OK, thanks! Almost 5/8" (Or almost exactly 15MM). I'm planning to build a deep hole drill jig that will guide a long drill bit to make that passage in an earlier block. I'll probably start out at about 1/4" and step-drill up with a few passes. It's basically the same process that's used to make a rifle barrel. There are a couple of designs of hydraulic roller lifters that would be a close fit to a stovebolt block, if I ever get around to fabricating a roller cam.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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