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Underbody inspection for rust (especially sills & arches)

Inspect MX-5 NB underbody rust-prone zones with emphasis on sill/arch areas and convertible-drain related moisture paths.

Difficulty
Intermediate
★★☆☆☆
Est. Time
45-120 min
Models
NB1 & NB2
Last Updated
2026-02-27

Before You Start / Safety

  • Inspect on a stable, level surface with the car securely supported.
  • Wear eye protection; rust scale and dirt can fall when checking seams.
  • Do not probe aggressively into structural seams with sharp tools.

Warning: “Do not operate the convertible top while moving.” https://www.youcanic.com/convertible-maintenance-tips/

Required Tools

  • Bright inspection light
  • Plastic trim tool or non-sharp probe
  • Small inspection mirror/borescope (optional)
  • Gloves and eye protection
  • Camera/phone for condition logging

Required Parts / Fluids

Inspection-only:

  • No parts required

If issues are found:

  • Rust treatment/repair materials as appropriate
  • Drain cleaning tool (for moisture-path checks)

Model-specific notes (NB1 vs NB2)

  • NB1/NB2 commonly corrode around sill/arch regions when moisture is retained.
  • Convertible drain condition and rain-rail path health are directly relevant to repeat dampness and rust progression near lower body areas.

Manual-reference quote: “Welcome to Mazda Miata Factory Service Manuals” https://www.mellens.net/mazda/

Community quote: “NB rust on sill - from inside or surface rust?” https://www.reddit.com/r/Miata/search/?q=NB+rust+sills&restrict_sr=1

Step-by-Step Procedure

1) Start with moisture-path context

Before judging rust severity, verify whether drains are flowing correctly. Chronic water retention can mask root cause if you only treat visible corrosion.

High-risk quote: “Hood drains can be a bit fragile, the rubber tube may crack/puncture.” https://www.pistonheads.com/gassing/topic.asp?t=798737

2) Inspect outer sill and wheel-arch seams

Use strong side-lighting to identify:

  • blistering paint,
  • seam swelling,
  • bubbling at arch lips,
  • prior filler/repair transitions.

Photograph each finding for comparison over time.

3) Check underside sill seam lines and jack-area vicinity

Look for flaky scale, seam split lines, and rust expansion around overlap joints. Distinguish light surface oxidation from layered delamination.

4) Check drain exits and nearby underbody zones

If drain outlets are slow or blocked, note nearby damp dirt trails or corrosion concentration.

High-risk quote: “Hood drains can be blocked and hold water, causing damage to the interior and corrosion.” https://help.flyinmiata.com/en_us/convertible-top-drain-maintenance-SJNTFQxfo

5) Probe gently, not destructively

Use a non-sharp probe to check whether rust is superficial or if metal thickness is compromised. Stop if seams feel weak and escalate to body-repair assessment.

6) Classify and plan action

  • Monitor: light surface rust only, no seam swelling/perforation.
  • Treat soon: scaling rust, seam expansion, repeat dampness.
  • Escalate repair: perforation, soft seams, recurring interior water ingress.

Community confirmation quote: “I think I probably dislodged/pushed it all apart when doing a monthly ‘check my drain tubes are clear!’” https://www.mx5nutz.com/threads/guide-drain-tube-repair-and-bonus-drainage-info.250370/

Torque Specs / Capacities

  • Not applicable for visual inspection only.
  • If panels/fasteners are removed for access, use year/VIN-specific FSM values.

Verification / Post-service checks

  • Re-run drain flow check after inspection.
  • Re-check photographed rust spots every 2-3 months (or after winter/salt season).
  • Confirm no new interior dampness behind seats or in sill-adjacent carpet zones.

Uncertainty / Open Questions

  • Publicly accessible sources in this run strongly support drain-health correlation with moisture/rust progression and careful non-destructive inspection.
  • A single fully extractable NB-only Mazda rust inspection map (exact measurement thresholds by panel area) was not available in this environment, so this guide emphasizes conservative condition triage and repeat documentation.

Images

No reusable, clearly licensed NB-specific rust-inspection images were added in this run.

Image Credits

  • No images used.

Sources

  1. Mellens.net — Mazda Miata Factory Service Manuals. Retrieved 2026-02-27. https://www.mellens.net/mazda/
  2. Flyin’ Miata — Convertible Top Drain Maintenance. Retrieved 2026-02-27. https://help.flyinmiata.com/en_us/convertible-top-drain-maintenance-SJNTFQxfo
  3. MX5Parts — MX5 Roof Drains: How to Clean & Maintain. Retrieved 2026-02-27. https://www.mx5parts.co.uk/mx5-roof-drains-how-to-clean-maintain
  4. PistonHeads — Best way to clear the drain holes?? - Mazda MX5/Roadster/Miata. Retrieved 2026-02-27. https://www.pistonheads.com/gassing/topic.asp?t=798737
  5. MX5Nutz Forum — Guide - Drain Tube Repair And Bonus Drainage Info. Retrieved 2026-02-27. https://www.mx5nutz.com/threads/guide-drain-tube-repair-and-bonus-drainage-info.250370/
  6. Reddit r/Miata — Search results: NB rust sills. Retrieved 2026-02-27. https://www.reddit.com/r/Miata/search/?q=NB+rust+sills&restrict_sr=1
  7. YOUCANIC — Convertible Top Maintenance Tips. Retrieved 2026-02-27. https://www.youcanic.com/convertible-maintenance-tips/