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Spark plug replacement

Replace spark plugs on Mazda MX-5 NB (1998-2005) with careful thread handling, verified gap, and conservative torque practice.

Difficulty
Beginner
★★☆☆☆
Est. Time
45-75 min
Models
NB1 & NB2
Last Updated
2026-02-26

Before You Start / Safety

  • Work on a fully cool engine before removing/installing plugs in the aluminum cylinder head.
  • Blow debris out of plug wells before removal.
  • Replace plugs one cylinder at a time to avoid wiring/coil mix-ups.

High-risk quote: “you should only remove and install spark plugs in a cool engine, because metal expands when hot and thread damage can happen.” (NGK installation guidance, retrieved 2026-02-26) https://ngksparkplugs.com/en/resources/spark-plug-installation

Required Tools

  • 3/8” ratchet
  • 16 mm (5/8”) spark plug socket
  • Extension(s)
  • Torque wrench (recommended)
  • Feeler gauge / wire gap gauge
  • Compressed air or blower
  • Optional: small amount of dielectric grease for boot interiors

Required Parts / Fluids

  • 4x spark plugs suitable for your NB engine variant (NB1 BP-4W / NB2 BP-Z3)
  • Optional: ignition leads/boots if degraded

Model-specific notes (NB1 vs NB2)

  • NB1 (1998–2000, BP-4W) and NB2 (2001–2005, BP-Z3 VVT) use similar plug replacement procedure.
  • Workshop-manual index sources list separate 1999–2000 and 2001–2005 engine manuals, so verify part number/heat range/gap by your exact year/market.

Step-by-Step Procedure

1) Verify plug type and target gap first

Use your year/engine documentation and plug manufacturer catalog before installation. Even pre-gapped plugs should be checked.

2) Clean plug wells before removal

Blow out all dirt around each plug well.

High-risk quote: “The spark plug holes must always be clean prior to installation… [otherwise] the spark plug will be under-torqued…” (NGK, retrieved 2026-02-26) https://ngksparkplugs.com/en/resources/spark-plug-installation

3) Remove one boot/lead at a time

Twist the boot slightly and pull straight up; do not pull by the wire itself.

4) Remove old plug carefully

If removal force feels abnormal, stop and re-seat before retrying to reduce thread-damage risk.

5) Inspect old plug condition

Check for oil fouling, carboning, cracked insulators, and cylinder-to-cylinder differences.

Community evidence (NB-specific troubleshooting context):

  • MX5Nutz users report plug appearance differences when chasing emissions/idle issues on NBs.

6) Install new plug by hand first

  • Start threads by hand until seated.
  • Final-tighten using torque wrench.
  • If you have to install without a torque wrench, use seat-type turn-angle guidance only as a fallback.

High-risk quote: “Torque is one of the most critical aspects of spark plug installation.” (NGK, retrieved 2026-02-26) https://ngksparkplugs.com/en/resources/spark-plug-installation

High-risk quote: “New Plug: 1/2~2/3 turn… Reused Plug: 1/12 turn” for gasket-seat plugs after finger-tight contact (Champion guidance, retrieved 2026-02-26). https://www.championautoparts.com/Technical/Tech-Tips/Spark-Plug-Torque-Recommendations.html

7) Refit boots/coils and repeat cylinders 2–4

Optional: very light dielectric grease inside boot (not on plug threads/electrode).

8) Double-check lead routing before first start

NB owners frequently report rough running/no-start after ignition-lead mix-ups. Confirm each lead is fully seated at both coil and plug ends before cranking.

Community quote (NA/NB forum context): “No mention of a possible mis-wiring of the spark plugs at the coil pack… return the plug wires to their proper places.” (Miata.net NA/NB archive thread, retrieved 2026-02-26) https://forum.miata.net/vb/showthread.php?t=279746

Torque Specs / Capacities

  • NB spark plug torque (compiled NA/NB references): 11–17 ft-lb (15–22 Nm).

High-risk quote: “Spark Plugs 11-17 (132-204 inch/lbs) 15-22” (The Apex Drag NA/NB table, retrieved 2026-02-26) https://theapexdrag.com/mx-5-na-nb-torque-assembly-specs/

Cross-check note:

  • NGK’s generic 14 mm flat-seat aluminum-head range (18.0–21.6 lb-ft) overlaps the upper part of the NB compiled value and supports using a torque wrench and seat/thread awareness.

Verification / Post-service checks

  • Start engine and confirm steady idle.
  • Drive lightly and verify no hesitation/misfire.
  • If roughness appears immediately, re-check connector seating and firing order.

Uncertainty / Open Questions

  • Public access to some major Miata forums/manual mirrors was limited by anti-bot controls in this environment.
  • Exact plug part numbers and replacement intervals vary by market/year/emissions package; confirm with VIN-specific documentation.
  • Anti-seize remains debated in community threads; follow plug manufacturer guidance first, and adjust torque strategy only when explicitly instructed by manufacturer.

Images

No reusable images were added in this run. I could not retrieve clearly reusable/licensed NB-specific procedural images from accessible sources.

Image Credits

  • No images used.

Sources

  1. BOFI Racing — MX-5/Miata Workshop Manuals – Including all Torque Settings (links to 1999–2000 BP4W and 2001–2005 BPZ3 workshop PDFs). Retrieved 2026-02-26. https://bofiracing.com/blog/mx5-miata-workshop-manuals/
  2. The Apex Drag — MX-5 NA/NB Torque & Assembly Specs (compiled spark-plug torque range). Retrieved 2026-02-26. https://theapexdrag.com/mx-5-na-nb-torque-assembly-specs/
  3. NGK — Spark Plug Installation (torque importance, cool-engine caution, hole-cleaning caution, seat-type torque ranges). Retrieved 2026-02-26. https://ngksparkplugs.com/en/resources/spark-plug-installation
  4. Champion Auto Parts — Spark Plug Torque Recommendations (seat-type tightening method and turn-angle guidance). Retrieved 2026-02-26. https://www.championautoparts.com/Technical/Tech-Tips/Spark-Plug-Torque-Recommendations.html
  5. wikiHow — How to Change Spark Plugs (general handling sequence and one-cylinder-at-a-time workflow). Retrieved 2026-02-26. https://www.wikihow.com/Change-Spark-Plugs
  6. MX5Nutz — Spark plug thread (NB-owner community discussion, including thread-treatment/installation practice debate). Retrieved 2026-02-26. https://www.mx5nutz.com/threads/spark-plug-thread.390970/
  7. MX5Nutz — Reading spark plugs (NB troubleshooting context; interpreting plug condition with drivability/emissions symptoms). Retrieved 2026-02-26. https://www.mx5nutz.com/threads/reading-spark-plugs.391244/
  8. Miata.net (NA/NB Archive) — No-Start Troubleshooting Guide (community troubleshooting thread; includes spark-plug gap mention and ignition lead mis-wiring caution in replies). Retrieved 2026-02-26. https://forum.miata.net/vb/showthread.php?t=279746