I understood that having the plug recessed into the head would retard the ignition process....Not by changing the timing of the spark but rather the rate of combustion, with similar results....Normally it wouldn't be recommended...However, if a bike was was built like that I assume fitting a longer plug would have the opposite effect...Ian
If there were an oiling problem with a particular engine putting a shorter reach plug in would mask the tip so keeping the oil away. Old dodge used for old engines.
