Short answer first: There are no spark plugs in an NV3550 gearbox. If you meant “replace the engine spark plugs on a Jeep Wrangler that is fitted with an NV3550 manual transmission,” below is a concise, step‑by‑step procedure from an experienced tech. No extra questions.

What you’ll need
- Correct replacement spark plugs (exact type/gap per engine/service manual). Common brands: NGK, Denso, Champion.
- Spark plug socket (with rubber insert) sized to the plug hex (use the socket that matches the plug — typically 5/8" or 13/16"; confirm your plug).
- 3/8" drive ratchet, 6" extension, universal swivel (helps reach deep wells).
- Torque wrench (ft·lb scale).
- Feeler gauge or preset gap tool.
- Dielectric grease (for boots) and small amount of anti‑seize only if the plug manufacturer recommends it.
- Spark plug boot puller (optional), compressed air or vacuum, shop rags.
- Penetrant spray (if boots/wires are stubborn), replacement ignition wires or coils if worn.
- Safety glasses, gloves, jack/stands if extra access needed.

Safety precautions
- Work on a cool engine. Never remove plugs on a hot engine — risk of thread damage.
- Disconnect battery negative if working around ignition coils/wires to avoid accidental spark.
- Wear safety glasses and gloves.
- Keep dirt out of the plug wells — blow out around each well before removal.
- Use a torque wrench for final tightening to avoid over/under torque and thread damage.
- Support vehicle safely with jack stands if you must get under it.

Step‑by‑step procedure (general for 2.5L or 4.0L Jeep engines)
1. Preparation
- Park on level ground, engine cool, parking brake set.
- Disconnect negative battery terminal if you’ll be removing coils/wires.
- Remove any obstructing engine covers or air intake components to access plug wells.

2. Clean area
- Use compressed air or a vacuum to remove dirt/debris from around each spark plug well. Keeping foreign material out of the cylinder is crucial.

3. Label/mark ignition leads or coils
- If the engine uses a distributor/plug wires or a coil pack that serves multiple plugs, mark each lead so firing order is preserved. If coil‑on‑plug (COP) each coil is usually one per cylinder—labeling still helps.

4. Remove ignition leads/coils
- For plug wires: grasp the boot, twist to break seal, pull straight out. Use a boot puller if needed.
- For coil packs/COP: unbolt the coil electrical connector, remove the mounting bolt(s), pull straight up.
- Inspect boots and connectors — replace if cracked or heavily worn.

5. Blow out wells again if necessary

6. Remove spark plugs
- Fit spark plug socket + extension + swivel on your ratchet. Insert into the well, engage the plug.
- Turn counterclockwise and slowly break the plug loose. Remove carefully (the rubber insert holds the plug).
- Inspect each plug for wear, fouling, oil, coolant signs — note any abnormal condition.

7. Prepare new plugs
- Verify the exact part number and gap for your engine.
- Check the electrode gap with a feeler gauge and adjust only if required (many modern plugs are pre‑gapped).
- If manufacturer allows, apply a very light coat of anti‑seize to the threads. Most modern plated plugs do NOT need anti‑seize — check packaging.
- Apply a thin dab of dielectric grease inside the boot (not on the plug electrode) to aid boot sealing and electrical connection.

8. Install new plugs
- Start threading by hand to avoid cross‑threading. If you feel resistance, back out and re‑start.
- Once hand tight, use the torque wrench to final torque per manufacturer spec. (If you do not have the spec immediately available, use a conservative range: many aluminum‑headed engines take roughly 10–18 ft·lb; steel heads higher — but confirm with the manual. Do not over‑torque.)
- If no torque wrench, snug and then an additional small fraction of a turn per plug condition is less reliable — get a torque wrench for correct job.

9. Reinstall ignition coils/wires
- Reconnect coils/wires in correct order. Push boots fully onto plugs until you feel/ hear them seat.
- Reconnect coil electrical connectors and any harness clips.

10. Reconnect battery and test
- Reconnect negative battery.
- Start engine and listen for smooth idle and no misfire. Clear any diagnostic codes if present and verify engine runs normally.

Common pitfalls and how to avoid them
- Cross‑threading plugs: always start by hand and avoid force.
- Over‑torqueing: use a torque wrench. Over‑tightening damages threads in the head.
- Working on a hot engine: threads can gall and you can be burned.
- Allowing dirt into the cylinders: always clean around wells before removal.
- Reusing worn wires/boots: replace if cracked, hardened, or arcing.
- Wrong plug gap or heat range: use the manufacturer’s specified part and gap.
- Using anti‑seize indiscriminately: many modern plugs are plated and don’t require anti‑seize; it can change torque readings.
- Not seating coil boots fully: results in misfires. Ensure full, clean contact.

Replacement parts/consumables typically required
- Correct spark plugs (4, 6, or as required by engine). Use OEM cross reference.
- Spark plug wires or ignition coils if cracked, resistive, or old.
- Dielectric grease, possibly anti‑seize (only if specified).
- New distributor cap/rotor if applicable and worn.

If you actually intended a different service on the NV3550 (speedometer gear, neutral switch, fill/drain plugs, vent, or internal service), that’s separate — the gearbox has no spark plugs.
rteeqp73

Hemi conversion using the stock Jeep 5 Spd This is a 2004 Jeep TJ we are putting a 5.7 L Hemi in and reusing the NV3550 trans.

NV3550 Rebuild My transmission failed on me. Time to go down the transmission rabbit hole. Also for the first part of the transmission rebuild you ...

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