Feel free to delete my post in general miata discussion.

So, maybe someone can shed some light on this situation for me:

I have two cranks, one from a previously turbo '96 and another from an n/a...believe it was 96 also.

They're sitting at the machine shop, and I plan on grinding the journals -.25mm to run my King XP's.

Turbo crank was getting ground down because rod journal #1 was .001 out of round. However, UV showed a ton of spiderweb cracking on rod journal 4 when dyed at my machine shop. Builder wouldn't send it out to get ground down.

So, I bought the second crank, looks beautiful...unmachined ever and has standard journals. That one ALSO is showing the start of this issue. You guessed it, almost in the identical spot on rod journal #4.

Is this a common issue on Miata cranks? My shop is pretty dumbfounded to be 2 for 2 on this same issue. I can't imagine most guys ever even test this. They just toss rods in and go for broke.

Thanks.

 

I guess the first thing i would consider is how many miatas with broken cranks or spun rod bearings you have seen? i cant say that i have seen any really. I did spin a bearing on a stock 1.8 many years ago but it wasnt on #4 and was due to other problems/abuse.

If it were me, i would toss in the new one you have and rock on. However i am cheap and dumb, so anything i say should be triple checked with people that have an IQ higher than a gopher.

 

I've never heard of the dye testing thing in the miata world and I've also never heard of anyone breaking a crank. Ever. I can't think of the last time I heard about a spun bearing either. I'm sure it's possible, but it's not something miatas commonly have a problem with.

 

I'm gonna agree with the above sentiment. its way too easy to get neurotic with this kind of stuff.

 

I know of 2 broken cranks. 1 at 350ish and 1 at 400ish hp

Otherwise they don't break often. And I don't see many people with cracks in theirs so not sure about that.

 

It's difficult to say, as the vast majority of us don't subject our crankshafts to anything resembling comprehensive inspection. We just mic 'em, determine whether they need to be turned down, and toss 'em back in the engine.

As 18PSI noted, crankshaft failures in the BP engines is extremely uncommon. At reasonable power levels (say, less than required to destroy the transmission within a few hours), they tend to last essentially forever. You have to get up into fairly high multiples of stock power before they start breaking, and even then it's one of the rarest failure modes of the engine. If I had to guess, I'd wager that the majority of us are running around with cranks that look like the ones you pictures, and will never know it.

 

I wouldn't worry. We would probably see this if we actually bothered to Magnaflux our cranks. We don't. And they don't fail.

We also don't turn the rod journals down because the crank surfaces are nitrided for durability and turning them removes the surface coating.

 

I've heard the coating is safe unless you grind .5mm or so. At a .25mm, which is a common undersize, I'm not concerned.

Thanks for all the feedback guys, we are gonna get crank #2 ground and use it.

 

the cranks aren't nitrided from the factory. However, they are induction hardened on number 4 rod and 5 main.