All three ZJ engines run the Champion RC12LYC (or NGK ZFR5N), gapped at 0.035". The 4.0L is a 15-minute job; the Magnum V8s take an hour because the rear plugs hide behind the exhaust manifolds.
The ZJ is one of the rare vehicles where every factory engine option runs the same spark plug. Champion's RC12LYC is the OEM unit — Mopar boxes it as part number 56027275. Gap them to 0.035" before installation, even brand-new in the box; plug gaps drift in shipping and a tight gap will make the 4.0L stumble on cold mornings.
NGK ZFR5N is the equivalent if you can't find Champions. Skip the platinum and iridium upgrades — these engines were designed for copper-electrode plugs, and the high-energy plugs don't last meaningfully longer in a low-compression motor like the 4.0L. Spend the money you save on a real coil pack and cap-and-rotor kit.
On the 4.0L straight-six, all six plugs are on the passenger side of the head, fully accessible from above. Pop the wires off one at a time (don't pull all six and try to remember the firing order — pull, swap, replace, move on). On the Magnum V8s, the front three plugs on each side come out from above; the rear plugs require either a long extension and patience, or removing a wheel and going through the wheel well. Both Magnum approaches work — the wheel-well route is faster after you've done it once.
Anti-seize on the threads is mandatory on the iron heads of the 4.0L — without it, you'll pull aluminum thread material out of the head on the next change. The Magnum V8s also have iron heads; anti-seize is still recommended. Use a light coat — too much and you'll over-torque the plug because the friction is misleading.
| Part | Vendor | Est. price |
|---|---|---|
| Champion RC12LYC plugs (4.0L I6) | Champion | ~$18 |
| Champion RC12LYC plugs (5.2L Magnum) | Champion | ~$24 |
| Champion RC12LYC plugs (5.9L Magnum) | Champion | ~$24 |
| NGK ZFR5N alternative | 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.