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.
