Spark plug threads

I have a lot of thread tables, I love the variation and amount of weird threads, think it might be a fetish?
In one old table there is a colomn for spark plug threads, they are M14x1,25, M18x1,5 and 7/8x18 threads per Inch.
My Harley cylinders for the ongoing engine build should have the later Harley option, M18 (kind of funny, M threads on a Harley back in mid twenties!).
Someone obviously "screwed it" on one of the plugs, and repaired it by increasing the hole to run 7/8 spark plugs.
I want two M18 spark plug cylinder`s, had to make and fit a bushing to bring the hole back to its former glory!

Today was known to be a day with no work shop time, I found it appropriate to mock up the engine, and dream!