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Old 04-07-2014, 06:24 PM   #1
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Default forum newbie; crank snapped

crank shaft broke. I'm looking for useful ideas....



I've done everything myself to this motor so to some that might be the whole story... but here goes...

ATI Super damper, MP62 hot side kit with 6rib belt (BRP kit sold by FM about 5 years back), no AC/PS. ~16psi boost (biggest pulley compatible with ATI... don't recall diameter .. will add that info tonight). lightened steel flywheel and stage-2 FM clutch, 235rwhp. wiseco +1mm pistons, carillo a-rods and salvage crank from FM. prior crank woodruff channel wore (that was pre ATI super damper ~4 years ago)

This car is race prep and track use only. TTB @NASA-SE and instructor HPDEs account for about 10 weekends/year. it is pretty much either IGN OFF or WOT.

so, I've been getting some vibration in the SC belt last few events. new belt had same vibration. I assumed the tensioner spring was getting weak. so I [perhaps foolishly] added tension by displacing the SC on the bracket away from the crank pulley. When you're at the track you do what you can... ordering parts doesn't compute... Just 1 washer increased the tension enough to reduce the vibration such that the belt wouldn't hop off the pulley. But, its worth noting the spring loaded tensioner was still necessary.

The vibration turned out to be wobble in the crank pulley. The wobble was very noticeable this Saturday after just 2-3 pulls in the neighborhood (I was chasing a vacuum leak)


I yanked the crank bolt and the whole pulley assembly just fell off in my hands.

From the looks of the metal in the break it appears to be torsion stress (and not shear)...

Q1: could this have been caused by too much belt tension? (I think not but I'm being open minded here)


Perhaps another fact is there was an electrical issue recently which caused voltage droop on track (at heat, after a few laps). The droop was low enough to cause serious spark issues (perhaps ECU resets). The power was jerking on and off while I was nursing her back into the PIT. Lots of instantaneous load deltas...


Q2: anyone ever seen a crank break like this?
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Last edited by chrislwood; 04-08-2014 at 07:04 PM.
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Old 04-07-2014, 06:35 PM   #2
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I've seen a whole bunch of keyways get stripped out, but never seen a crank nose completely snap like that.
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Old 04-07-2014, 07:28 PM   #3
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That's pretty impressive.
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Old 04-08-2014, 01:29 AM   #4
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The SC pulley diameter is: 148mm

Here are shots of the bearings.



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Old 04-08-2014, 10:05 AM   #5
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Welcome, chrislwood. Check out the Miata Gallery and post up some pics!
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Old 04-08-2014, 12:48 PM   #6
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Quote:
Originally Posted by chrislwood View Post
The SC pulley diameter is: 148mm

Here are shots of the bearings.



Don't care, I want to see pictures of the broken crank.
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Old 04-08-2014, 07:03 PM   #7
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Hah... welp, I'll post the picture *again* (in part because you asked nicely). I was curious why it disappeared after 12 hours.



It *was* after the first sentence... carrying proper/intended effect..
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Old 04-08-2014, 07:22 PM   #8
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wow, that's a new one.
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Old 04-08-2014, 07:59 PM   #9
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I've heard of early 1.6's doing this -- after they wear the keyway, develop a substantial wobble and owner keeps running it.

Couldn't tell from your sig. Year?

It is impressive. You need a turbo.
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Old 04-08-2014, 08:00 PM   #10
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It's gotta be from too much belt tension on the blower. Scrap it and buy a snail!
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Old 04-08-2014, 08:31 PM   #11
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It is a 1995. The keyway is pristine (at least what I can see of it). I'll have the crank-tip pressed out to get a better look before sending the ATI off to get rebuilt.

I have considered moving to a turbo but cannot justify the expense. 'course I'm almost there with all the other mods. Personal preference.. I like the whine and I like the low end torque.

At least I'm happy to know this is not common.
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Old 04-08-2014, 08:50 PM   #12
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