The International Cessna^® 170 Association

This last weekend I was reminded of something not discussed recently.

I was changing out my failing oil temp gauge with a new unit (upcoming article in a future quarterly) and decided to perform a compression test and spark plug cleaning.

I found my front (#5 and #6) cylinder spark plugs were heavily loaded up with lead deposits. (I've been out of TCP for a while.) They weren't beyond saving with a good cleaning with a spark plug vibrator.... and was a reminder this is likely due to those cylinders habits of running cooler than others. I decided to rotate them to the middle cyls (#3 and #4).

But the event also reminded me to bring up another reason to rotate spark plugs.... wear-pattern.

Whenever you remove your spark plugs, keep/place them in an orderly fashion so as to easily identify which cylinder they've been servicing and which position (uppler/lower) they've been.

Take a look at them for signs of oil burning, or rich-running, temperature-range, etc., etc...... but also, with regard to massive-electrode type plugs such as those most commonly used, look at the center-electrode and inspect for out-of-round or elliptical shape of that electrode. This is indicative of reverse-firing ....a normal occurance...... of magneto ignition systems.

Every "alternate" spark generated by the magneto reverses in polarity from the previous one. This means that one spark plug will experience the spark jumping normally from the ground-electrode (the one attached to the threads of the spark plug) to the center electrode,... while the next spark plug receiving a spark from that magneto will experience a spark jumping from the center electrode to the ground-electrode.
While this is normal operation... it has an odd effect, that of causing the center electrode to lose mass as the electrons jump across to the ground electrode they bring with them a little bit of electrode material from where they began the jump. This material is lost in the process and ultimately ends up out the tailpipe until the spark plug is worn into that elliptical shape, eventually worn so far it must be thrown away.

Those expensive spark plugs can benefit from increased service-life if they are rotated to an alternate position in order to spread the wear out amongst all the spark plugs. For example: if you place the spark plugs in a spark plug tray (you can make your own if you really want to save money, by using a step-drill-bit or "UniBit" to drill 12 holes in the side of a 1-gallon metal can such as an empty WD-40 gallon can. This way you even have a nice handle on one end to carry them around the shop to the worbench.)

Drill the holes in two straight rows of 6 holes each, and number them in cylinder-pairs, and label them for UPPER and LOWER positions.

Now when you place the used spark plugs in the holes, their electrodes can be examined for wear and condition.

You will notice (provided your engine is set up per the original ignition distribution-pattern) that, for example, cyl #1 upper and lower plugs may have nice round center electrodes....while cyl #2 center electrodes may be elliptical.
After inspecting, cleaning, and re-gapping those plugs, you can swap #1 upper with the #2 lower postion, and the #1 lower with the #2 upper position. The same can be done with the rest of the plugs in the engine. Look at each set, and determine the rotation necessary to spread out the wear pattern among all the plugs more evenly.

Clean the plugs with a spark plug media blaster (inexpensive blasters are available from discount tool stores...but don't get carried away with this...you can take many hours of life away from a good plug by over-aggressive use of a blaster) and re-gap them to .016". Use the proper tool or you risk damaging that expensive plugs' porcelain tip causing rejection. Spruce sells inexpensive tooling for these jobs, and also sells an inexpensive spark plug vibrator/cleaning tool to get the lead out of the deep recesses of the porcelain tip area. (Careful... don't hurt that tip....and DON"T FORGET, blow out the residue with compressed air, you DON'T want that abrasive lead-bromide powder down in your engine!)

While you've got those plugs cleaned up, use an Ohm Meter to check resistance from the center electrode to it's opposite end inside the "well" where the ignition-lead "cigarette" contacts. That resistance should be 5K Ohms or less, or the spark plug internal resistor is failing, and may cause poor ignition and irregular engine performance.
Also, be certain to check the porcelain tip for cracks or damage. ANY is cause for rejection.

Reinstall the plugs after you've lubricated the threads with anti-sieze, keeping the lube off the electrodes. (I use Champion spark plug lube, or Permatex Anti-seize.) The plugs should be installed with 300-360 in. lbs/25-30 ft lbs of torque. Use new copper gaskets, or heat the old ones cherry-red with a propane torch and drop them into water to anneal them.

Do not overtighten the ignition-leads, as the ignition-lead nut is aluminum and can be easily stripped. A very light brush of anti-seize is acceptable on that area of the spark plug barrel-threads...but be certain not to contaminate the interior well or the ignition lead "cigarette", which should remain clean.

You may be able to put off that $250 expenditure for spark plugs for another year or so.

Aryana wrote:I don't have my manual handy, but are you sure the plugs are torqued at 50 ft lbs? That seems higher than I remember from the book...but I could be wrong!

