Is this the big nose crank?

[mathia]

So I've been reading about the failures of the small nose crankshaft that supposedly came in the '91 Escort GT as well as the '90-'91 B6 Miata and the '94 BP Miata. This link covers the details of the problem: http://miata.net/garage/crankshaft.html

There are some good pics of failed crankshafts here: http://www.miataclub.org/orlando/94crank/

I have a '91 Escort GT and I'm in the middle of putting a GTX turbo on it so this wasn't something I was happy to hear about. However, the miata article has this pic of the crank bolt for the small nose crank:


And this is the bolt on the miata big nose crank:


You can see the big nose crank bolt has a much larger shoulder. My bolt looks like the big nose bolt:



Can anyone familiar with this confirm that I have the large nose crank? I don't have calipers to measure it, but the size of the shoulder on the bolt looks like the large nose crank bolt and just using a ruler it looks like it's about the 1.455" the larger bolt is (the smaller bolt is 1.128"). I 'll be taking the timing belt pulley off next and then I should be able to measure the crank, but I might not be able to do that till next weekend so I was hoping someone could tell by the bolt. It certainly looks like the big nose crank bolt, but those pics are from the miata. Thanks!

[FE3-323]

its a big nose.... there are only 2 types of BP DOHC cranks, regardless of what you may hear. the only other exception is a GTR crank possibly being of a different lighter material.

you don't have to worry about problems with the BP & crank failures. (Unless you horribly mess up reinstallation)
Simply put, mazda redesigned all cranks B6/BP with a small nose, to better design using a better & or longer keyway.

The miata people have sparked fear into BP people with that retarded b6 crank failure posting. its a common misconception to think that BP's actually have an issue like the early b6's did.

crank/woodruff-key/keyway failures are due to improper reinstallation or TQ'ing & or the lack there of sealant/threadlocker to prevent backouts.

[Lex]

My GTX had the long nose crank. The crank out of a 94 tracer I rebuilt had the big nose. They were different (big versus small bolt) but none were the small nose which has a much shorter snout.

[FE3-323]

seriously lex, if you can take pictures to proove theres 3 cranks i'll listen

small nose/small bolt- same thing

[ryan1]

i have a few 91 escort gt's none have had any problems with the crank, but my sister bought a pro lx and it was a 90 or 91 with a auto and developed crank walk, lex pics would be great.

[FE3-323]

crank walk is due to thrust washers, it has nothing to do with the snout itself. im sure you know that tho
of the 6 BP engines i've dissasembled i've only seen 2 types of cranks excluding the GTR crank

[therieldeal]

i have a GTR crank

and i took pics for you, so technically youve seen that too.

[Lex]

I have only seen the long nose and big nose cranks, I have not see the dreaded small nose. My GTX had the long nose (full keyway entry with small bolt) and my new motor has the known big nose with large bolt.

[FE3-323]

your confusion lex, must be with the b6 & the stupid miata nonsense. in BP form there really isn't a small nose which had a partial key way (as id like to call it) barely touching the crank in comparison to a big nose.
BP's truely only have small bolt & large bolt (to simplfy things) there is only 1 change to a BP bottom end in 99' miatas that had been of error/concern/problematic. i never really investigated it throughly i just know to stay away from them. (memory is fuzzy as to truely why i think it was crank walk or main caps?????)

Drastic stress factors for those truely worried about keyway problems arrises from overloading the crankpulley system beyond its true manufactured use. or an improper install. Given these are more crucial to the early design simply because the pulley is directly attached to the timing cog, not just a small portion of the keyway like a big nose.

a supercharger COULD posibly hold a significant negative effect upon the force loading the keyway.
As could overtightening a belt & running for prolonged use. or a malfunction forcing any which belt to sieze & snap ( i.e. something drastic goes wrong)

sorry to seem agressive in the matter its
just my 2 cents

Jon, i think you forgot that i bought a GTR motor & stupidly parted it out

[mathia]

Thanks for all the responses guys. So correct me if I'm wrong, but from what I gather there are three different crankshafts used between the 1.6L DOHC B6 and the 1.8L DOHC BP. There's the small diameter short length snout crankshaft which all the '90-'91 Miata guys complain about the keyway in crank getting screwed up on in their B6 motors. Then there's the small diameter but longer length snout crankshaft which is supposed to fix the issue as the longer keyway completely covers the woodruff key making a more solid connection. Finally there is the larger diameter or big nose crank which is what most BP motors have and it does not have the keyway issue.

bpt323, you're saying that the BP motors that have the smaller diameter crank only got the longer one and so there's no need to worry even if my BP has a small diameter crank.

Looking at the size of the shoulder on the bolt I have it seems to me that based on the pics of the Miata B6 bolts, I must have the big nose crank. Or am I wrong?

Does any one know the diameter of the small versus big nose cranks? I'm gonna pull of the timing belt pulley tomorrow. Regardless of wether or not I need to worry about the keyway in the crankshaft snout, I need to know which crankshaft I have so I can get the right oil pump. Thanks.

[Lex]

There is a thread on here listing the shoulder diameters.

[therieldeal]

based on the bolt shoulder size you can tell which crank you have

[mathia]

OK, I found the thread where the diameters of the crankshafts are listed:
http://www.wihandyman.com/forum/show...nose+bolt+size


Small nose crankshaft: 22mm diameter, short keyway
Long nose crankshaft: 22mm diameter, long keyway
Big nose crankshaft: 27mm diameter

That thread also lists the part number for the 22mm diameter shafts front seal as 223012.

Sorry I missed that thread in my initial search. Only the miata article lists actual sizes of the bolts. I'm just not sure if those sizes apply to the BP motor as that article refers to the B6. I had hoped someone would see the pics of my bolt and be able to say oh yeah, that's the larger bolt that's in the big nose crankshaft.

Hopefully I'll get the timing belt pulley off today and just measure the damn crankshaft and I'll report back so others will know what to expect when they see the bolts in my pics.

[naprotejay]

Here's how I look at it/think of it (using the common miata.net terms):

All Mazda B motors (BP, BPT, B6, B5) of the early 90's came with a 'short-nose' crank up untill ~1991.5

In ~1991.5, they changed the design to what's called the 'big-nose'.

The 'long-nose' NEVER came in a factory car; instead it was a factory replacement/fix for a failed 'short-nose'.

My 'short-nose' BP crank went 225K miles and several timing belt changes without a problem. (should post a pic of it)

The B6 'short-nose' cranks as found in the Miata seem to have a much higher (or much more advertised) than normal failure rate compared to the FWD 'short-nose' cranks.............. why? well just a couple of observations that may or may not have something to do with it:

- the BP DOHC 'short-nose' crank (in the Protege at least) is forged and the B6 Miata is cast.
- the FWD Protege BP's came with a MBSP, the B6 Miata did not.

You have a 'big-nose' crank. The bolt, the components on the front of the crank, and the 'B6S8' stamping on the front of your oil pump all tell me that.

[Lex]

I have to say that my GTX motor from Japan came with that miata.net refers to as the long nose.