[Walt Fricke]

One thing you can do is keep turning the motor over, and watching to see if these new values move around a bit. The numbers of teeth on the gears and chain wheels are such that it takes quite a number of revolutions for the same chain link to be on the same tooth of any of the gears. A friend tells me he expects to see some change, though he hasn't attributed this to differences in tension.

Did you take your "nicely within spec" readings after you had torqued the big nut to its full book value?

The locating pin system has slop in it. If a guy is lucky, you can hit on a pin location where you can keep the timing where you want it even without tightening the nut. But usually that reading comes with the pin not tight up against the sides of the holes in the direction of chain pull. Which means it is easy for tightening the nut to move things a bit.