Crank Pulley Thrown...Again!!

Well I was at the track Sunday trying to adjust a few things on the car when I threw the crank pulley at redline. It sheared off the 4 heads on the bolts that hold it on. Any ideas on how to keep it in place? This is not the first time this has happend either. This problem has previously surfaced 3 other times. All bolts are torqued to spec, and lock tight on bolts (that are grade 8 quality). I am getting frusturated because this problem cost me 300 dollars to get new parts for, and I am bored of spending money on the same problem over and over. HELP ME!!

Parts needed to fix:

New pulley
Pulley boss
Crank Pos. Sensor
Arp fasteners
Timing wheel
Aligning nipple

Not sure why you keep having this problem. I could see it on a SC car, but not Turbo. What is redline on your motor? Did you have the pulley assembley ballanced? I'm just throwing things out there as I can't figure out why this would happen.

The bolts weren't chome plated, by any chance? That would make them brittle, if not done properly.

Another thing I have found. The grade 8.8, that some of the parts stores carry, don't seem to be equal to standard grade 5 quality. Stainless bolts are also slightly less than grade 8 quality.

One last thing.

Aircraft bolts have a better sheer strength than grade 8 bolts by quite a bit. The only problem would be finding Metric aircraft bolts, since almost all of the planes in the U.S. are built with SAE sizes. The Airbus and European makes would be the exeption.

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Originally Posted by Radio-Active

One last thing.

Aircraft bolts have a better sheer strength than grade 8 bolts by quite a bit. The only problem would be finding Metric aircraft bolts, since almost all of the planes in the U.S. are built with SAE sizes. The Airbus and European makes would be the exeption.

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Starwares in Arlington has a good selection of MIL standard bolts. You might want to give them a try.

There has got to be vibration issue in the crank assembly. I've driven/raced Miatas about 600,000 miles and have never had, seen, or heard of this happening. Has this engine been build with "parts"? I'm betting it is out of balance. Since this is a '99, has the thrust bearing runnout ever been checked. Lots of engines were replaced for bad ones.

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Originally Posted by MadMerv

There has got to be vibration issue in the crank assembly. I've driven/raced Miatas about 600,000 miles and have never had, seen, or heard of this happening. Has this engine been build with "parts"? I'm betting it is out of balance. Since this is a '99, has the thrust bearing runnout ever been checked. Lots of engines were replaced for bad ones.

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He is running a fully built motor from Flyin Miata, and is putting down around 400hp at the wheels.

Specs on the bottom end from another thread:

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Complete Flyin Miata built long block including:
FM Lightened Knife edged and Balanced Crank
Carrillo H Beam Rods
ARP Lower end studs
FM Billet Main Crank Girdle
Weiseco 9.1 Compression pistons
ARP Head bolts

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Quote:

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Originally Posted by Titus

He is running a fully built motor from Flyin Miata, and is putting down around 400hp at the wheels.

Specs on the bottom end from another thread:

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Which... with all due respect to FM... means that it has been "built with parts..."

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Originally Posted by tailchaser

Which... with all due respect to FM... means that it has been "built with parts..."

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Yep... I think the first thing I would do is call FM and see what they say. It may be a problem they have seen before and have a good idea of what is causing it.

OK... I knew I had read about this before... sounds like FM has found some bolts for $10 from APR that FM is recommending specifically for high HP cars that have problems shearing off crank bolts.

With all due respect to Radioactive and miatamoto, I don’t think changing bolt spec is going to make any difference in this situation. While MILSPEC fasteners have a higher yield strength than SAE Grade 8 or metric Grade 10.9, you’ve got four bolts holding that pulley on, which means that each one only sees ¼ of the total shear load on the joint. I don’t know what size bolts you’re using, but I’d expect the belt to fail long before one of those bolts would in pure shear.

So, if pure shear isn’t causing the failure, what is? My guess would be vibrational fatigue. You need to take all of the pulleys in the system to a reputable machine shop and have them inspected not only for balance, but also for variations in groove height and both lateral and radial runout. If the height variation exceeds about .008” or the total indicated runout in either direction exceeds .015”, I’d have the pulleys remachined or replace them with one that meets those specs.

Making sure the pulleys are balanced is one step in the process (similar to checking wheels and tires for balance). However, the pulley can be perfectly balanced and yet still cause vibration problems if it is misshapen – this is akin to a tire being “out of round”, yet perfectly balanced. Checking the runout on the pulleys will help eliminate this potential problem. I’d also agree with Merv’s suggestion to check the runout on the crankshaft.

In addition, here are a few other things you can do to increase the life of a belt driven system:

• Make sure all fasteners are properly torqued – we can track the vast majority of out field fatigue failures to undertorqued fasteners, but overtorqued ones will drastically reduce fatigue life as well.
• Make sure belts are properly tensioned. You can buy a tension gauge at most auto shops, and the belt manufacturer should be able to provide you with a tension spec for the belts you’re using.

Hope that helps,

I doubt he can check to see if the pulley being out of round is the cause. ;)

I like the explanation of the pointy board of using too long of bolts. That would cause a definite vibration. The crankshaft should have been inspected for run out, thrust bearing gap and bearing fit when the engine was built.

Wow... Iain took the words right out of my mouth. ;)

If you replaced the pulley bolts with even 2 mm longer than stock it will keep the crank pulley from setting properly on the crank base. The threaded base that mounts to the crank with the one large bolt is designed so the 6 x 1.00 bolts do not extend pass the back. If they do then the pulley will not properly be mated to the base and will wobble until it loosens the bolts and then breaks them. That is exactly what happen to you.

We know this pulley arrangement well since we use a lot of SC overlay pulleys and as far as the hardware, the factory bolts with a little Blue Lock-Tite will work just fine.

Gary

I agree with Gary.
The supercharged car are adding way more shear both by adding to the leverage by adding a pulley and by adding the extra tension of the SC belt. We use the stock bolts with no problems. (knock on wood)
I don't think a turbo car (know matter what HP) should have any more force on the crank pulley than stock. Are you using the stock bolts? If not, are they too long? If you are using the stock bolts then there is something seriously out of shape or balance in your engine. I'd talk to FM and I'd want a better fix than use some stronger bolts. For what it's worth, that's my 2 cents.