cranks

[DaveR]

Hi Folks

Just read the article on cranks on Miatanet
(http://www.miata.net/garage/crankshaft.html) and it sounds pretty scary
to someone looking for an early MX-5. Just how big a problem is it?
Maybe this is why the last one I looked at had had a replacement
engine? Should I start looking for a 92 or later model? What's the
local wisdom?

Thanks

Dave

[...]

[Michael & Kate Cooper]

Hi Dave

If you can find out when they improved the crank bolt then you will be able
to check the vin number to ensure you buy a car with the newer bolt. I
think it was mid 1991. I believe that the problem starts when the bolt gets
loose, so if it has stayed tight throughout its life then there shouldn't be
any wear on the key-way. As it gets loose and wears, the difference in
performance changes so slowly that it is hard to notice. With our car, it
got steadily worse over the course of a year, then we took the engine apart
and did the loc-tite fix. The improvement was instantly noticeable.

If you drive enough MX5s in your search for the perfect one then you should
be able to notice a lack of power in a certain car, and it could be due to
the crank bolt problem. If it feels good, then it's either got the new bolt
design, or it hasn't worn. If the bolt hasn't worn then I think that all
you have to do to keep it that way is check that it is tight, I guess every
few months.

Good luck

Kate

[ianbell]

Hi Dave

From info that I have, the early model short-nose cranks that could have a

problem had 4 slots in the pulley and the later, improved, long-nose cranks
had a pulley with 8 slots. Also the info indicates that cars below VIN
number 209446 may have the short-nose crank. I don't know if this info is
100% accurate because I have a 4-slot pulley on a car VIN no 306100. There
is also a bolt shoulder difference too - 1.128 inches and 1.455 inches for
the stronger crank. But until the pulley bolt is removed and the tip of the
crank can be seen, you can't really be sure what crank is in the car.
My MX5 has 145k on it and still goes well.
Ian
CHN433

[Dez]

This crank pulley/bolt "problem" isn't limited to one particular year. I
have a 94 & have found that if the previous maintenance provider doesn't put
the key into the keyway properly, AND a new bolt torqued correctly, the bolt
WILL come loose an allow the pullet & key to flog out the keyway.
Fortunately for me the loose bolt & pulley was noticed in time & a fix with
the loctite method with a new bolt cured the issue. Quite frankly, I
believe the "crank problem" is somewhat over-rated however, some issues have
been documented therefore some caution should be made. But if you find a
nice car, can't confirm it's maintenance pedigree regarding the bolt, it is
a relatively cheap fix for peace of mind. It is not a thing I personally
would lose any sleep over. Good luck with your search.