Pull the wires one at a time, swap each Champion RC12LYC (gapped to 0.035") with a dab of anti-seize on the threads, torque to 27 ft-lbs, and call it done in under an hour.
The AMC inline-six is one of the friendliest engines on the planet for a spark plug change. All six plugs sit in a straight line along the passenger side of the head, each one exposed and reachable without removing a thing. The whole job is 30 to 45 minutes the first time, and 20 minutes after that. Plug life on copper Champions is 30,000 miles; platinum aftermarket equivalents can stretch to 60,000 miles but the engine doesn't run any better on them — the HEI-spec ignition on the 4.0L makes great spark either way.
The factory plug is the Champion **RC12LYC** (or **RC12YC** on early YJs) — both interchange. Gap is **0.035"** on engines with the distributor cap (which covers every YJ); set the gap with a feeler gauge or a wire gap tool before installing. NGK and Autolite make crossover plugs that work fine — what matters is the heat range, which is what the 12 designates. Don't run plugs cooler than the OE heat range unless your engine is built; you'll foul them at idle.
Pull one wire at a time and replace one plug at a time — that way you cannot mix up the firing order. The boots get stiff with age; twist before pulling. Use a 5/8" spark plug socket with a rubber insert to hold the plug, and a short extension for clearance under the hood. Apply a small dab of anti-seize on the threads (a thin film, not a glob — too much and the torque reading gets thrown off). Thread by hand for at least three turns to confirm you haven't cross-threaded the aluminum head — this is the most common failure point for a plug change. Torque to **27 ft-lbs**.
While you're in there, look at each old plug as it comes out. White ash = lean, oily black = rich or burning oil, dry black = running rich or short trips. A normal plug pulled from a healthy 4.0L will be tan to light brown. If one plug looks different from the other five, that cylinder has a problem worth chasing.
| Part | Vendor | Est. price |
|---|---|---|
| Champion RC12LYC (set of 6, copper) | Champion | ~$18 |
| Champion RC12YC (set of 6, copper) | Champion | ~$18 |
| NGK ZFR5N (set of 6) | NGK | ~$28 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.