The 4.0L takes six plugs, gapped at 0.035", torqued to 27 ft-lb. Copper plugs (Champion RC12LYC or NGK ZFR5N) are the OEM choice and run cleaner than premium platinums on this old engine. Job takes 30–60 minutes with a 5/8" plug socket.
The 4.0L straight-six uses a straightforward distributor-and-coil ignition through 1997, then coil-on-distributor with cap-and-rotor through 2006. Either way, the plugs sit in a row down the passenger side of the engine, fully exposed, no intake to remove. This is the most straightforward spark plug job on any modern engine.
**Plug choice.** The factory plug is the Champion RC12LYC copper. NGK ZFR5N is the direct equivalent. Both gap to 0.035" out of the box. Service interval is 30,000 miles on copper, 60,000 on platinum (NGK ZFR5F-11), 100,000 on iridium (NGK IZFR5F-11). On an engine this straightforward, copper is the better answer: cheaper, runs cooler, more manageable to read for diagnostics.
**Why not premium plugs?** The 4.0L was designed for copper. Iridium plugs sometimes cause cold-start hesitation on this engine because the precious metal tip has a slightly higher firing voltage requirement and the older ignition coil isn't always up to it. If your TJ runs perfectly on copper and you switch to iridium and get rough idle, that's why.
**Gap matters.** Pre-gapped doesn't mean correctly gapped. Always verify with a wire gauge — 0.035" should slide through with light drag. If you can shove a 0.040" gauge through too, the plug is no good (don't try to close the gap on iridium; the tip chips).
**Anti-seize warning.** Most modern plugs ship with a nickel-coated thread that does not need anti-seize. Adding anti-seize will cause you to overtorque (the lubricant reduces friction, so your torque wrench reads correct but the actual clamp load is too high). If you must use it, a tiny dab on the threads only.
| Part | Vendor | Est. price |
|---|---|---|
| Champion RC12LYC OEM copper (qty 6) | Champion | ~$18 |
| NGK ZFR5N copper (qty 6) | NGK | ~$22 |
| NGK ZFR5F-11 platinum (qty 6, longer life) | NGK | ~$35 |
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.