Hello, Aryana!
I was proofreading my msg and confirming with my tech book and didn't realize I'd posted this in the public area already....I thought I was still "previewing"....and apparently we were both working and reading at the same time. The correct info is in the article 300-360 in lbs or 25-30 ft lbs.

I have also found online an article by Champion. They offer a different rotation pattern, but the concept is the same and should take into consideration that their rotation pattern may not meet YOUR engine's ignition polarity pattern. A visual inspection of YOUR plugs is necessary to correctly rotate them.

I always check the electrode resistance as well. Tempest (formerly Unison/Autolite) recommends replacing plugs having more then 5k ohms. Break down of the resistor may cause high CHTs and intermittent roughness.

Personally I have always used the rotation recommended by Continental since it's their engine.

N2255D wrote:Personally I have always used the rotation recommended by Continental since it's their engine.

That is the same rotation offered by Champion. The only problem is... it does not consider magneto polarity/electrode erosion based upon firing order. In order to address that issue, I believe it's best to inspect them as they are removed...then rotate them Top-to-Bottom/Bottom-to-Top in exchange with alternate-polarity firing cylinders (which is very evident when the center electrodes are examined.)

I like to negate the whole problem by putting in fine wire or Iridium plugs. If you look at the cost over the life of the engine you generally change plugs if you are smart, about every 400 hours and the cost of fine wires is about the same as having Irudiums over that span. Damn who wants something with four syllables.

OOPS typo

gahorn wrote:

N2255D wrote:Personally I have always used the rotation recommended by Continental since it's their engine.

That is the same rotation offered by Champion. The only problem is... it does not consider magneto polarity/electrode erosion based upon firing order. In order to address that issue, I believe it's best to inspect them as they are removed...then rotate them Top-to-Bottom/Bottom-to-Top in exchange with alternate-polarity firing cylinders (which is very evident when the center electrodes are examined.)

After I inspect them I also clean and check the gap before I rotate and reinstall them.

If you guys ever get around to running on mogas, (and believe me, it's coming) you can finally quit having to clean your spark plugs.

Had 5 plugs that failed the resistance test today during the annual. All had good looking electrodes. Don't recall total hours on the Champions but if I had to guess, I'd say around 200. Maybe I'm a sucker for Tempest's tester but I am going to put in all 12 new Tempest plugs. I thought I had noticed just a bit of roughness lately so will be interesting to see if I note any difference.

Frank - I saw on another post you were thinking of going that route too. Did you?

Kyle, some 450+ hours ago (July 2006) I replaced my old Champion plugs (after mag overhauls they ran really bad) with Unison plugs. I have been well satisfied with those spark plugs every since. At my most recent annual in April they still looked good with no football electrodes. I have not tried checking each plug as has been recommended in other posts, but I plan to continue using these spark plugs until they either show extreme wear or the ole Continental starts not running smooth. What will I replace them with? Not sure. Will see what results others report with Tempest and Champions.

It's unfortunate that our fearless federal friends require that we are limited to using the ridiculously expensive spark plugs that they have approved. Strangely enough, the experimentals mostly seem able to make automotive spark plugs work just fine. You can buy a set of twelve for about the price of one fine wire aircrafrt plug.
In fact the whole ignition system is terribly outdated. When was the last time you had an automotive ignition system failure, or replaced the spark plugs, with a modern automobile ignition system.
How efficient would an automobile engine be if the ignition fired at 25 deg. BTC no matter what the running condition was. You can buy a brand new Cirrus airplane and the ignition system is almost as sophisticated as a Model A Ford except that you have no control over the spark advance.

Based on the 200 to 400 hours and they need replacement I would for sure only use fine wires. The ohm test is a good way to check as well. Also there is a go no go gauge that is usually free at trade shows for AFTER you gap your plugs. If they go through the hole they are beyond limits.

Not to mention, when was the last time you sent your automotive ignition system in for a 500 hour overhaul for a few hundred bucks?
I have two Fords, a '96 and a '99, with 4 liter V-6 engines, each one having over 150K miles, and neither has ever had any work done on their ignition systems.
When I was a little kid, my dad had a 1916 Hupmobile with an Atwater Kent ignition system utilizing a rotary ignition switch. This switch was desigined to reverse the polarity of the ignition system each time it was roated 90 degrees clockwise to turn it on. See, they even knew about polarity wear way back then.

blueldr wrote:...When I was a little kid, my dad had a 1916 Hupmobile with an Atwater Kent ignition system utilizing a rotary ignition switch. This switch was desigined to reverse the polarity of the ignition system each time it was roated 90 degrees clockwise to turn it on. See, they even knew about polarity wear way back then.

Geesus....NOW I understand you a lot better!