My 1999 Crankshaft needs to be resurfaced says the machine shop. He says I have to use 0.25 bearings. The builder says that resurfacing the crank will remove the nitrate coating on the crank and eventually lead to crankshaft hairline cracks on boosted cars.

What is the consensus here? Is it bad idea to resurface the crank?

 

I'd buy a used one, getting it turned will mean smaller bearings with less area to spread out the load, slightly weaker crank that can flex more, and lik ehe said possible loss of surface treatments. If you're going to drive it hard/boost it, I'd get another crank. If it's a DD that you just drive like a normal person, probably doesn't matter.

 

A nitrided crank surface is a good thing for long term wear prevention but isn't a necessity. That being said, .25 is huge. Usually a crank is turned .010 or .020 to clean it up after a spun bearing. And that is really rare on Miatas because the crank surfaces are nitrided.

Anything over ~.030 is usually a throw away on the American stuff I'm familiar with. I don't know what Mazda's machinability limits are.

 

I'm pretty sure the machinist meant 0.25mm or 0.010in.
That's a pretty common grind. I just had mine done and am not worried about it at all if that's any comfort.

 

definitely meant mm, not a 1/4"

 

Often it's standard operating procedure to grind a crank, I'd get a 2nd opinion from a Miata specific expert to make sure the crank really is bad.

 

 

its cheaper to get a known good crank than pay the machineist to grind it then have it nitrided.

Side note: number 4 rod journal and the last main bearing section of the crank are induction hardened. very special care has to be taken when nitriding this section to take into account for this. iv seen some less favorable things happen in this situation