B series Miata throttle shaft failure

copyright 949 Racing 2006-2023

Failure mode
The B6 and BP (1.6/1.8) NA NB Miata engines can experience throttle shaft failures with sustained use at 7,000 rpm or above. The shaft has a countersunk  section for the plate screws. The sharp edge of that countersink creates a stress riser. A crack forms there and eventually the shaft fractures completely. When it does the screw head pops off and gets sucked into the engine. Sometimes you get lucky and the screw head skips happily through the combustion chamber out the exhaust and you never see it. Usually though it has a fight with one of your valves. The valve always loses.

How we fix it
The throttle plate is a brass alloy. It tends to absorb oil that seeps out and makes it difficult for adhesives to do their job. It’s critical to thoroughly clean the plate in a solvent and rinse with isopropyl alcohol to make sure it is immaculately clean before you apply the epoxy. Just a smooth lump over the screw heads and fanning out a bit onto the throttle plate. Use as little as possible. The idea is to catch the screw heads if they snap off or the shaft snaps. We went to the 3M website and looked for a two part epoxy that would be vibration resistant and bond to metal. Ended up using 3M DP420 and it’s worked well. There might be other epoxies better suited to the task.

Other options
In practice, the epoxy seems to add enough stiffness that the plate and screws don’t ever snap. Another option is the Skunk2 throttle body. The Skunk2 has its own problems and is overall less reliable than the OEM throttle body out of the box. The design is flawed, requiring modification to the throttle stop, return springs and loctiting every fastener. But it does have an unbreakable throttle shaft. We do not offer instructions, guidelines or pictures on the Skunk2 mods, that’s a DIY thing for an experienced mechanic. Of the two options we prefer the OEM throttle body with epoxied plate over the modified Skunk2. Cheaper, easier and it works.

Mass dampers
Some of you might recall that the JDM NB’s had a 7,500rpm redline. Mazda addressed the harmonic vibration by adding a 339 gram mass damper to the throttle body. Those dampers tend to come apart with lots of track use and are discontinued by Mazda.