Miata Mailing List: December 1994, Message #108
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   From: manson@fred.net
Subject: Spark Plugs Notes
   Date: Sun, 4 Dec 1994 15:34:49 -0500

Miateers,
I just wanted to mention a couple of items that I discovered while 
replacing plugs yesterday.  These data might be helpful for somebody out 
there.  As it might be significant for data collectors, Mimi had 54480 
miles and 18 months on the road at the time of the change.
(1) The Enthusiasts Manual (I didn't check my workshop manual) indicated 
that spark plug torque should be 15-32 N-m (newton-meters).  I 
discovered that while installing the plugs that the compression washers 
start compressing at about 15-20 N-m.  They continue to compress for 
about 1/2 to 3/4 of a turn (rough estimate) and then the torque starts 
increasing again.  Through that compression the running torque climbs to 
as much as 25 N-m.  When I installed mine, I torqued until until ir rose 
from 25 N-m to about 30 N-m to be sure that the seating of the 
compression washers was adequate.  The concern here is that if one were 
to follow the Enthusiasts Manual explicitly, one would not fully compress 
the compression washers before meeting the criteria of the plug torque.  
It may be that the washers don't have to be fully compressed, but I felt 
that it was the best configuration to prevent loosening later in life.

(2) The manual directed that the plugs be gapped to .039 to .043 in.  The 
new NGK plugs right out of the boxes were gapped right at about .040 so I 
left them alone after checking each one.  I assumed that starting them 
out on the low side would give them a smidgen longer operational life.

(3) I noticed a significant pattern on the old plugs.  Identifying the 
plugs as 1-4, starting from the back (firewall end) of the engine, plugs 
2 and 4 had larger gaps and were not quite as dark.  Their gap was about 
080.  Numbers 1 and 3 were darker and were not as heavily worn with gaps 
about .060 (rough estimate based on the others and the spec'd .040 gap of 
the new ones).  This alternate wear supports the alternate current flow 
discussion (real fact, not theory), in that in one direction of current 
flow more of the center electrode would wear away.  The difference in 
gaps was strictly due to the center electrode wear (the side electrode 
was a fairly good shape with no significant erosion).  I really can't 
speak for the coloration difference.
Later,
Tom