Both the AMC 258 and 304 use Champion RN12YC (or equivalent) at 0.035 in. gap and 26–30 ft-lb torque into cast iron heads. Anti-seize is optional but recommended on a 40-year-old aluminum-free engine that has seen seasonal humidity.
This is the second-cheapest tune-up you can do, and the one that most often gets skipped because the engine "still runs." The 258 will tolerate worn plugs for tens of thousands of miles before it stumbles. By the time it stumbles, the cap, rotor, and wires are probably also tired. Fresh plugs every 20,000 miles on a daily-driver CJ-7 is the standard interval; every 5,000 if it sees serious dust or short trips.
The 258 firing order is **1-5-3-6-2-4** (front to back: cylinders 1, 2, 3, 4, 5, 6). The 304 V8 is **1-8-4-3-6-5-7-2** — passenger bank is 1, 3, 5, 7 front to back; driver bank is 2, 4, 6, 8 front to back. Pull and label one wire at a time so you don't have to relearn the order. If wires are cracked, ceramic insulators are oil-stained, or the boots feel hard, replace the whole set. NAPA Belden or Accel 5mm wires are the standard upgrade.
Stock plug for both engines is the Champion RN12YC copper. The "RN" prefix means resistor, nominal heat range; the "12Y" is the heat number; the "C" denotes copper core. NGK BPR5ES is the direct cross-reference and many owners prefer it for a sharper spark in worn engines. Avoid platinum or iridium plugs unless your ignition is upgraded — the stock points-or-Motorcraft ignition doesn't put out the voltage to clean a fine-tip plug.
Gap is **0.035 in.** on both engines. The factory tolerance is 0.033–0.038 in. The 258 with HEI distributor (1978+ on most Jeeps) can run 0.040 in. if the coil is healthy, but stay at 0.035 if you've got original ignition.
| Part | Vendor | Est. price |
|---|---|---|
| Champion RN12YC copper plug (AMC 258 stock) | Champion | ~$4 |
| Champion RN12YC copper plug (AMC 304 stock) | Champion | ~$4 |
| NGK BPR5ES (cross-reference) | NGK | ~$4 |
| Permatex anti-seize compound | Permatex | ~$7 |
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.