The fifth tutorial in the curriculum, and the only one in Powerplant. Spark plug servicing is
artifact-verified (S3), it is cheap to practice, and it teaches something larger than itself:
a set of used plugs is a diagnostic map of an engine.
The course work is done in the shop, on the running engine, during the Powerplant in-person course. Nothing for this unit is posted to you. Practicing beforehand on a set of used plugs is worthwhile and optional.
You need: a set of used aviation spark plugs — any A&P or maintenance shop has a box of them — a plug cleaner or approved abrasive method, a gapping tool, a wire gauge set, and a torque wrench. In the shop, all of it is provided.
Read PP-H Procedures Procedure 3 first.
Session 1 — Read before you clean
This session is the point of the tutorial and it can only be done once per set.
- Lay the plugs out in cylinder order, top and bottom, exactly as they came out.
- Photograph them.
- Look at each one and write down: electrode color, deposit type and amount, electrode wear, insulator condition.
- Compare them to each other. That is where the information is.
What you are looking for:
| What you see | What it suggests |
|---|---|
| Light gray-brown, even, across the set | Normal |
| Black, dry, sooty | Rich mixture, or excessive idling |
| Black, wet, oily | Oil control — rings, guides, or that cylinder |
| White, blistered, or eroded electrodes | Too hot — heat range, mixture, or timing |
| Lead deposits, hard gray lumps | Fuel lead and operating temperature |
| One plug different from the other seven | That cylinder. Investigate it |
The last row is the whole exercise. Absolute appearance tells you something; difference across the set tells you much more, and it is available only if you kept them in order.
Session 2 — Cleaning without damage
- Clean by the approved method — abrasive blast in a proper plug cleaner, or as the manual specifies.
- Do not overdo it. Excessive blasting erodes the insulator nose and embeds media.
- Blow out thoroughly. Media left inside goes into the cylinder.
- Inspect the insulator nose for cracks and chips — a cracked insulator is scrap, not cleanable.
What will be wrong on your first attempts:
| What you will see | What it means |
|---|---|
| Insulator nose looks eroded or rough | Over-blasted |
| Media still in the plug | Not blown out — this will damage a cylinder |
| Electrodes rounded off | Worn beyond limits, or damaged during cleaning |
Session 3 — Gapping, and the electrode you may bend
- Measure the existing gap with a wire gauge, not a flat feeler.
- Adjust to the specification using the correct tool.
- Only ever bend the ground electrode, never the center electrode — bending the center cracks the insulator.
- Re-measure.
- Repeat on ten plugs until it is quick and consistent.
Check — a wire gauge, not a flat feeler. Electrodes wear to a curved profile and a flat gauge reads the widest point, giving a gap that is wrong in the direction that matters.
Session 4 — Torque, and the thread
- Fit a new gasket every time. A reused gasket does not seal or conduct heat properly, and heat path is what sets the plug's operating temperature.
- Apply anti-seize only where the manual specifies, only on the threads, and never near the electrode end.
- Torque to specification with a torque wrench (GEN-E Procedures Procedure 4).
- Do it ten times.
What you are learning: a spark plug is a heat path as well as an electrical component. The gasket, the thread contact and the torque all determine how fast heat leaves the plug — and therefore its effective heat range (PP-H Concepts §4).
Over-torque distorts the plug and damages the head. Under-torque leaves it running hot.
Session 5 — Rotation, and why
Read the manual's plug rotation scheme — top to bottom, cylinder to cylinder — and work out why it exists.
The answer: plugs in different positions run at different temperatures and accumulate deposits differently, and rotating them evens out the wear so the set ages together rather than one plug failing early.
What you hand in, in the shop
| Element | Artifact |
|---|---|
S3 |
Eight serviced plugs, cleaned, gapped and tested, in a labeled tray in cylinder order |
| Your Session 1 notes and photograph, with your reading of what the set says about the engine |
The notes matter as much as the plugs. A student who services eight plugs perfectly and cannot say what they indicated has done maintenance; one who can has done diagnosis.
License
CC BY-SA 4.0 (Licensing).