Making Spark Plugs

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The top cover was originally made from Bakelite or ebony, but as I did not have either material, I made my top cover from Tufnol, a manmade compressed fibre.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)

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.

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