Mine is over the oil filter on the K3C engine and measures 45mm. my plug blew due to old age and I changed it last Jan but installed a lower wattage unit as that was all I could get on short notice.
I since upgraded for a 600 watt as the 300 watt just would not heat enough in our cold climate of Quebec where it drops to - 25 celcius at times.
With the 600 watt I only need about 0ne hour of preheat for my Mitsu to fire up.
