I fitted 18mm to 14mm adaptors to my 1932 Flyer for two reasons, I could only get 18 mm plugs in short reach. The head requires long reach so as not to shroud the plugs and there is a much bigger range of plugs in 14 mm. You must check the length of thread in the head before fitting long reach plugs as if any thread extend into the combustion chamber it will carbon up and damage the head threads upon plug removal. I never have any plug problems. I used to fit equivalent to NSK 6 but now use 5’s.
I have also fitted extended nose plugs as I have plenty of clearance between head and piston.