Just made a two-piece basswood saya for my munetoshi 240 and I made it a smidge too wide. the knife won't fall out with light shaking but harder shaking will make it fall out and I can wiggle the knife a tiny bit side to side. It's not a perfect fit BUT...
I was wondering if I could use steam or something to make the sides of the saya curve in a tiny bit and hug the blade a bit more snugly? eh? eh?
Putting any water inside - hot or cold - will raise the wood grain and could give some improvement but it's also risky. Wet wood expands and contracts pretty aggressively. Depending on the grain alignments, it could warp the whole saya, pop the glue joints, of if the glue joints are stronger than the wood fibers, cause other damage. As a reference, slightly off topic: if you put a piece of veneer on just one side of a piece of plywood instead of both, as thin as it is, the expansion and contraction when it gets wet from steam of glue can be enough to warp a 1/2 thick piece of plywood noticeably!
If you try it, I'd add only a little water at a time...let it dry thoroughly than test... repeat a few times but go slowly.
Not a help for an already made saya - but couple things that I find helpful for future reference:
1. When you glue the saya halves consider using a liquid hide glue instead of normal wood glue, epoxy etc. Hide glues are used for instruments because they're reversible without destroying the rest of the piece. The glue hold is extremely strong but heat it up above 125 or so degrees and you can pop it right open again without destroying your work. Being able to re-open and adjust a saya later (especially if you ever thin your knives or round over spines etc) can be really useful. I use this approach on all sayas I make and a variation on it for any handle mounting with rare woods where I want them to be removable.
2. To adjust fit: With the same hide glue, I will glue a thin strip of veneer (5in long, 1/2 inch tall, 1/32 thick) inside one side of my sayas for what I call a "rub rail." The idea is to give myself a little extra friction that is adjustable later (if wood shrinks or the knife has been changed). I glue the strip maybe a quarter inch down from the spine and run it at least half the length of the blade. I'll sand it to fade the edges in to the saya to prevent and edges from causing scratches but the center will be near the full thickness of the veneer. This "rail" is more compressible than the saya itself and more likely to swell a little with moisture so it helps keep a good friction fit. Later, if I ever need to change the fit - because the rail has worn out, the knife was thinned etc...I'll open the saya, remove the rail and replace it. I came up with this method for fancy hardwood sayas with inlays and other details because so much time can go into making them. It's a bit of extra work to add the veneer rail into something utilitarian but it does make getting a perfect fit easier in any case.
I think I wrote a detailed explanation of the method a year or two back in this subforum and also posted pics of couple fancy inlayed sayas for two heiji's I did with it ...years later, they're still a tight friction fit.
