You're right on the Gleaner; You thresh with concave clearance. You crack with excessive rotor/cylinder speed. If it's not threshing, close down the concaves. Having them tight requires more cylinder speed to keep from slugging but there is a sweet spot where it gets completely threshed without cracking. I helped a couple farmers, one year. One had a IH Rotary, one had a Gleaner N5, and I had an L2 with badly worn concaves and cylinder bars. Cutting a hard threshing wheat, the N5 had a very clean sample. Red one was running a standard rotor and had a lousy sample and very heavy returns. My L2 did a poor job because of the worn bars and concaves, but it was still cleaner than the IH. Next year, I changed bars and put new concaves ahead of the grate. Then I could thresh also. Still no comparison with the Gleaner rotaries though.

Redman is right on the Neepewa wheat. Gets tougher yet if it's burned into the hull with drought. At the time we raised it, I was using a C2 Gleaner. No need to worry about filler plates with that one, as it had a closed bottom concave. It threshed it OK, but it was a struggle. The next year, dad stumbled on an offering of Cargill seeds. Our very first semi-dwarf variety named "Bounty". Short straw that stood without lodging, kernels were held tightly in the head, so they didn't shell in the field, but it still threshed very easily and cleaned up nicely. Not having to ingest all the straw that comes with a lodged crop made a big difference in what the C2 could do in a day, too. Needless to say, the Neepewa didn't stay around long, and we became converts of the semi-dwarf varieties.