I use Champion D9s. They are 18 mm. No adapters, no detonation, I used adapters for years with all kinds of plugs with nothing but trouble. Went to d16 plugs but they were too hot and I got detonation above 11000 ft.. Went to d9 and problem solved... No problems up to FL 180....
Tailwinds
Doug Rozendaal
F1 io-540. 9.5-1 pistons.