How does coolant get in the Oil????

You want to minimize variables, since the drop in piston height could be very subtle:

1. Ensure end of dowel (assume hardwood) is cut very square, maybe end of purchased dowel, not recut. Carefully knock off any burrs on edge of cut.

2. Top of piston is somewhat uneven, so try as much as possible to put the dowel in exactly the same on all. Check for perp with speed square. Press dowel against wall of spark plug hole in same way, while pressing down.

3. Use something like rigid back razor blade, or spare box-cutter blade, pressed firmly on flat against spark plug hole lip (if possible), to make nick in dowel. Use cylinder #4 to make nick, as it’s least likely to have bent rod.