For those of you keeping score at home, I picked up my new-to-me 2010 NC2 Miata about six weeks ago. Since I bought it from a dealership, the past maintenance records are about as easy to find as a unicorn in the while—aka, they're MIA. The car has only 29,000 miles on the clock and looks like it’s been well cared for. The one thing I do know? When the last oil change was done. Everything else? A complete mystery. Time to roll up my sleeves and make sure nothing important is running on pure optimism.
This is also the perfect chance to get the NC in the air and see what’s lurking underneath—hopefully nothing too exciting.
Sizing Up the NC:
It goes without saying that a 91.7-inch wheelbase isn’t exactly limousine-length. While that makes parking a breeze, it also meant my QuickJack 5000TL was working with just enough room to fit—tight, but not impossible.
The car reportedly has never been in an accident and spent most of its life in a state that doesn’t feast on cars with the dreaded rust worm. A quick look underneath backs up the claim—no signs of an accident, just a tiny bit of light surface rust on the subframes, and none at all on the chassis itself. The frame rails are straight as an arrow, most of the bolts look like they’ve never been touched, and there’s not a single leak in sight. All in all, this Miata looks like it spent most of its life tucked away in a pampered garage. Perfect—disaster averted!
Upon closer inspection, I found only two modifications made by previous owners. The first, which I knew about when I bought the car, is a fairly tasteful weighted, queue-ball-style shift knob. It’s a nice piece, but I must be one of those rare NC owners who actually enjoys the OEM knob—so I swapped it back to the original within days of bringing the car home.
Upon opening the hood, I discovered the second modification made by a previous owner:
Dear previous owner: whoever you are, you sir, are a gentleman and a scholar! I don’t know who you are, but this is one of the few modifications I’m genuinely happy to see on the car: a powder-coated black Moroso Aluminum Coolant Expansion Tank! For those who don’t know, NC Miatas are notorious for having a plastic coolant expansion tank that tends to crack over time. Given the age of the car, this was one modification I was planning to tackle sooner rather than later, as they're known to be ticking time bombs—and this guy saved me the trouble! If I knew him, I’d buy him a beer for his efforts!
As a bonus, the previous owner seems to have labeled the coolant expansion tank with a note that says "5 year 8/21." Now, I’m no genius, but I’m guessing the label indicates when the coolant was last changed (August 2021) and the recommended interval before it should be changed again. According to the factory manual, coolant should be changed initially at 10 years (regardless of mileage), and then every 5 years after that. Oh, I really like this guy.
Looking over the rest of the car, I didn’t see any indication that the stock intake, exhaust components, or suspension had ever been touched. This makes for the perfect canvas to work with, where the only signs of modification are a tasteful shift knob and a high-quality Moroso expansion tank—addressing the NC’s biggest Achilles' heel from a reliability standpoint. So far, things are definitely on the right track!
Building Spark in Our New Relationship
With the car inspected, it was time to quit procrastinating and get to work. First order of business: have the spark plugs ever been touched?
I know what you're thinking—the car only has 29,000 miles on it, so why bother changing the spark plugs? Fair point. But it's also 15 years old. While the plugs are probably fine, some sources claim they can wear out not just from mileage but from age as well. Mazda’s maintenance schedule doesn’t mention a time-based replacement, only recommending new plugs at 60,000 to 75,000 miles, depending on driving habits. But in my mind, they’re relatively inexpensive and easy to swap—translation: cheap insurance.
So, I ordered a set of OEM NGK Iridium spark plugs from the fine folks at Flyin’ Miata.
Now, here’s where things got interesting. The recommended stock gap is 0.050-0.053”—which made me do a double-take just to make sure I wasn’t losing my mind. That’s a wide gap.
Most spark plug manufacturers do a solid job of pre-gapping OE plugs, but I always double-check with feeler gauges—gently, to avoid damaging the less-than-burly electrode. The only problem? I didn’t have a single feeler gauge that wide. So, with a bit of math and some creative stacking, I mashed a couple together to match the spec. Sure enough, they were all perfectly gapped from the factory.
Installing plugs on the NC is a simple, straightforward affair—no convoluted parts removal or fancy swivel sockets required. Yeah, I’m looking at you, BMW.
First step? Remove the OEM plastic engine cover. It pops off in seconds—just lift it up and pull it off. Easy.
Depending on your trim level, there’s a good chance your NC came from the factory with a strut tower brace—like mine did. Unfortunately, that brace conveniently blocks access to the rearmost spark plug. But here’s where Mazda’s engineers earned their gold star: they actually designed the OEM strut tower brace with a removable center section just to make this job easier. What a legend!
All it takes is removing two 14mm nuts, and the center section lifts right out with ease. Simple, effective, and a rare moment of automotive engineering that actually makes life easier.
Next up, each coil pack is secured with an 8mm bolt. Remove the bolt, and the coil pack pops right out—easy as that.
For removal, I like to use a long, magnetic, thin-walled 5/8" spark plug socket. I prefer the gentle pull of a magnet over wrestling with an internal rubber grommet, and the thin-walled design helps with tight clearances—even if that’s not an issue on the NC.
