When was it...must have been 1994...when the Japan Foundation sent me around the world offering up one or two day shakuhachi repair courses. Counting that and over 30 years of shakuhachi repairing, a large number of shakuhachi bindings have been accomplished. Here's a general outline of a couple of satisfactory processes to get a split shakuhachi closed and bound up in durable fashion.
First, the crack should be closed. It has happened more than once that a split shakuhachi has arrived here with many metal hose clamps spaced along its length and tightened to close the crack. Sorry, no success with that. Other shakuhachi have arrived here wrapped with steel wire, also unsuccessful. Many other folks have written asking how to close a crack. Thinking about it one should be able to come up with the reason why the bamboo split in the first place...excessive drying! To close any crack in bamboo, and cracks wide enough to accept a wooden pencil are not uncommon, simply replace the lost moisture. The most recent repair had a wide crack which took three days of resting in a damp environment to close.
Sometimes cracks just won't close completely without a little help from something other than moisture, but there should be some place where the crack is completely closed. This often happens when a crack extends the whole length of the upper half of the shakuhachi. It will be closed at the joint, the end away from the utaguchi, because a binding is normally under whatever ornament has been used at the joint. Start binding there, using one or the other of the methods described below or some other method, placing the binding between the ornament and the 4th finger hole. Then work toward the utaguchi end of the bamboo. Applying a tight binding should close the crack completely or very nearly so. Sometimes it happens that while working the crack will begin to open up. In this case, make a couple of bindings and then place the bamboo back in a damp environment to reclose the crack. Make a couple more bindings the next day or whenever the crack has closed. Eventually you'll get to the end of the work.
For rattan bindings square cornered grooves are cut into the bamboo at intervals along the length of the crack. These grooves can be any width (2 bu is common but 3 bu or even 4 bu or whatever can also be used). The grooves should be deep enough to accommodate some string which is wrapped round and round the bamboo in the bottom of the groove, some filler of some sort (used to fill the groove so the finished surface of the filler is below the surface of the bamboo by an amount equal to the thickness of the rattan) and the rattan itself.
Be very careful to not make these grooves too deep. I've seen more than a few flutes with distortions to the bore, little compression rings, at the location of each binding.
By all means, never apply any sort of that instant drying super-glue to cracks. It goes way down inside the crack and is almost impossible to remove. Many people use it on cracks but after doing so the crack will probably never close completely.
When making the structural string binding, wrap the string over itself for the first few rounds and at the end pull the string under itself for the last few rounds. If the string tension is sufficient it will not slip. The same technique is used for wrapping the cosmetic rattan. Use a small cross peen hammer or other suitable tool to eliminate the bumps, flatten them, where the rattan goes over itself at the beginning and end. Lastly, apply some sort of liquid adhesive that will seep down through the spaces between the rattan to the surface of the filler and also fill any other spaces. More than one application may be necessary. Be sure to wipe off the excess adhesive before it hardens. Urushi is the material of choice for this in Japan.
After all the bindings have been done, place the bamboo in a dry environment, a kotatsu works really fine, to open the crack a bit. Then fill the crack with some sort of filler that will eventually harden. Keep it in mind that the bamboo will move with changes in humidity so the repaired crack is expected to open and close a bit from day to day.
A more simple and structurally stronger method of binding is to use ordinary string directly on the surface of the bamboo. After the bindings are in place paint them one or more times with some sort of liquid adhesive to hold them in place.