Thought about it some more and had an idea. I'll share it if anyone is interested. Cut the stem out of the old tube and shaved the rubber back from the end opposite the threads. Chucked it up in the cordless drill and run it in and out of the air chuck (turning in both directions) and wallowed it out. Checked frequently, by slipping it on by hand, until it was not so difficult to slide on the valve. Still clamps/seals very snug. If you try it, hold the valve so the bits of rubber that are removed fall out of the valve, not into it. Kind of speeds up the break in period!