Trapped compression ratio for premium gas is indeed a typical 7.5:1, and most engines use a bowler hat shaped chamber.
A normal prop is 18x8 - 18x8, for diesel about 20x8, if you can harness the high cylinder pressures.
A CM6 plug is M10x1 fine metric thread, not the standard M10x1.5
Garden plugs are M14x1.25 fine metric thread