Rob made a study of which types of 18mm spark plugs work best in his 1928 Model 18 Norton. I would like to add that in my experience a somewhat hotter plug (usually identified by a lower number) works better if you still run a piston without an oil control ring. Here we go:
NKG A-6, A-7 and AB-7
Champion D9, D10, UK10, 6-COM
Bosch M175 T1, M-7A
KLG M60
AC 83 COM
Lodge HBV
