finally, I will cut the thinned repair pieces to the final desired length before doing the final assembly and repair.
Note: for cutting blanks and repair pieces I use a miniature chop saw that turns a 3 inch abrasive disk. Quick and easy.
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I will have the inner plug sized, the outer sleeve sized and prepped and have every thing ready to go.
For the outer sleeve I will always start with the outer sleeve placed in the smaller side of the break, and slid well above the break.
I will coat the inner plug and ends of the break with 5 minute epoxy and slip the rod together making sure that the inner plug stays in its desired position. Then, with the outer sleeve well above the break, I will coat the outside of the rod with a thin layer of epoxy well up the rod so that I can simply slide the outer sleeve down the rod which insures excellent coating of adhesive on both the outside of the rod blank and the inside of the inner sleeve. I slip everything together, do a check on alignment, do a clean up of the outside of the rod blank of any adhesive and let the adhesive setup. Then, I do the end of outer sleeve thread wrap and thread coating work. Most times the total repair takes about 30 minutes - excluding drying time for adhesive and thread finish.
The vast majority of the repairs have a negligible effect on the fishing characteristic of the fishing rod.
