My 1931-402 currently has been running on some old Champion D14's. The plugs are about worn out and I'd like some input on the proper replacement plug. The metal portion on the D14's are black, which looks good. I've been told that using a Champion 516 is a good plug as is an Autolite 386. In looking at some old specs I found that Indians of this era used a Champion 6-con, which have shorter, more vintage look, but I don't know if 6-con would be recommended for a rebuilt engine.
Obviously I'd like something as close to original as possible. If anyone has and experience or recommendations it would be greatly appreciated.