I have been running my 1936 Model 50 (Affectionately called 'Norma' ) now for a number of years, on 18mm Champion D16 spark plugs. They have not proved to be totally reliable in that time, having various failures. Over the weekend, I was let down again, with an open circuit Champion spark plug that had only covered about 60 miles. I changed it for a new one, and off she went again. As I intend to be riding her as much as possible, I thought I would try changing manufacturers.
From what I could find out an NGK equivalent plug was an AB6, so purchased a couple of these. When comparing them next to the Champions, apart from the Hex being a slightly different size (20.8mm compared to 23mm - why ? Another of life's mysteries ☺!) the NGK is ever so slightly shorter in the body (from what I can find out online, 12mm instead of 12.7mm), meaning the spark would be slightly higher in the combustion chamber ? Would this matter ? (Please forgive my ignorance in these matters ) - I guess my real question is - Is the NGK plug, the correct one to use in Norma ? Or would you suggest / recommend another type of plug or manufacture, in your experience ?
Thanks in advance,
Richard (& Norma)
I used an AB6 in my Iron engine Rudge with success. You may find the extra exposure of thread in the chamber to increase the tendency to pre-ignition if the motor runs hot and the thread may carbon up and need clearing if a longer plug is substituted. But given the usual classic usage and extra care it would not worry me.