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Making Spark Plugs

April 8, 2015

Ron Fellowes famously rode his 1910 FN four cylinder 14,606km from Nepal to Belgium in 2012. You can buy his book about the trip here. I’ll have a review of the book in a few days, but first I wanted to share his notes about making his own spark plugs! He mentioned in the book that he had a little bit of trouble with the modern plugs he carried on his trip and decided to make his own when he returned home. Fascinated by the idea of home made sparking plugs, I asked him for more information…



“During my journey I met Michel Bovy, who had original spark plugs fitted to his FN4. I admired them, took lots of photos and said I would like to replicate them. Next day, Michel brought me working drawings that his friend had made the night before.



I used hex steel to make the lower body which I threaded to 18mm and an internal taper of 2 degrees was machined to form a seal between the mica washers. The mica washers were purchased from India. These were of .2 + .1 thickness, mounted on a mandrel and ground with a matching 2 degree taper. I drilled cooling holes in the appropriate places.


The top cover was originally made from Bakelite or ebony, but as I did not have either material, I made my top cover from Tufnol, a manmade compressed fibre. The centre electrode is turned from a Grade 8 bolt.




To adjust the gap (firing point), mica washers are either added or subtracted from the centre electrode.


When I used the mica spark plugs I was happy with the performance, but I found the heat range a little high. I’ve been studying the drawings closely and feel I need to remove some of the mica washers in the cover end to make them run a little colder.


It’s not time consuming to adjust the electrode gap but I do need to put a dummy centre electrode in from the spark plug terminal end to keep the two degree taper in order. I am running .018inch gap.”



  (click on the photos to see larger images)DSC_0076

Ron is continuing to test the new plugs with rides in Australia. I’ll ask if he has any new comments in the coming months.

Click here for about 50 more How To articles.


Above photos are copyright 2014 Ron Fellowes.


early plugs

8 Comments leave one →
  1. April 8, 2015 9:34 am

    Thanks Pete! Very interesting.

  2. Anonymous permalink
    April 8, 2015 10:15 am

    Pete: A friend built up a 4-cyl engine for a Curtis “Jenny” and made his own plugs (which did NOT work). I’d like to forward your article to him if I may. Also, I can’t figure out where the .018″ gap is.
    Thanks in advance, Jim A.
    p.s. the Jenny motor was since stolen from his shop.

    • April 8, 2015 10:22 am

      What used to be the hex head of the bolt has been turned on a lathe to be round. It fits inside the big hexagon steel body, with .018″ clearance from the edge of the bolt head to the interior of the spark plug body. So there isn’t any discrete point where the spark will originate, but it could happen anywhere in the 360 degree circle. The last image on the page that shows the dimensioned drawing can be blown up bigger to see some details.

  3. Alan in Roberts Creek permalink
    April 8, 2015 12:03 pm

    Brilliant! I wonder if plugs like this would make the Velocette run better.
    Alan in Roberts Creek

  4. April 8, 2015 2:57 pm

    A fun little project. Mica is almost transparant so it is also used in fuel stove windows and some suppliers can provide sheets of mica. Mica was an essential mineral during WW2 and was secretly mined in Sweden in some remote places in the mountains. More can be read about mica here:

  5. brad kenyon permalink
    April 8, 2015 5:19 pm

    I’m reading Ron Fellowes book on my iPad / Kindle as time permits.
    Suprisingly well written – Highly inspiring pre-Cannonball reading, IMO.
    His account of crossing Pakistan/Iran with the bare minimals self contained on his Century old FN exposes me to anther level of true pioneering-endurance-Adventure riding.
    Inspiring indeed.

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