In that case, I'm going to guess that your machinist fitted the valve stems a little too close in the guides. What we used to consider to be "normal" clearances on valve guides, pistons, bearings, etc. are called "worn out" these days. Machinists who are accustomed to working on today's rice rockets are prone to tighten things up too much on the old stovebolt engines. Stem-to-guide clearance of around .0015" or so is just about right for stovebolts.
Jerry