While these plugs don’t require anti-seize, I always thread them in carefully by hand to avoid the nightmare scenario of cross-threading—because turning a simple install into a day-long, heart-attack-inducing ordeal is not on my to-do list. Once the plugs are seated, it’s time to break out the torque wrench. According to my internet sleuthing, they should be torqued to 11-17 ft-lbs.
You'll have to excuse the blinding light from my LED wand. It has magnetic attachments that I usually use to hang it from the hood—except, you know, minor issue… aluminum and magnets don’t exactly get along. Swapping the Air Filter With the new spark plugs installed, I figured I’d swap in a fresh air filter while I was still poking around the engine bay. This is about as simple as it gets—just pop off the two clips on the airbox, and it opens right up. Easy peasy.
Upon removal, it looks like the previous owner actually ponied up for a genuine OEM air filter—and to my surprise, it was in pretty decent shape. This suggests he was likely on top of maintenance, which only makes me like him more.
Rather than shelling out big bucks for the OEM filter, I decided to pinch my pennies and go with a Denso air filter for a fraction of the price. Partly because Denso is an OEM supplier for most Japanese manufacturers, and partly because I don’t plan on keeping the stock intake forever—so spending extra on an OEM filter felt like a waste.
Fresh Drivetrain Fluids
It was time to crawl under the car and start getting dirty. Mazda recommends changing both the manual transmission and differential fluids based on either time or mileage, depending on your driving habits:
Schedule 1: Change both every 48 months or 60,000 miles. Schedule 2: Change both every 24 months or 30,000 miles.
Either way, they’re both easy to access and replace, with essentially identical processes.
Starting with the transmission, both the drain and fill bolts are 24mm and located on the left side of the car/transmission (driver’s side in the USA).
Pro tip: ALWAYS make sure you can remove the fill (upper) bolt before attempting to break loose the drain bolt. The last thing you want is to drain all the fluid, only to realize you can't refill it. That’s a bad day.
When you remove the lower drain bolt, be prepared for the fluid to shoot out like a fountain at the Bellagio. I didn't quite anticipate the fury and pressure that would come gushing from the drain hole, and as a result, it shot straight past my drain pan and onto the hoodie I was wearing. If you’ve ever smelled old gear oil, you know it’s a scent that lingers. I spent the rest of the morning walking around smelling like I’d taken a stroll through the Bog of Eternal Stench. While the bolt head sizes for the transmission plugs are the same (24mm), the bolts themselves are not. If you take a closer look, you’ll notice that the lower drain plug (pictured on the left, below) has a magnetic element. This magnet helps catch tiny metal particles that naturally occur due to friction in the transmission—kind of like a little guard dog keeping the gears safe from excessive wear.
Before reinstalling the transmission plugs, make sure to replace the crush washers. These are designed to be one-use items, so reusing them could lead to leaks. It's a small step that saves you a big headache down the road.
After reinstalling the lower drain plug, I used a fluid extractor to fill the transmission with 2.2 quarts of Ford Motorcraft XT-M5-QS full synthetic manual transmission fluid. You’ll know when to stop adding fluid when it starts weeping out of the upper fill hole like a toddler who just had his favorite toy stolen by a raccoon. Once that’s done, it’s time to bring out the torque wrench again and tighten the transmission drain and fill bolts: torque the drain plug to 29-42 ft/lbs and the fill plug to 19-28 ft/lbs. The process for the differential is essentially identical—two bolts: an upper filler and a lower drain. The difference this time? The bolt sizes. The upper bolt is 23mm, while the lower bolt is 24mm. Same idea, just a slight variation in size.
And just like the transmission, the lower bolt has a magnetic element to catch metallic particles. They also both have crush washers, just like the transmission.
To refill the differential, you’ll need just under a quart of GL-5 75w90 fluid—specifically, about 0.7 quarts. I went with Redline 75w90 gear oil. Once you’ve topped it off, torque both bolts to 29-40 ft-lbs for a secure seal. Overall, this was a pretty quick and easy morning’s worth of work to make sure the NC is up to par on maintenance. Definitely a satisfying way to spend a few hours keeping things running smoothly.
It should be noted that there are a few other items I’d recommend checking if you're buying an NC with no maintenance history:
Engine oil is an obvious one. I didn’t bother changing it since the oil was replaced just before I took delivery and still looks brand new. No need to fret—I'll make sure it gets fresh, high-quality, high-temperature resistant oil before any track work happens.
Coolant is another important one. As mentioned earlier, Mazda recommends changing it at 10 years/100,000 to 120,000 miles initially, and then every 5 years/60,000 miles after that. Since the previous owner thoughtfully labeled the new Moroso coolant expansion tank, I believe I’ve got at least another year before it needs to be swapped again, which matches the current condition of the coolant. So, for now, I’ll leave it as is.
It’s also a good idea to check the drive belt and consider replacing it. While Mazda doesn’t specify a replacement interval, for a car this age, it’s something worth looking into. The belt still seems serviceable, but I might swap it out in the near future when I have an excuse to start removing things. With the NC seemingly running well and all caught up on maintenance, I suppose it’s time to stop talking about performance mods and actually get to the business of making the car faster. Let’s see if I can back up all that talk with some real results!