Before You Start / Safety
- Work only on a fully cold engine.
- Collect coolant in a sealed container and dispose of it correctly.
- Do not open the cooling system when hot/pressurized.
Warning: “It won’t be a good day if you singe off your eyebrows … turn off your car and let it cool down before you open up the hood.” https://www.wikihow.com/Replace-a-Thermostat
Required Tools
- 10 mm socket/wrench (typical thermostat housing hardware)
- Ratchet + short extension
- Drain pan
- Gasket scraper/plastic razor + clean rags
- Funnel
- Torque wrench (low range)
Required Parts / Fluids
- Thermostat (NB1/NB2 compatible)
- New thermostat gasket / seal
- Correct coolant for NB (plus distilled water if using concentrate)
- Optional: fresh radiator cap if old cap is suspect
Model-specific notes (NB1 vs NB2)
- Procedure is fundamentally the same for NB1 (1998-2000) and NB2 (2001-2005).
- Cooling-system total capacity is commonly cited around a little over 6 qt for NB, but a thermostat service drains only part of the system.
Manual-reference quote: “1999-2001 Mazda Miata Service Repair Manual” and “2005 Mazda Miata Service Repair Manual” are listed in the Mazda Miata FSM archive. https://www.mellens.net/mazda/
Step-by-Step Procedure
1) Confirm the thermostat is actually the likely fault
Highway-overheat with A/C can also be airflow, fan, cap, or radiator efficiency issues. Do a quick system check before replacing parts blindly.
Community quote: “Before you replace the thermostat… Highway + A/C = overheating… Air blockage … Aux cooling fan was not kicking on.” https://forum.miata.net/vb/showthread.php?t=236876
2) Partially drain coolant (cold engine)
Drain enough coolant so the thermostat housing can be opened without major spill. Expect some residual coolant loss when opening the housing.
3) Remove upper hose and thermostat housing
Loosen/remove housing fasteners carefully; older hardware can seize. Avoid snapping bolts by using controlled force.
Community quote: “Also, be aware that taking the thermostat bolts out can be risky.” https://forum.miata.net/vb/showthread.php?t=515684
4) Remove old thermostat and inspect orientation
Note exactly how the old thermostat was positioned before removal.
High-risk quote: “Notice how it’s oriented, with a small hole at the top … That’s called the jiggle pin, and is used to help get air out of the system.” https://sites.google.com/site/redcapsmiata/home/tutorials/maintenance/mazda-miata-coolant-thermostat-replacement
5) Clean mating surfaces thoroughly
Remove all old gasket material and corrosion without gouging housing surfaces.
High-risk quote: “Be careful, scraping and damaging the metal surfaces can introduce gouges that allow coolant to leak out!” https://sites.google.com/site/redcapsmiata/home/tutorials/maintenance/mazda-miata-coolant-thermostat-replacement
6) Install new thermostat + gasket correctly
Install with correct orientation (jiggle pin at top when required by the thermostat design). Refit housing evenly.
Community confirmation quote: “Remember to keep the Jiggle Pin in the thermostat at the 12:00 position when you install it, so air can escape the engine block.” https://forum.miata.net/vb/showthread.php?t=236876
7) Refill and bleed cooling system
Refill radiator, run engine to operating temperature, and top up as air purges. Recheck when cold after first drive.
Community quote: “Run it until it comes up to temp, make sure you have ‘burped’ all the air out, and keep the reservoir level topped up for a few days” https://forum.miata.net/vb/showthread.php?t=515684
Torque Specs / Capacities
- One independent Miata guide reports thermostat housing fastener torque around 15-22 ft-lb; verify against your year-specific FSM before final tightening.
- NB system capacity is commonly discussed as a little over 6 qt total, but thermostat replacement typically requires only partial refill + bleeding.
Manual-reference quote: “Mazda manual states 15-22 ft-lbs for the nut and bolt to tighten it back together.” https://sites.google.com/site/redcapsmiata/home/tutorials/maintenance/mazda-miata-coolant-thermostat-replacement
Verification / Post-service checks
- No active seepage at housing seam, upper hose joint, or drain point.
- Cabin heat output is normal (basic flow check).
- Coolant level stabilizes after 1-3 heat cycles.
- No persistent coolant odor after residual spill cleanup.
Community confirmation quote: “Check the radiator level when cold a few times … Any leak will show up as a reduction in the levels in the expansion tank and/or coolant under the car.” https://forum.miata.net/vb/showthread.php?t=593901
Uncertainty / Open Questions
- In this environment, full direct extraction of NB-specific FSM thermostat pages/torque tables was limited.
- Community consensus strongly supports jiggle-pin orientation and careful bleeding, but coolant concentration advice varies by climate.
- Treat the torque value above as provisional until matched to your exact year/VIN factory manual.
Images
No reusable images were added in this run. Forum media and third-party how-to images reviewed did not provide clear reuse licensing suitable for this repository.
Image Credits
- No images used.
Sources
- Mellens (Mazda FSM archive) — Mazda Miata Factory Service Manuals. Retrieved 2026-02-26. https://www.mellens.net/mazda/
- wikiHow — How to Replace a Thermostat (with Pictures). Retrieved 2026-02-26. https://www.wikihow.com/Replace-a-Thermostat
- Redcap’s Miata Garage — Thermostat Replacement. Retrieved 2026-02-26. https://sites.google.com/site/redcapsmiata/home/tutorials/maintenance/mazda-miata-coolant-thermostat-replacement
- MX-5 Miata Forum — Thermostat replacement (NB discussion). Retrieved 2026-02-26. https://forum.miata.net/vb/showthread.php?t=236876
- MX-5 Miata Forum — Is NB coolant change so simple? Retrieved 2026-02-26. https://forum.miata.net/vb/showthread.php?t=515684
- MX-5 Miata Forum — Coolant odor after radiator replacement (NB discussion). Retrieved 2026-02-26. https://forum.miata.net/vb/showthread.php?t=593901