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 Posted: May 19, 2019 06:44AM
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.. and then there's the gap between the rotor and the dizzy cap's four outer posts. No idea what it actually is, but I'm sure Doug has the free time to work it out.

.

"Hang on a minute lads....I've got a great idea."

 Posted: May 18, 2019 06:41PM
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I expect that when the resistance is attributable only to the fit of the parts (i.e. high resistance due only to the gap and corrosion between components) there is almost no effect on the spark.  The spark is already strong enough to jump plug gaps of 0.025" or so.  The small gap between the components won't add significantly to that.

Doug L.
 Posted: May 18, 2019 02:56PM
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The day after you first posted about spark plugs, I dissected some plugs and came up with the same conclusion as you did.  The question now would be, does the varying resistance, do anything to the actual spark, from the spark plug. o resistance = good spark, 20K ohm = less spark??  Maybe it would important, to a very highly tuned engine??

 Posted: May 12, 2019 07:18PM
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Quote:
Originally Posted by dklawson
I spent the evening in the garage after making my post. I figured out what's going on.
Thanks for your research. That's good news. One less thing I have to fuss with.

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: May 12, 2019 06:46PM
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I spent the evening in the garage after making my post. I figured out what's going on.

The electrode is fired in the ceramic, the ceramic is swaged into the threaded body, and finally the plug terminal is screwed and glued into the top of the ceramic. The top of the electrode is sized to just fit in a hole on the bottom of the plug terminal. The tolerance between the electrode and and the plug terminal has to be tight but lose enough to allow assembly. 

I removed the plug terminal from four of my high-resistance plugs, did what I could to clean the bits, then placed a few strands of fine copper wire into the hole of the plug terminal, and screwed it back together. Measurements showed that the resistances had dropped from mega-ohms to nothing. I am not proposing fixing plugs... only suggesting that the high-resistance I was measuring was largely due to the assembly method and perhaps some minor internal arcing between the electrode and the plug terminal. The film thickness and clearance was obviously less than the spark plug gap so the plugs were functioning normally.

Because of what I saw with the plugs I took apart (Champion and NGK) I don't think I would put any faith in pass/fail resistance measurements.  I think the only real test would be to look at the color of the spark that a given plug can generates.

Doug L.
 Posted: May 12, 2019 04:29PM
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Good question, Doug. Looking forward to the discussion.

 

Michael, Santa Barbara, CA

. . . the sled, not the flower

      Poser MotorSports

 Posted: May 12, 2019 04:01PM
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This is not the typical “what spark plug is best for my car” question.  This is a “have you ever measured” question.  It is not specific for any make or model of car.  It is something that is bothering me.
 
I have a car that is giving me some running problems so I cleaned the old spark plugs and then decided to measure the resistance of all the high tension ignition parts.  The resistor plug wires measured 500 to 1000 Ohms depending on length.  The dizzy cap measured fine with no shorts between terminals.  Then there were the spark plugs.  
 
These are standard plugs (not resistor plugs) so I expected to measure Zero (0) Ohms between the plug’s terminal and the center tip at the firing end.  I only found Zero resistance on about 3 out of 10 plugs.  (Once I started making measurements I brought out ALL my old spark plugs to measure).  Apart from the handful of plugs that measured low resistance, MOST of my used plugs measured several hundred thousand Ohms or more.  
 
All these plugs were presumed working when removed.  I have never had an engine start misfiring due to a failed plug so apparently the resistance I measured does not affect the firing of these plugs… much.
 
So have you ever measured the resistance of new and old spark plugs?  Have you ever seen a published value that says “if the resistance is above X don’t reuse the plug.”?  Please share your thoughts and experiences.
 
Thanks,
Doug L.

 

Doug L.