What Spark Plug Do You Run In Your Modified Race Flat Head? Thoughts on J17LM, J19LM

#1
Folks (all 6 of you or likely more who still run flat heads),
I would like to hear people's thoughts on what spark plugs they run in their modified
high performance flat heads. Since the 1940's, auto and small engine flat heads used
the Champion J and CJ series of spark plugs, J-8 (heat range 8) being the most common.
Today the J-8 is replaced with the J8C.
In later years, Champion introduced the J17LM (heat range 7) and
J19LM (heat range 9) specifically marketed to lawn mowers (hence the "LM") and small engines.
When the two LM spark plugs were introduced, J17LM was specified for Tecumseh
flat heads of that time period and J19LM was specified for Briggs flat heads. Later on,
Tecumseh also specified the J19LM for their flat heads. I think the motivation for this was
so that the dealers would have to stock only one part number of spark plug for all
flat heads and so that the technicians no longer had to take the time to look up each
engine. In later years, both Tecumseh and Briggs specified J19LM. In looking over
the Tecumseh spec by each flat head engine, the very old flat heads specify Champion
J8C, the middle years of flat heads specify J17LM and the later years specify J19LM.
(I realize that Tecumseh has specified there own Tecumseh spark plug part number
at various times and so does Briggs specify a Briggs spark plug part number.) (Not sure
who made or makes the spark plugs labelled with Tecumseh part no. or who currently
makes the spark plugs labelled with the Briggs part no. I would not be surprised if it
is a Chinese company.)

As a practical matter, it looks like, today, any flat head that comes into a shop receives a Briggs
part number or a Champion J19LM (or RJ19LM) spark plug.

Two questions
1. What spark plug (brand and part #) do you like to run in your high performance flat head, to achieve the
desired color of the electrodes? Do you run a cooler spark plug such as a Champion J6C
(heat range 6) or CJ-6?

2. Can someone help pinpoint the date that the Champion J17LM and J19LM were introduced?
I am thinking that the J17LM and J19LM were introduced sometime in the late 70's to the early 80's.
Here is why I think so. During the late 60's and early 70's when I was working on flat heads,
Champion J-8 was the go to spark plug. In the early 70's, I am 99% certain the J17LM and
the J19LM were not around yet. However, my Champion Spark Plug Application catalog, dated 1984,
shows J17LM and J19LM, which means the J17LM and J19LM were around by then.

I am wondering if cost motivation was the reason for the introduction of the J17LM and J19LM.
Compared to the original flat head spark plug of J8C, the J17LM and J19LM are shorter (like the
CJ series) and the LM spark plugs have thinner electrodes (both electrodes).

Another factor might be
since the 1970's, electronic ignition has been introduced on flat heads, lead has been removed
from gasoline, and ethanol added to the
gasoline which may have also been the reasons for the introduction of the new LM spark plugs.
 
Last edited:
#2
I use tge j19Lm in my mild motor 95-3 cam bp'ed raptor block by trick connection racing billet rod 22mm chikuni it seems to like it it needs new rings now but it was fast
 

Top