So I was going to glue the crack, buy some thin brass strips and bond them to the inside of the scales, then I had an idea. I remembered that I had a whole load of waxed cotton from wrapping my kamisori so I did this on the off chance that it might keep the crack closed:

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The glue seemed to hold this time, anyway...

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...I used three washers on each side to balance the scales; it took quite a bit of hammering but I'm pretty happy with the tightness now.

Tomorrow I'll try and hone it up, at last!