I just performed similar fix on a 1950 Gretsch snare and tom with significant inner ply separation -- after LOTS of internet research and thought -- here's what I did:
1. find a good round section of the shell and trace the inner diameter on the 3.5" side of a 2 x 4. Slowly cut through the arc with a jig saw and inspect when you're done (blade wander). Perform 2 straight cuts on the 2 x 4 -- one inch from where the arc hits the side of the board on both sides of the arc. Now you have 2 pieces that you can use for clamping each side of the shell in the affected zone of separation.
2. Since I could not easily pull the plies together, I made a 'make-shift' steam heater. I used my son's cold mister and blew this moisture into the intake of my wife's hair dryer and onto the affected area. I don't know if this is safe, but worked fine. After heating the ply for about 1 minute, I clamped the zone with some Robogrips and the blocks I used for 24 hour. Now I'm ready to glue.
3. I went to Woodcraft and found this really neat "Chair Doctor" glue kit with a syringe and 3 different needle sizes. I used Titebond III inside this syringe. The Chair Doctor glue said it expanded -- not what I wanted to do.
4. If I could inject glue from the edge near the bearing surface, that's where I did it. Sometimes I used my pic set (Craftsman) to wedge the plies apart enough to fit the needle into. One zone was in the middle of the shell and I bought a 1/16" drill bit and drilled at angle and again carefully used one of my pics to separate the plies slightly.
5. Sometimes if I needed the smallest needle size, I'd blow the hairdryer on the Titebond III glue to reduce the viscocity. Again, I don't know if this was safe to do, but worked fine.
6. After injecting glue, wipe excess with damp paper towel. Clamp joint and again wipe the glue that was extruded from clamping. Careful not to glue the blocks to the shell -- sometimes I put some duct tape on the blocks so I could separate after drying, but never had an adhesion problem. Leave clamped for 24 hours.
7. Clean the syringe and purge needles with water.
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