DIY Sparkplugs
In 2012 when I arrived in Belgium, I met a fellow FN four
enthusiast, Michel Bovy. I admired his original mica spark plugs and
took photos, saying at the time I would like to see if I could replicate
them. Lo and behold, the next day Michel presented
me with working drawings of the spark plugs that a friend of his had
created the night before. I was amazed how detailed the drawings were
and kept them safe until two years later when I began about making a
set.

I googled mica washer suppliers in India and received several
responses. Despite their small size and little weight, the postage was
expensive and a one-off tool needed to be made to cut the washers to
size. 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. 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The top cover was originally made from Bakelite or ebony, but as I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. On the
last run I did on the FN I used the spark plugs
from my 2012 journey because I had constant stop-start city traffic and
330kms to travel with no time to make adjustments. When I get time to
adjust the heat range I will let you know if the change improves their
performance.