There's an interesting thread in the customization section where ghost and others discuss the vulnerabilities and stress points inherent in the tsuka of a katana. http://forum.sword-buyers-guide.com/viewtopic.php?f=18&t=4860&start=20
A couple of points that were brought up were the importance of a tight ito wrap to help keep the wood from splitting along the grain and a good snug nakago/tsuka fit to more evenly distribute the stress placed on it.
I was recently remounting an old Masahiro blade ("General sword", my very first sword, actually) with a leftover tsuka core from another katana. Normally, this would be a very bad idea but I did this to try my hand at ito wrapping and blade polishing on a cheap old guinea pig first. I figured I'd end up with a sword that would be suitable for display and a little dry handling but not cutting. The tsuka that came with the Masahiro had been severely cracked from the get go, even before they had wrapped it. You could clearly see that they made a half assed effort to keep it intact with some sort of wood putty. The glue that they used to affix the plastic "samegawa" as it turned out, was EXTREMELY TOXIC
and caused my left hand to painfully swell for a couple of days.
Watch out for this. Wear gloves when handling unknown adhesives.
After drilling new mekugi holes in the alternative tsuka core that lined up with the existing holes in the nakago, I pinned the naked tsuka core onto the blade and gave it a few good swings to test the fit. There were definitely some creaking and knocking sounds, indicating a loose nakago/tsuka fit. I tried shimming with a strip of thick paper and gave it a few more swings. I could tell the fit was still not good and I noticed to my surprise, the tsuka now had a 3" crack running along the mune side, towards the back end.
As I sat in dismay, ready to scrap the project altogether, an interesting solution came to mind. I'd heard of people epoxying their tsukas on permanently to solve nakago movement but I wanted to be able to disassemble the sword in the future. This is what you can do:
1. With the habaki, seppas and tsuba fitted, completely wrap the nakago (tang) of the blade in aluminum foil and secure it with some mild glue like elmer's. Use just one layer, and make sure it conforms tightly to the surface of the nakago. The glue should be between the overlapping foil, not between the foil and the nakago, so you'll be able to remove the foil like a sleeve.
2. Seal whatever gaps and mekugi holes in the bare wood tsuka core tightly (carved cork should do nicely) and fill it with a good amount of wood glue through the opening... it should take up around 1/4 or more of the hollow space in the tsuka, depending on how much extra play you think is in there.
3. If you have a crack in there somewhere, tilt your tsuka around until the area is well saturated with the glue. Then keep the tsuka upright until most of the glue settles to the bottom again. Put the fuchi on the tsuka.
4. Take your blade with the fittings and SLOWLY insert the foil wrapped nakago into the tsuka until it's fully seated. If you have a crack, the glue should not only completely fill up any empty space left in the tsuka but the pressure should cause some of it to ooze out through the crack. There'll probably be some overflow so be prepared for a bit of a mess and have some damp paper towels handy. Don't let the glue get all over the seppa & tsuba. A little extra foil sticking out should keep the gunk at bay or have a little gap in one of the mekugi holes to give the excess glue somewhere to vent.
5. If the pressure from the glue has caused the crack to expand, tightly wrap the tsuka with a strip of cloth to keep the crack closed. Store your katana upright and allow 1 or 2 days for the glue to dry a bit and check the mekugi holes in the tsuka to see if the glue around those areas are dry. If so, hammer in the mekugis. Then allow another 4 to 6 days for the wood glue to dry completely.
This should give you a very snug nakago/tsuka fit and secure any cracks as well. It worked for me. I still wouldn't do any cutting with my remounted and rewrapped Masahiro (maybe a little light cutting) but the fit is now rock solid and I can still disassemble it if needed. Wood glue is very strong and keeps most of its volume as it dries. DON'T try this with stuff like Gorilla or Python glue. Those glues expand dramatically as they dry and that can be very bad. Completely replacing a cracked tsuka is obviously the best and safest choice but this might be a decent fix for a light duty katana. Hope this helps.