How to bulletproof an ND Miata transmission

Name an iconic sports car with a slick-shifting, bulletproof manual transmission. The answer is always Miata, right? Well, except for the latest generation.

A key ingredient to Mazda’s recipe for classic roadster performance has been lightness. And after the NC generation bulked up a bit, the ND design team was tasked with a so-called gram strategy to find weight savings bit by bit. As heralded in these pages, they succeeded in producing a fantastic new car in 2016 that harkened back to the lithe and light NA and NB generations.

The ND Miata was an immediate hit with enthusiasts, especially those who participated in motorsports. In addition to wide adoption for autocross, that year also saw the inaugural races in the Global MX-5 Cup road racing series for mostly stock versions of these cars.

Almost immediately, problems surfaced with the six-speed transmissions. Autocrossers were breaking second gear while road racers were stripping third and fourth. For the MX-5 Cup road race cars, a relatively quick fix involved a hardened gear set by EMCO. For everyone else, Mazda issued a TSB covering transmission replacement within the warranty period, with a policy for coverage even for cars modified up to Street Touring specs for autocross.

 Behind the scenes, Mazda quickly set about beefing up the more fragile components of the gearbox, revising the full build combination four times. These final V5 boxes slowed down the breakage considerably, but failures still persisted, especially when they were operated “in a spirited manner.”

Our original ND1-chassis Triple Threat Miata project car stripped fourth gear at the track at the 10K-mile mark, and Mazda replaced it under warranty with a V5 version. That one lasted through the time we traded the car on the updated model a few years later.

With the introduction of the 180-horsepower, ND2 version of the MX-5 in 2019, further driveline tweaks were added. Most impactful were a dual-mass flywheel, which softened the shift points mechanically, and a revised engine tune, which did so digitally. Warranty claims for ND2 transmissions plummeted to normal levels. Success! Or so it seemed.

Fast-forward to today, and ND1 owners are still at risk, especially when cars are modified to higher power levels. And since these cars are out of warranty, the only solution is replacement at owner cost, which is $2300 from a dealer just for the part. 

The MX-5 Cup series, though, moved on to a full-race SADEV sequential gearbox, with new builds yielding fresh OE V5 transmission takeouts at a reduced cost–$1500 plus $350 shipping–through Flis Performance. Alternatively, documented competitors can order replacement transmissions through the Mazda Motorsports support program at a cost of $1350 plus $350 shipping.

[Track test and review: ND-chassis Mazda MX-5 Cup Car]

While rare, occasional ND2 failures do still turn up. Case in point: Online forums are full of reports of a recent spate of synchro failures repaired under warranty.

So, what about our current Triple Threat Miata, the test mule for many of our tire tests? For six years, its gearbox seemed to be indestructible despite the heavy track use. But we finally got bit again during a recent outing. There was nothing unusual about the session until third gear suddenly left the chat. Fortunately, all of the other gears still worked, and we safely drove the car home, skipping around the redundant neutral position.

What to do? Take our chances on a fresh one from either Flis or Mazda Motorsports? What about those other recent failure reports for new transmissions? Would lightning strike twice? 

Enter Walter Motorsports.

After draining the old fluid, we found a chunk of transmission attached to the magnetic drain plug–an ominous sign.

Next stop: Walter Motorsports, a shop that offers track-ready transmissions for Miatas.

What went wrong with our transmission? Removal, disassembly and analysis of our broken transmission didn’t take long–maybe an hour. 

Lee Walter has spent the last decade developing a reputation as the preeminent rebuilder of Miata transmissions for motorsports use. His boxes are found in many a Spec Miata, including those competing at the highest levels, plus higher-horsepower Miatas for other applications. Three stages of durability are offered for all NA, NB and NC Miata generations, with the strongest supporting engine outputs exceeding 300 horsepower.

Recently, Lee took up the ND cause, analyzing the shortcomings of the stock box and developing a comprehensive strategy for a bulletproof product that would withstand motorsports use even at higher output levels. Given the fragility of the ND stock components, Walter Motorsports only offers the Stage 3 setup. It features a replacement gear set made from a stronger alloy, which is then cryo treated and shot peened for additional strength. Higher-quality bearings are used, while the case itself is modified for stronger, larger hardware to reduce case flex. Ratios are the same as OE, so there’s no direct performance benefit other than durability.

Given how much we use our Triple Threat Miata, the Walter Motorsports rebuild seemed to be the best option. While a little more than twice the cost up front of either of the two stock-box sources, it avoids future interruptions in our tire testing program or inconvenient failures while out of town. As they say, buy once, cry once, right? 

Making the decision even easier was Walter Motorsports’ new location in Fort Worth, Texas, not too far from Austin’s Harris Hill Raceway, site of much of our tire testing. 

The root cause of our failure was this thrust washer that positions third gear. Additionally, a piece of it jammed against this oil guide, breaking off the mounting tab in the process. That thrust washer is one of the components replaced with a stronger unit in the Stage 3 rebuild, so this failure is unlikely to reoccur.

The rest of our transmission guts were then disassembled for cleaning and rebuilding. 

Several years back, Lee began to have some health issues, causing his production to stagnate. Greg Clifton, a longtime amateur racer and successful business owner, partnered with Lee to take over the company operations, retaining the founder as a technical consultant. Part of that arrangement involved moving the bulk of the business from Atlanta to Fort Worth.

Due to high demand, typical turnaround time for a rebuild is four weeks. Once all of the parts are on hand, though, the actual build only takes a few days. Walter Motorsports does keep viable cores around, so you don’t have to send in your broken box for work to begin. Simply return it for credit once the new box goes in.

Naturally, our breakage occurred at the worst possible time–just before several track days. Not only did Greg work with us to juggle the build order, but he also offered to install it so we could more easily make our trip deadline. 

One of Greg’s other businesses, Tejas Restomods, shares space with the transmission rebuild shop. While its primary work is upgrading typical American muscle cars, its most experienced tech has a background in racing Mazdas, especially Miatas. Team MER was once a dominant force in Spec Miata and Pro Spec Miata, and Greg has partnered with the team’s former owner, Darin George. We’d have one of the best Mazda techs in the country doing the work.

With everything lined up, we towed our ailing MX-5 3 hours north to meet Greg and the guys at the shop. The entire swap took less than 3 hours, with a shop tour and plenty of photography along the way. As racers often do, we swapped stories while the work progressed. Once it was complete, a quick test drive around the local roads assured that all of the gears in the rebuilt box worked. Then it was back on the trailer and home for dinner. 

Then the new transmission could be placed in the car. 

Over the next several days, we performed the break-in process, which consisted of street driving with deliberate shifts for a total of 500 miles. We even went to the track, where we focused on using third and fourth gear at moderate loads for an hour. 

The final step was a fluid change with the specified Motul Competition 75W-140 gear oil. Note that this is substantially higher viscosity than the Mazda factory fill but is the only fluid Walter recommends. In addition to the viscosity increase, there are specific protective additives unique to this oil. It does have the side effect of notchy shifting when cold, but once warmed it feels quite normal.

With break-in completed, we headed back to the track for a real test. Within a few circuits, lap times were right back where we left them, and we soon forgot about the whole episode. 

That is, until the bill came. It’s hard to put a price on peace of mind. Though our wallet is lighter, we are now confident that our transmission is the last thing we need worry about. Thanks to Walter Motorsports and Tejas Restomods, Miata is indeed always the answer.

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