Are miata cranks factory hardened? If yes how do we know. I have seen some say yes, some say no. Others say it is induction hardened yet a few say it’s only induction hardened in a couple places. Still others on the internet state it is both induction hardened and nitrites from the factory. Anyone have any evidence or literature on this?

My crank needs to be turned .010 on the mains and .020 on the rods. Only one rod journal was questionable if .010 would clean it so they reccomended just going to .020 so that we can assure we get the bearing clearance I want. The machine at said he didn’t see anything that led him to believe it was nitrided. .030 is the standard accepted service limit so I’m not worried about .010/.020. And even better the bigger cut is on the rods instead of the mains.

From my understanding the nitride usually penetrates to .030 or so so if the cra k was factory nitrided it should still be “hardened” and if it wasn’t then it’s a non issue.

Power goal is 300hp. At this level the machine at said with a forged crank nitriding isn’t needed.

Any facts, experience or opinions would be welcomed. $150 to cut/polish and another $20 to jetwash it...cheaper than buying another used crank and way cheaper than new.

Thanks y’all.

 

I paid $300 for a polish, balance, and nitride hardening. The shop that came recommended to me was Marine Crankshaft in Santa Ana, CA. Service was top notch. I'm sure if you give them a call and ask they'll give you a straight up answer.

 

stock crank is forged then induction hardened on the 4th rod journal and 5th main journal. After a basic grind.10/10 you do not need to nitride the crank. although, i will say i have seen a spun bearing on a nitrided miata crank and the crank won.

 

Ok, thank you for the info. I appreciate it. I am cutting the crank 10 on the mains and 20 on the rods. The guy before didn’t treat the engine well. Do you think it will be fine without additional gardening like this?