07/14/2026
This comes from DJS Aviation's newsletter. A great publication that's always interesting. Since it addresses spark plugs, I thought I would pass it along. djsaviationllc.com
The Tech Desk
What Your Spark Plugs Are Trying
to Tell You Right Now
Every piston engine carries a set of small, threaded witnesses screwed directly into each combustion chamber. They fire thousands of times per flight hour under conditions of intense heat and pressure, and in doing so they record — in carbon deposits, electrode wear, and color patterns — a precise account of how that engine has been operated. Most owners hand them to a mechanic at annual who cleans, gaps, and reinstalls them without a word. That is maintenance. Reading them first is intelligence.
July is the right month for this conversation. Summer heat, high CHTs, and the rich ground-run mixtures that accumulate during long Texas taxi sequences all leave fingerprints on plug electrodes and insulators. If you know what you are looking at, a used spark plug tells you more about combustion health than any single gauge in the cockpit. Here is what to look for — and what each reading means.
▶ Reading Your Plugs: What Each Pattern Means
✓ Light tan or gray deposits — the correct answer
A plug with a light tan or gray insulator nose and minimal electrode wear is running at the right temperature in the right mixture. This is what a healthy, properly operated engine produces. If all your plugs look like this, the engine is content. Keep doing what you are doing.
✓ Dry, sooty black carbon — the rich mixture signature
Black, fluffy, dry carbon fouling is the unmistakable result of a mixture that has been running too rich for too long. In July, the most common cause is prolonged ground operations at full rich — the exact scenario playing out at airports with long taxi sequences in 100°F heat. A plug fouled this way will misfire under load. The fix is leaning correctly during taxi and ground runup. The plug is the proof you have been skipping that step.
✓ Wet, oily deposits — oil is getting into the combustion chamber
A wet, shiny plug with oil contamination on the insulator is a different conversation entirely. Oil past worn or stuck piston rings, a failing valve guide seal, or a breather system issue can all put oil into a combustion chamber. This reading does not fix itself. It is the engine telling you something specific — and the answer is not a new plug. It is a diagnosis of where the oil is coming from.
✓ Eroded or rounded electrodes — the plug is past its useful life
Electrodes erode with every firing. A sharp, square center electrode fires reliably and consistently. A rounded, pitted, or eroded one requires higher voltage to bridge the gap, stresses the magneto, and increases the probability of misfire under high-power or high-altitude conditions. Aviation spark plugs are typically serviced at 100-hour intervals and replaced at 500 hours or per manufacturer limits. If yours are approaching those numbers and the electrodes look worn, replace them. Do not gap-and-go your way through another season.
✓ Rotate plugs on the correct firing-order sequence — not just top-to-bottom in the same hole
Most shops clean plugs and reinstall them in the same hole. Better shops swap top to bottom. The correct procedure does both of those things and one more: it rotates plugs to the next cylinder in the firing order, which reverses the electrical polarity at the electrode. That distinction matters more than most owners realize — and more than most shops will tell you.
Great Stuff. We will resistance test, clean, gap, pressure test, clean threads and paint your plugs. Before sealing them in a package, we'll put on a new gasket, l**e the threads (don't forget to clean the l**e off before installing) and drop in a desiccant pack for some additional dryness. Let us know how we can help you and thanks to DJS Aviation LLC in Cleburne, TX KCPT for the great write up.
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